- 8 Function of Carbohydrates in Our Bodies
- The Background on Carbs
- What Do Carbohydrates Do? A Look at the Function of Carbohydrates in The Body
- How Carbohydrates Function…
- How to Maximize the Function of Carbohydrates
- Carbohydrates 101: The benefits of carbohydrates
- Bread is a type of carbohydrate
- Types of carbohydrates
- Digestion of carbohydrates
- Glycaemic index
- Examples of Carbohydrates
- Simple Carbohydrates
- Complex Carbohydrates
- SAT / ACT Prep Online Guides and Tips
- What Are Carbohydrates and How Do They Work?
- Why Do Some People Choose to Limit Carbs?
- Examples of Carbohydrates: Sugars, Starches, and Fibers
- 47 Examples of Carbohydrates: Simple vs Complex
- Review: What Exactly Are Carbohydrates?
- What’s Next?
- Functions of Carbohydrates
- Function of carbohydrates
- Simple vs. complex carbohydrates
- Sugars, starches and fibers
- Good carbs vs. bad carbs
- Glycemic index
- Carbohydrate benefits
- Carbohydrate deficiency
8 Function of Carbohydrates in Our Bodies
Between the macros there’s a lot up for debate, leaving many of us confused as to the function of carbohydrates, fats and proteins in our bodies.
Generally speaking, we’ve been told that many carbs are a no-go if you want to lose weight, gain muscle, or you know, get the healthiest, most nutrient dense diet possible.
But let’s be real, how many of you know the ins and outs of the humble carb? Like how carbohydrates function, carbohydrates food sources and how they affect our bodies?
Let’s stop making assumptions and dive deep into the nitty gritty. Here’s a look at what makes a carb a carb, when you should avoid them, and when to keep ‘em around.
Use this table of contents to skip to whatever section you’re looking for:
The Background on Carbs
Carbs have a wide-ranging scope of physical effects (both positive and negative). But before we delve into that, let’s break things down a little…
What are Carbohydrates?
Well, a carb is a macronutrient—a sugar, starch or fiber found in grains, fruits, vegetables, and dairy products. They’re hard to avoid and they’re one of the most basic food groups humans need to stay alive.
Basically, carbs get their name from their chemical makeup—containing carbon, hydrogen, and oxygen. Once we eat a carb, our bodies convert it into glucose (one of our body’s primary energy reserves) —which is transported to our cells via insulin. Once there, they’re able to execute…
The Main Function of Carbohydrates
The main function of carbohydrates is to provide fuel for the nervous system and keep muscles on the move.
Which sounds simple, but it’s an extremely complicated process that involves almost every system of the body. We’ll delve into this more below. There are 3 types of carbohydrates…
The 3 Types of Carbohydrates:
Carbs can be found in just about everything we eat, but not all carbs are the same. Here’s a quick breakdown of where are carbohydrates found:
A complex carbohydrate, found in vegetables, grains, and beans. Starchy foods, while often maligned, provide the body with key nutrients like iron, calcium and B complex vitamins. But, going overboard on the potatoes, rice, and breads can lead to the dreaded food coma or make you feel sleepy after eating.
While sugar is relatively familiar, the definition of this type of carb is more expansive than the white stuff you put in your tea…
So, what is sugar? Sugar, can be classified as any number of sweet, colorless, water-soluble compounds. Sugars are found in fruits, vegetables, milk and more—making up the simplest group of carbs—easily identified by the “-ose” suffix they carry around.
Sucrose (or table sugar) is the most common type—found in soft drinks and processed foods. Other sugars include fructose, which comes from fruit and lactose, which comes from milk.
Fiber also falls into the complex carbohydrate category. Fiber isn’t digested by the body—passing through the stomach, mostly intact. While this sounds a bit strange, it’s worth pointing out that fiber keeps you full, regular, and satisfied after eating.
These healthy carbs can be found in fruits, vegetables, whole grains, and cooked dry beans/peas. Beyond digestive health, fiber may have a positive effect on cholesterol.
Good Carbs vs Bad Carbs. Different Examples of Carbs and How They Function
Most scientists agree that not all calories, or more specifically, carbohydrates are created equal—specifically because different types of carbohydrates function differently in our bodies (source) —Some filling us with the nutrients, while others may lead to common health issues.
Here’s another means of categorizing carbohydrate foods: based on their chemical structure:
To put it simply, simple carbs are made from just one or two sugar molecules. Simple carbs include glucose, sucrose, fructose, and lactose.
Now, while simple carbs are often considered the bad guys, the truth is a little more nuanced than that. Simple carbs found in fruit or dairy products are considered healthy for most people (in moderation), while foods containing processed or refined sugars such as table sugar (sucrose) are not exactly the best choice. To understand the function of simple carbohydrates, may not be ideal.
Complex carbs also contain sugar molecules but are present in the form of long chains (3 or more). Which means, these carbs take longer for the body to digest and absorb.
These are often referred to as “good” carbs, and include items like beans, peas, whole grains, and fiber-rich fruits/vegetables. In addition, these foods contain many essential nutrients like fiber, vitamins, and minerals.
Complex carbs may also be refined. Which means the high fiber parts of the grain are removed–think white bread, potato chips, and baked goods. The refining process “simplifies” the carb, stripping it of its health benefits.
When it comes to carbohydrates, luckily, it’s pretty easy to tell the good from the bad.
However, you can’t look at a list of carbohydrates, and categorize them as the traditional good vs bad, per se—rather refined vs. natural.
Per Medline, most carbs should come from complex carbs and naturally occurring sugars (found in milk, fruit and veggies).
What Do Carbohydrates Do? A Look at the Function of Carbohydrates in The Body
Seeing as carbohydrates are one of the 3 macronutrients needed by our bodies, they’re in charge of many basic functions. To name a few function of carbohydrates:
Blood sugar and insulin
Eating carbohydrates is a quick way to increases blood sugar and stimulate insulin production—but is this a good thing? How does insulin work? And what is blood sugar?
It’s relatively simple: You eat carbs which are broken down by your digestive tract into simple sugars. These are absorbed into the bloodstream as glucose i.e. blood sugar.
Glucose gets transported to the body’s cells via insulin—a hormone made in the pancreas—which is then used by our cells as fuel (source). Once glucose is moved out of the bloodstream and into the cells, our blood sugar levels go back down (source).
Sounds great, but, this system doesn’t run smoothly for everyone, and for those people, controlling blood sugar is a regimented and arduous task.
The first scenario in which this system doesn’t run smoothly is seen in people with a…
1. Lack of insulin responsive cells or production (source)—which leaves glucose and insulin levels at a heightened level after eating. Overtime, this high demand on insulin producing cells can wear them out to the point of preventing further insulin production.
This is where insulin resistance, type 2 diabetes, heart disease, weight gain and other blood sugar/insulin perpetuated maladies arise.
2. On the other hand, hypoglycemia, or low blood sugar is the result of blood sugar dropping too low. This can happen if your body uses glucose too quickly, or it’s released into the blood stream slowly/not at all (source)—hypoglycemia is rare in non-diabetic patients (source).
Which leads us to the autoimmune disease with a similar blood sugar response: Type 1 Diabetes, in which the pancreas produces little to no insulin (source).
Healthy blood sugar and insulin levels are key to maintaining a healthy body. Whether you have a health issue related to regulating your blood sugar or not, it’s important to understand the foods that can spike glucose levels.
Scientists are continually finding more evidence centered around maintaining health blood sugar levels and the wide range of effects it has on our bodies—from mental to physical health.
Carbohydrates function in energy production by supplyingd our bodies with 4 easily digestible calories (energy in food) per gram, making them our body’s main source of energy (source)—that said, they’re not the only source of energy, but that’s a convo for another article.
We already touched on insulin and glucose/blood sugar regulation above—from this process, whatever glucose isn’t immediately used for energy is sent to the liver or muscles in the way of glycogen for later use, or is stored as fat (source).
Simple carbs are used quickly as an energy source because their minimal molecular structure is easy for the body to break down. This means a quick spike in energy, followed by, sadly, a vengeful crash.
On the other hand, complex carbs are made of multiple sugar molecules—which takes more time for the body to break down into energy, therefore your blood sugar won’t go through such highs and lows.
Triggers appetite, hunger and fullness
A study conducted on the moods of grumpy people reaching for an afternoon snack found that carbohydrates increased their brain’s productions of serotonin more than other food groups (source)—consuming protein, unfortunately, didn’t have the same effect.
Serotonin is known as the “feel good hormone”. Once activated in the brain, it works to stimulate sleep, regulate blood pressure, control your mood, appetite and your sensitivity to pain.
Which sounds totally great—but there are some drawbacks…
Because of serotonin’s ability to control your mood, many people lean on the overconsumption of carbohydrates to feel better—essentially using this food group as a drug (source). Hence the term “carboholics”.
As if that wasn’t enough, our bodies have another set of hormones known as the “hunger hormones”—ghrelin and leptin. Ghrelin is the hormone that makes you feel hungry, while leptin suppresses your appetite.
This study evaluated ghrelin and leptin levels after patients consumed various meals—each high in either protein, fat or carbs.
The result of the high carb meal? At first, ghrelin levels were strongly suppressed—then they rebounded, with a vengeance—making subjects hungrier than they were prior to eating—scientists then concluded that consuming carbs can make you hungry, and crave carbs again…
…which explains why many of us have a hard time calling it after just one cookie.
Side note, and a topic for another article—the study found that protein was the best suppressor of ghrelin (source).
We already discussed serotonin, but here’s a little more on carbohydrate’s effect on our mood…
Serotonin is made in the brain, where it performs its main functions (source). The production of serotonin is stimulated by a handful of nutrients combined with serotonin’s precursor tryptophan—an essential amino acid (meaning the body can’t make its own and it must come from diet) (source).
Foods high in tryptophan are protein-rich foods, such as meat, dairy fruit and seeds (source). So, where do tryptophan-lacking carbohydrates come into play?
Our brains are protected from the passage of certain substances by a filtration process called the blood-brain barrier. As you can imagine, there’s constant competition for amino acids to pass this barrier, making it difficult for tryptophan to get through. But, when you consume carbs, insulin is secreted, which works to decrease the level of competing amino acids in the blood, making way for the tryptophan to cross the barrier (source).
Ergo, consuming carbohydrates helps facilitate the transport of serotonin’s precursor tryptophan to the brain.
This has led to a few opinions on carbohydrate’s effect on our moods…
1. One side is carb consumption is directly correlated to good moods:
MIT researcher Dr. Wurtman suggested that when you stop eating carbs, your brain stops regulating serotonin—and carb consumption is the only natural means of stimulating production of this hormone. She continues to say that…
“a meal like pasta or a snack of graham crackers will allow the brain to make serotonin, but eating chicken and potatoes or snacking on beef jerky will actually prevent serotonin from being made” (source).
To me, this is outdated research, and I tend to agree more with recent studies that show…
2. Consuming the right carbs is necessary for mental health, and sugar can negatively affect your mood.
We already know that not all carbs are created equal—and a growing body of research suggests a strong link between diets high in refined carb and depression. To name a few:
1. This study conducted over a span of 22 years found that men who consumed 67 grams of sugar or more daily were 23% more likely to be diagnosed with depression.
2. Per this study, depression in postmenopausal women may be directly links to diets high in refined carbohydrates.
Fiber—a carbohydrate found in fruit, vegetables, whole grains and legumes—is essential for healthy digestion.
There are two types of fiber, soluble and insoluble, both of which are in most plant-based foods—ergo, eating a variety of high-fiber foods will help you receive the most health benefits (source).
Fiber is known as the bulk that moves everything through our digestive tracks—because, unlike proteins, fats and other carbohydrates, it’s indigestible and passes through our systems mostly intact—adding roughage to your bowels while keeping you full, regular, and satisfied after eating.
That said, as you know from the above, not all carbohydrates are fiber. There are also sugars and starches…
…And studies show that diets high in refined sugar have a slower digestion, due to the effects refined sugars have on our gut bacteria.
Prebiotics and Colon Health
On top of fiber and its significant role in our digestive health, carbohydrates bring another group of beneficial foods to the table…
Prebiotics are nondigestible carbohydrates that stimulate the growth or activity of bacteria in the colon. This ultimately improves digestive health. Unfortunately, current research on this group of foods is limited, it’s been suggested that they may:
- Reduce symptoms of IBS
- Protect against colon cancer
- Enhance uptake of certain minerals
- Lower some risk factors of cardiovascular disease
All prebiotics are fiber, but not all fibers are prebiotic—Looks for them in foods such as leeks, asparagus, and chicory.
The brain and its abundance of nerve cells, demand more energy than any other organ. In fact, they require half the glucose in our bodies! (source)
Which leads many of us to believe that when the body lacks an adequate amount of glucose, the brain and its functions (thinking, learning and memory) are affected.
But, that’s not necessarily true…
Studies have shown that people with an increased level of ketones in the body—experienced by low carb dieters who switch from using sugar as a main source of energy to fat—may have positive effects on memory and neuro function.
In addition, many low carb meal plans tend to be high in healthy fats such as omega 3s—which play a crucial role in brain functionality
A little more worrisome is the increased body of evidence pointing to a link between elevated glucose levels and an increased risk for dementia and cognitive deficiencies (source + source)
Another controversial subject in the way of carbohydrates is the affect they may or may not have on our weight.
On one side, we’ve got fiber. Fiber is considered an important tool for weight management as it fills you up, and keeps you satiated.
What’s more, a study published in the Annals of Internal Medicine examined 240 patients with metabolic syndrome and found that those who simply increased their intake of fiber were able to lose more weight.
That said, fiber isn’t the only type of carbohydrate…
The other side of the argument is that reducing carb intake is the fastest way to stabilize glucose and insulin levels—which raised levels of insulin can program the body to gain weight (source).
As mentioned above, when we eat more carbs than our cells require for immediate energy, they’re stored as glycogen or as fat. That said, building these reserves without expenditure leads to excess weight.
Finally, increasing total carbs means increasing total calories. Calories in, calories out—traditional wisdom applies.
How Carbohydrates Function…
How Carbohydrates Function When You Eat Too Many
Carbohydrate examples are most typically linked to cookies, cakes, late night pizza binges, you know, basically anything that can derail a diet and send you into a food coma.
So, it should come as no surprise that hitting the carbs too hard can lead to some pretty serious health effects. Here are the potential affects, more information above:
- Spike in blood sugar and insulin levels
- Food comma
- Triggers appetite
- Carb consumption as a means of comfort
- Refined sugar and slowed digestion
- Potential weight gain
- Negative effects to cognitive health
How Carbohydrates Function On a Low Carb Diet
There are several reasons to limit your carbohydrate foods intake — here are some of the potential benefits you’ll see when you stick to low carb meals:
- Easier to regulate blood sugar and insulin
- Increased energy, after the initial “hump”
- Keeps you full longer
- Potential weight loss
- Improved cognitive health
- Less cravings
And while there’s a whole slew of terrifying bodily functions taking place after going on a full-fledged carbohydrate foods binge, there are some risks—or rather, unpleasant effects that can take place.
Fearing carbs uniformly goes against the wisdom of the good old “five-a-day” advice—leaving out veggies and fruits that provide key nutrients like vitamins, minerals, fiber, and antioxidants. A deficiency in carbohydrates can lead to:
- Not So Fresh: The bad news about relying primarily on fat and protein? The lack of carbs builds up ketones, which can cause bad breath. If you’re planning to go super low carb, stay fresh by incorporating plenty of sugar free gum.
- You’re constipated: Another issue with losing the carbs is, losing the critical fiber that goes along with it. And with fiber comes, well, regularity. Keep your system running smoothly by making sure you’re getting enough vegetables, fruits, and whole grains.
- You’re Wiped Out: While a low carb diet can get rid of those nasty sugar highs and lows, no carbs means no energy in a lot of cases. If you’re feeling sluggish, it may be time to reevaluate your fruit and vegetable intake. You may also be experiencing the carb flu, which is your body transitioning from sugar as an energy source to fat—which typically lasts about a week.
How to Maximize the Function of Carbohydrates
Beyond the carbohydrates definition that divides the two into simple and complex, there are a few other ways you can capitalize on the benefits of this macronutrient…
1. Eat Healthy Carbs
At this point of the article, this may seem like a giant “duh”, but it’s worth mentioning. To maximize the benefits of carbs in your diet, eat the right ones.
2. What Are Macronutrients and How to Balance:
The three macronutrients I’m referring to are: carbs, proteins and fats.
We’ve gone over the function of carbohydrates, but they rely on other nutrients to help maximize their potential.
What is the Function of Fats: Fats essentially provide a backup energy source for your body to use when carbs aren’t available—which is why low carb diets are linked to weight loss. Fats help maintain your body temperature and absorb nutrients.
What is the Function of Lipids: The function of lipids in the body is diverse, but these compounds play a role in energy storage and provide energy needed for a number of internal processes. The main function of lipids is to power our muscles.
What is the Function of Proteins: Proteins are present in every cell of your being, from your fingernails to your skin and organs. The main function of proteins is to regulate processes in the body, like breaking down food and transporting materials throughout the system.
3. Rethinking/How to Quit Sugar
Because the carbs in sugar are simple, it’s safe to say, breaking up with sugar could be a good move for your health. But there’s more to it than cutting out notorious sugar foods like candies, cakes, and pints of ice cream.
A few steps for making the change:
1. Start by cutting out things like soft drinks and added sugar in your coffee and tea, and go from there.
2. Read food labels! Look out for things like evaporated cane juice, corn syrup, sucrose, dextrose, fruit juice concentrate, and more.
3. Eat whole foods—packaged tomato sauces, condiments, salad dressings, and store-bought baked goods are often packed with the sugar. Go for plain vegetables, whole grains like brown rice and quinoa, and unseasoned meats. Seasoning yourself can lead to more health-conscious decisions.
4. Skip the juice—fruits are loaded with sugar, but also are rich in fiber. Eaten whole, fruit sugars are not a problem for most people (in moderation of course).
While a low or no sugar diet can be hard to get used to, finding a meal plan that works for you is crucial. For more tips on ditching simple carbs, here are some of our favorite low carb recipes.
4. Look at Net Carbs
If you’d like to try a low carb diet, but are worried about some of these effects, focus more on net carbs. Net carbs are the grams of total carbs in any given food, minus the total grams of fiber. The thought here is that you only consider the difference as part of your overall carb intake.
The Function of Carbohydrates and What’s Right For You?
Now that you know the function of carbohydrates, it’s up to you to decide what your body needs.
Whether you’re diabetic, overweight, or looking to amp up your workout routine, a low carb foods lifestyle might be a good fit—but it’s worth mentioning that balance is the key to your success—we might sound like a broken record, but diverse nutrients are the only way to go.
For others, it might be worthwhile to cut back on simple carbs and focus on getting the five-a-day fruits and veg from whole food sources. Everyone’s body is different and requires a different balance to function properly.
Have you ever tried a low carb diet? If so, let us know what your experience was like, or feel free to share any favorite recipes or food hacks.
Note from the Author:
When I started writing this article, it was a huge struggle. I thought of a function a positive trait—and man, have I been mad at carbs lately. No, I’m not entirely anti-carb, but I’m also not their number one fan.
With my recent life changes—all of which are centered around this one macronutrient—I’ve spent late nights browsing PubMed, talking to doctors, and reading everything I can find about carbs…
…And the most important lesson I’ve learned after all this research is this: whether you simply want to know what carbs do (and you’re currently snacking on a plate of cookies) or you haven’t touched a sugar/starch in years, there is not one simple answer for every person.
Figure out your body, what works best for you, and what you need to function. I know a low carb diet is how I need to eat, and as a diabetic who’s been sick most of her life, it’s worth sticking to this lifestyle to remain a much healthier and happier person!
That said, I bring to you a collaboration of my research on carbohydrates, can’t wait to hear from you, let us know in the comments below what brought you here.
Carbohydrates 101: The benefits of carbohydrates
Though there are many benefits of carbohydrates you need to make sure you’re eating them in moderation. A carbohydrate-intensive diet can cause high blood sugar and unwanted weight gain. But it’s important that you eat an appropriate amount of healthy carbohydrates in order to meet your body’s nutritional needs and maintain a healthy weight.
What are carbohydrates?
Carbohydrates are one of three macronutrients – along with proteins and fats – that your body requires daily. There are three main types of carbohydrates: starches fiber and sugars. Starches are often referred to as complex carbohydrates. They are found in grains legumes and starchy vegetables like potatoes and corn. Sugars are known as simple carbohydrates. There are natural sugars in vegetables fruits milk and honey. Added sugars are found in processed foods syrups sugary drinks and sweets.
Why do you need carbohydrates?
Carbohydrates are your body’s main source of energy: They help fuel your brain kidneys heart muscles and central nervous system. For instance fiber is a carbohydrate that aids in digestion helps you feel full and keeps blood cholesterol levels in check. Your body can store extra carbohydrates in your muscles and liver for use when you’re not getting enough carbohydrates in your diet. A carbohydrate-deficient diet may cause headaches fatigue weakness difficulty concentrating nausea constipation bad breath and vitamin and mineral deficiencies.
What are some healthy sources of carbohydrates?
To reap the benefits of carbohydrates you should choose carbohydrates that are loaded with nutrients. Christie Ferriell a registered dietitian and nutrition manager at Reid Health recommends that you get at least half of your carbohydrates from whole grains. Ferriell notes that “whole grains provide fiber that helps you feel full and satisfied with smaller portions.” Ferriell recommends that you try making quinoa pilaf with tofu and vegetables a heart-healthy recipe containing fiber- and protein-rich quinoa from Reid’s I Heart Cooking program.
Healthy carbohydrate-rich foods (containing 12 grams of carbohydrates or more per serving) include
- Whole grains: quinoa, amaranth, barley, brown rice, oatmeal, whole-grain pasta and whole-grain breakfast cereals
- Fruits: berries, citrus fruits, melons, apples, pears, bananas and kiwifruit
- Starchy vegetables: sweet potatoes, yams, corn. peas and carrots
- Legumes: lentils, black beans, pinto beans, navy beans, chick peas and soybeans
- Milk products: low-fat milk, plain yogurt and soy yogurt
Healthy foods lower in carbohydrates (less than 10 grams per serving) include
- Nonstarchy vegetables: leafy greens, spinach, cabbage, asparagus, tomatoes, broccoli, cauliflower, green beans, cucumbers, peppers, zucchini and mushrooms
- Nuts and seeds: pumpkin seeds, sunflower seeds, almonds, cashews, walnuts, peanuts and pistachios
- Soy milk and tofu
How many grams of carbohydrates do you need?
The Dietary Guidelines for Americans 2010 suggest that most adults get 45 to 65 percent of their calories from carbohydrates. Since carbohydrates contain four calories per gram you should consume 225 to 325 grams of carbohydrates on a daily basis if you’re following a 2000-calorie diet.
According to the United States Department of Agriculture you should consume at the very least the recommended dietary allowance (RDA) of carbohydrates which is 130 grams for adults 175 grams for women who are pregnant and 210 grams for women who are breastfeeding. According to the Dietary Guidelines for Americans 2010 women should consume 25 grams of fiber on a daily basis while men should consume 38 grams of fiber on a daily basis.
What if I have diabetes?
If you have diabetes you should see a doctor or dietitian who can help you plan meals to control your blood sugar. Though your daily carbohydrate requirements are the same as those for someone without diabetes it’s important to avoid eating too many carbohydrates in one sitting. The American Diabetes Association suggests that you should limit your intake to about 45 to 60 grams of carbohydrates at each meal.
The bottom line
You should avoid added sugar processed foods refined grains (like white bread) sodas other sugary drinks and sweets as much as possible. To help you look and feel your best you should choose nutrient-dense healthy carbohydrates.
Want to learn more about Carbohydrates?
Try the New Nourish You! We’ll help you reach your personal goals plus our group nutrition education experts will give you the support you need to stay on track. Together our program helps you dive into nutrition behavior management exercise and healthy recipes.
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Bread is a type of carbohydrate
Our bodies need carbohydrates for energy. Carbohydrates are present in all fruit and vegetables, breads and grain products, and sugar and sugary foods.
It is best to choose carbohydrate-rich foods that are healthy and full of dietary fibre. Try to limit your intake of snack foods, as they are high in kilojoules, saturated fat, sugars and salt, and have very few nutrients.
Types of carbohydrates
Sugars, starches and some types of dietary fibre are carbohydrates. Sugars include:
- glucose — in fruit, honey and some vegetables
- fructose — in fruit and honey
- sucrose — from sugar cane
- lactose — in all types of milk including breast milk
- maltose — in malted grains
Starches are also known as complex carbohydrates. Starches can be found in:
Starches are also found in cereal products such:
- breakfast cereals
- burgul (cracked wheat)
Dietary fibre is found in many different plant foods including:
- wholegrain foods
There is not enough sugar in fruit and milk to be a problem. But the sugars and starches in many snack foods are highly refined. You should only eat biscuits, sauces and confectionery in small amounts.
Digestion of carbohydrates
Carbohydrates are digested in your small intestine. They are broken down into simple (single) sugars such as glucose and fructose.
These sugars are absorbed into the bloodstream and can be used for energy. Some sugar is converted to glycogen and stored in the liver. Between meals, liver glycogen is converted back into blood glucose as an energy supply.
Glycogen is also stored in muscles for muscle activity. Carbohydrates not used for energy or glycogen storage are converted to fat.
The speed at which the carbohydrates in a food are digested to glucose is called the food’s glycaemic index (GI) value.
Low GI foods are broken down slowly. The glucose, or energy, from their carbohydrates is released into the blood over several hours. Low GI foods prolong digestion due to their slow break down and may help with you feel full for longer.
High GI foods are digested rapidly and give you a blood glucose spike. Slow digestion is better than fast digestion, so low GI foods are generally better than high GI foods. This is important if you have diabetes — it helps keeps your blood glucose levels stable.
It’s important to look at the overall nutritional value and carbohydrate content of a food item, not just the GI. Junk food with a low GI is still junk food.
Carbohydrates have six major functions within the body:
- Providing energy and regulation of blood glucose
- Sparing the use of proteins for energy
- Breakdown of fatty acids and preventing ketosis
- Biological recognition processes
- Flavor and Sweeteners
- Dietary fiber
Providing energy and regulating blood glucose
Glucose is the only sugar used by the body to provide energy for its tissues. Therefore, all digestible polysaccharides, disaccharides, and monosaccharides must eventually be converted into glucose or a metabolite of glucose by various liver enzymes. Because of its significant importance to proper cellular function, blood glucose levels must be kept relatively constant.
Among the enormous metabolic activities the liver performs, it also includes regulating the level of blood glucose. During periods of food consumption, pancreatic beta cells sense the rise in blood glucose and begin to secrete the hormone insulin. Insulin binds to many cells in the body having appropriate receptors for the peptide hormone and causes a general uptake in cellular glucose. In the liver, insulin causes the uptake of glucose as well as the synthesis of glycogen, a glucose storage polymer. In this way, the liver is able to remove excessive levels of blood glucose through the action of insulin.
In contrast, the hormone glucagons is secreted into the bloodstream by pancreatic alpha cells upon sensing falling levels of blood glucose. Upon binding to targeted cells such as skeletal muscle and brain cells, glucagon acts to decrease the amount of glucose in the bloodstream. This hormone inhibits the uptake of glucose by muscle and other cells and promotes the breakdown of glycogen in the liver in order to release glucose into the blood. Glucagon also promotes gluconeogenesis, a process involving the synthesis of glucose from amino acid precursors. Through the effects of both glucagon and insulin, blood glucose can usually be regulated in concentrations between 70 and 115mg/100 ml of blood.
Other hormones of importance in glucose regulation are epinephrine and cortisol. Both hormones are secreted from the adrenal glands, however, epinephrine mimics the effects of glucagon while cortisol mobilizes glucose during periods of emotional stress or exercise.
Despite the liver’s unique ability to maintain homeostatic levels of blood glucose, it only stores enough for a twenty-four hour period of fasting. After twenty four hours, the tissues in the body that preferentially rely on glucose, particularly the brain and skeletal muscle, must seek an alternative energy source. During fasting periods, when the insulin to glucagons ratio is low, adipose tissue begins to release fatty acids into the bloodstream. Fatty acids are long hydrocarbon chains consisting of single carboxylic acid group and are not very soluble in water. Skeletal muscle begins to use fatty acids for energy during resting conditions; however, the brain cannot afford the same luxury. Fatty acids are too long and bulky to cross the blood-brain barrier. Therefore, proteins from various body tissues are broken down into amino acids and used by the liver to produce glucose for the brain and muscle. This process is known as gluconeogenesis or “the production of new glucose.” If fasting is prolonged for more than a day, the body enters a state called ketosis. Ketosis comes from the root word ketones and indicates a carbon atom with two side groups bonded to an oxygen atom. Ketones are produced when there is no longer enough oxaloacetate in the mitochondria of cells to condense with acetyl CoA formed from fatty acids. Oxaloacetate is a four-carbon compound that begins the first reaction of the Krebs Cycle, a cycle containing a series of reactions that produces high-energy species to eventually be used to produce energy for the cell. Since oxaloacetate is formed from pyruvate (a metabolite of glucose), a certain level of carbohydrate is required in order to burn fats. Otherwise, fatty acids cannot be completely broken down and ketones will be produced.
Examples of Carbohydrates
- Galactosamine hydrochloride
- Mannosamine hydrochloride
- Galactose-6-O-sulfate sodium salt
- Acetyl-D-glucosamine-6-O-sulfate sodium salt
- Glucosamine-2-N, 3-O-sulfate sodium salt
- Acetyl-D-galactosamine-4-O-sulfate sodium salt
- Galactosamine-2-N-sulfate sodium salt
- Mannose-6-O-sulfate sodium salt
- Acetylneuraminic acid
- Arabino – 1,4-lactone
- Lyxono – 1,4-lactone
- Ribono – 1,4-lactone
- Threono – 1,4-lactone
- Methyl uronate
- GlcNAc L- asparagine
- Xylosylated L- serine
- Manosylated L- serine
- Galactosylated L- serine
- Galactosylated L- tyrosine
Foods with Simple Carbohydrates
- Baked goods (including bread) made with white flour
- Candy bar
- Carbonated drink
- Corn syrup
- Fruit juice
- Fruit preserve or jam
- Whole milk
- Plain, full fat yogurt
- Most packaged cereals
- Pasta made with white flour
- Table sugar
- Fructo – oligosaccharides (FOS)
- Galactooligosaccharides (GOS)
- Gentianose – a trisaccharide
- Maltotriose – a trisaccharide
- Mannan oligosaccharides (MOS)
- Raffinose – a trisaccharide
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You’ve likely heard people talk about carbohydrates or carbs, whether it be on TV or in your day-to-day life. But what exactly are carbs? What are examples of carbohydrates, and why do so many people try to limit them in their diets?
Here, we’ll explain how carbs work and give you numerous examples of complex carbohydrates, in addition to examples of simple carbohydrates.
What Are Carbohydrates and How Do They Work?
Carbohydrates (or carbs for short) are molecular compounds made of carbon, hydrogen, and oxygen atoms. They are also called saccharides, a group in chemistry that includes sugars, starches, and cellulose.
Saccharides can be divided into three categories:
- Monosaccharides are the smallest and simplest carbohydrates, often referred to as single sugars. This group includes glucose, fructose, and galactose.
- Disaccharides are slightly bigger carbohydrates that form when two monosaccharides, or two sugars, react. They are more commonly found in nature. This group includes lactose, maltose, and sucrose.
- Polysaccharides are the biggest carbohydrates and are formed by a chain of reactions of monosaccharides. This group includes starch, glycogen, and cellulose.
In terms of diet, carbs are the starches, fibers, and sugars found in a variety of foods, such as grains, vegetables, fruits, and dairy products. Eating carbohydrates gives us energy in the form of calories. When you eat foods with carbs in them, your body turns these carbs into a type of sugar called glucose, thereby raising your blood sugar level and giving you more energy.
Carbohydrates are one of three macronutrients, meaning they are one of three types of food groups humans should be eating in large quantities in order to maintain a healthy diet and build energy (the other two macronutrients are protein and fats).
Typically, carbs are counted in grams. You can check the label of any packaged food to see how many carbs are in it. The National Institutes of Health says that the average adult should consume about 135 grams of carbs per day; however, it’s important to note that “each person should have their own carbohydrate goal.”
In food science, carbs are divided into two main categories: simple and complex. Here’s the main difference between these two categories, as explained by an article in Live Science:
“The difference between the two forms is the chemical structure and how quickly the sugar is absorbed and digested. Generally speaking, simple carbs are digested and absorbed more quickly and easily than complex carbs.”
Simple carbohydrates are monosaccharides and disaccharides (one to two sugars), and include foods with naturally occurring sugar in them, such as fruits. As stated above, simple carbs are generally digested faster, giving you a brief, immediate spike of energy but making you feel hungry again sooner.
Complex carbohydrates, on the other hand, are polysaccharides (three or more sugars) and include starchy foods, such as cereals and bread. Unlike simple carbs, complex carbs are digested slower, giving you long-term energy and making you feel full longer.
Thinking of trying a low-carb diet? Read on to learn what this entails.
Why Do Some People Choose to Limit Carbs?
Although carbs are an essential part of any healthy diet, many people choose to limit them in order to lose weight. These people will often focus on restricting refined sugars in pastries and other sweets, as well as starchy, refined foods such as white rice and white bread.
“Good” carbs are generally said to include any naturally occurring sugars in fruits and veggies, and whole-grain, fiber-rich foods. However, as some experts note, it’s not that carbs themselves are bad but rather how often you eat carbs and what types of carbs. Others look at carbs not in a good vs bad dichotomy but in terms of “whole” vs “refined,” with whole being the healthier option.
Nevertheless, the prevailing view is that simple carbs are unhealthy and complex carbs are healthy. This is due to the fact that simple carbs are digested faster and don’t provide you with as much long-lasting energy as complex carbs do.
Many people follow low-carb diets in which they focus on eating fewer (sometimes almost no) carbs and eating more proteins and fats. The ketogenic, or keto, diet, for instance, is a popular low-carb, high-fat diet that’s said to lower blood-sugar levels and help keep weight off.
Ultimately, regardless of your take on how healthy (or unhealthy) carbs are, it’s universally agreed that carbohydrates are an important part of a healthy, balanced diet.
Examples of Carbohydrates: Sugars, Starches, and Fibers
As mentioned, carbohydrates as a group consist of sugars, starches, and fibers. But which foods fall into which of these three categories? In other words, what foods are considered starchy? What foods have lots of fiber in them? And what foods are high in sugar?
Here are some examples of carbohydrates in the sugar, starch, and fiber categories:
- Naturally occurring sugars in fruits, vegetables, and milk/milk products
- Refined sugars in sweet foods such as pastries, syrups, cookies, and soft drinks
- Dried beans, such as lentil beans and kidney beans
- Grains, such as rice, oats, and wheat
- Whole-grain foods, such as bread, cereal, and pasta
- Some fruits and vegetables, particularly those with skins you can eat, such as apples and corn
- Nuts, such as peanuts, walnuts, and almonds
47 Examples of Carbohydrates: Simple vs Complex
As we know, carbs can be classified into two categories: simple and complex. Below, we give you examples of simple carbohydrates as well as examples of complex carbohydrates.
Examples of Simple Carbohydrates
- Raw sugar
- Brown sugar
- Corn syrup/high-fructose corn syrup
- Fruits (naturally occurring sugars)
- Vegetables (naturally occurring sugars)
- Milk and milk products (naturally occurring sugars)
- Refined sugars in the following:
- Soda and other sweetened beverages
- Pastries and other baked treats/sweets
- Breakfast cereals
- Fruit juice concentrate
Examples of Complex Carbohydrates
- Beans (kidney, garbanzo, black, white, pinto, etc.)
- Leafy greens
- Pasta, including whole-wheat pasta
- Rice (white and brown)
- Starchy vegetables such as the following:
- Butternut squash
- Sweet potatoes
- Bread (white, whole grain, etc.)
- Whole-grain cereal
Review: What Exactly Are Carbohydrates?
Carbohydrates, or carbs, are a big part of a healthy, balanced diet and consist of foods that have sugars, fibers, and/or starches.
Most people divide carbohydrates into two categories: simple vs complex. Simple carbs are digested faster and give you a quick burst of energy but don’t keep you feeling full long, whereas complex carbs are digested slower, giving you longer-lasting energy.
The general rule of thumb for a healthy diet is that you should try to limit your daily intake of simple carbs and eat more complex carbs.
Many refer to simple carbs as “bad” carbs and complex carbs as “good” carbs. However, as experts have noted, this highly dichotomous view of carbs is not that accurate, as some complex carbs contain refined sugars (such as white rice and white bread) and are generally viewed as unhealthy, while other foods with naturally occurring simple sugars (such as fruits and vegetables) are argued to be healthy options.
In the end, eating everything in moderation—including simple and complex carbs—is one of the best ways you can ensure you’re taking good care of your body!
Humans eat carbohydrates for energy, whereas plants use photosynthesis to get their nutrients. Learn more about the photosynthesis equation and how it works.
Did you know that when you eat food, it gets turned into energy—meaning that the mass of the food you ate hasn’t actually disappeared? Read our guide to learn everything there is to know about the law of the conservation of mass.
You know how carbohydrates work but what about the human brain? Check out our expert guide to learn about the different parts and functions of the brain.
Have friends who also need help with test prep? Share this article!Hannah Muniz About the Author
Hannah received her MA in Japanese Studies from the University of Michigan and holds a bachelor’s degree from the University of Southern California. From 2013 to 2015, she taught English in Japan via the JET Program. She is passionate about education, writing, and travel.
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Functions of Carbohydrates
- Carbohydrates provides energy and regulation of blood glucose.
- It will prevent the degradation of skeletal muscle and other tissues such as the heart, liver, and kidneys.
- It prevent the breakdown of proteins for energy.
- Carbohydrates also help with fat metabolism. If the body has enough energy for its immediate needs, it stores extra energy as fat.
- Carbohydrates are an important component of many industries like textile, paper, lacquers and breweries.
- Detoxification of physiological importance is carried out to some extent with carbohydrate derivatives.
- Agar is polysaccharide used in culture media, laxative and food.
- Carbohydrates form a part of genetic material like DNA and RNA in the form of deoxyribose and ribose sugars.
- Hyaluronic acid found in between joints acts as synovial fluid and provides frictionless movement.
- They help make up the body mass by being included in all the parts of the cell and tissues.
- Adequate storage of hepatic glycogen helps in detoxifying a normal liver.
- They form components of bio-molecules which have a key role in blood clotting, immunity, fertilization etc.
- Carbohydrates is basically the main fibre of the diet or provide the bulk fibre for better digestion.
- Carbohydrates help clear gut and prevent constipation.
- Starch is the form the food is stored in plants.
- It provides sweetness to foods.
- Pectine and Hemiceliulose are the structural carbohydrate in plant cell walls.
- It plays important roles in cellular recognition processes.
- Chitin forms the cell wall of fungi and the outer schelitone of insects.
- Murine is a structural carbohydrate in bacterial cell wall.
Carbohydrates are the sugars, starches and fibers found in fruits, grains, vegetables and milk products. Though often maligned in trendy diets, carbohydrates — one of the basic food groups — are important to a healthy diet.
“Carbohydrates are macronutrients, meaning they are one of the three main ways the body obtains energy, or calories,” said Paige Smathers, a Utah-based registered dietitian. The American Diabetes Association notes that carbohydrates are the body’s main source of energy. They are called carbohydrates because, at the chemical level, they contain carbon, hydrogen and oxygen.
There are three macronutrients: carbohydrates, protein and fats, Smathers said. Macronutrients are essential for proper body functioning, and the body requires large amounts of them. All macronutrients must be obtained through diet; the body cannot produce macronutrients on its own.
The recommended daily amount (RDA) of carbs for adults is 135 grams, according to the National Institutes of Health (NIH); however, the NIH also recommends that everyone should have his or her own carbohydrate goal. Carb intake for most people should be between 45% and 65% of total calories. One gram of carbohydrates equals about 4 calories, so a diet of 1,800 calories per day would equal about 202 grams on the low end and 292 grams of carbs on the high end. However, people with diabetes should not eat more than 200 grams of carbs per day, while pregnant women need at least 175 grams.
Function of carbohydrates
Carbohydrates provide fuel for the central nervous system and energy for working muscles. They also prevent protein from being used as an energy source and enable fat metabolism, according to Iowa State University.
Also, “carbohydrates are important for brain function,” Smathers said. They are an influence on “mood, memory, etc., as well as a quick energy source.” In fact, the RDA of carbohydrates is based on the amount of carbs the brain needs to function.
Two recent studies published in the journal Proceedings of the National Academy of Sciences have also linked carbs to decision-making. In the studies, people who ate a high-carbohydrate breakfast were less willing to share when playing the “ultimatum game” than those who ate high-protein breakfasts. Scientists speculate this may be caused by baseline dopamine levels, which are higher after eating carbohydrates. This doesn’t mean carbs make you mean, but underscores how different types of food intake can affect cognition and behavior.
Simple vs. complex carbohydrates
Carbohydrates are classified as simple or complex, Smathers said. The difference between the two forms is the chemical structure and how quickly the sugar is absorbed and digested. Generally speaking, simple carbs are digested and absorbed more quickly and easily than complex carbs, according to the NIH.
Simple carbohydrates contain just one or two sugars, such as fructose (found in fruits) and galactose (found in milk products). These single sugars are called monosaccharides. Carbs with two sugars — such as sucrose (table sugar), lactose (from dairy) and maltose (found in beer and some vegetables) — are called disaccharides, according to the NIH.
Simple carbs are also in candy, soda and syrups. However, these foods are made with processed and refined sugars and do not have vitamins, minerals or fiber. They are called “empty calories” and can lead to weight gain, according to the NIH.
Complex carbohydrates (polysaccharides) have three or more sugars. They are often referred to as starchy foods and include beans, peas, lentils, peanuts, potatoes, corn, parsnips, whole-grain breads and cereals.
Smathers pointed out that, while all carbohydrates function as relatively quick energy sources, simple carbs cause bursts of energy much more quickly than complex carbs because of the quicker rate at which they are digested and absorbed. Simple carbs can lead to spikes in blood sugar levels and sugar highs, while complex carbs provide more sustained energy.
Studies have shown that replacing saturated fats with simple carbs, such as those in many processed foods, is associated with an increased risk of heart disease and type 2 diabetes.
Smathers offered the following advice: “It’s best to focus on getting primarily complex carbs in your diet, including whole grains and vegetables.”
Sugars, starches and fibers
In the body, carbs break down into smaller units of sugar, such as glucose and fructose, according to Iowa State University. The small intestine absorbs these smaller units, which then enter the bloodstream and travel to the liver. The liver converts all of these sugars into glucose, which is carried through the bloodstream — accompanied by insulin — and converted into energy for basic body functioning and physical activity.
If the glucose is not immediately needed for energy, the body can store up to 2,000 calories of it in the liver and skeletal muscles in the form of glycogen, according to Iowa State University. Once glycogen stores are full, carbs are stored as fat. If you have insufficient carbohydrate intake or stores, the body will consume protein for fuel. This is problematic because the body needs protein to make muscles. Using protein instead of carbohydrates for fuel also puts stress on the kidneys, leading to the passage of painful byproducts in the urine.
Fiber is essential to digestion. Fibers promote healthy bowel movements and decrease the risk of chronic diseases such as coronary heart disease and diabetes, according to the U.S. Department of Agriculture. However, unlike sugars and starches, fibers are not absorbed in the small intestine and are not converted to glucose. Instead, they pass into the large intestine relatively intact, where they are converted to hydrogen and carbon dioxide and fatty acids. The Institute of Medicine recommends that people consume 14 grams of fiber for every 1,000 calories. Sources of fiber include fruits, grains and vegetables, especially legumes.
Smathers pointed out that carbs are also found naturally in some forms of dairy and both starchy and nonstarchy vegetables. For example, nonstarchy vegetables like lettuces, kale, green beans, celery, carrots and broccoli all contain carbs. Starchy vegetables like potatoes and corn also contain carbohydrates, but in larger amounts. According to the American Diabetes Association, nonstarchy vegetables generally contain only about 5 grams of carbohydrates per cup of raw vegetables, and most of those carbs come from fiber.
Good carbs vs. bad carbs
Carbohydrates are found in foods you know are good for you (vegetables) and ones you know are not (doughnuts). This has led to the idea that some carbs are “good” and some are “bad.” According to Healthy Geezer Fred Cicetti, carbs commonly considered bad include pastries, sodas, highly processed foods, white rice, white bread and other white-flour foods. These are foods with simple carbs. Bad carbs rarely have any nutritional value.
Carbs usually considered good are complex carbs, such as whole grains, fruits, vegetables, beans and legumes. These are not only processed more slowly, but they also contain a bounty of other nutrients.
The Pritikin Longevity Center offers this checklist for determining if a carbohydrate is “good” or “bad.”
Good carbs are:
- Low or moderate in calories
- High in nutrients
- Devoid of refined sugars and refined grains
- High in naturally occurring fiber
- Low in sodium
- Low in saturated fat
- Very low in, or devoid of, cholesterol and trans fats
Bad carbs are:
- High in calories
- Full of refined sugars, like corn syrup, white sugar, honey and fruit juices
- High in refined grains like white flour
- Low in many nutrients
- Low in fiber
- High in sodium
- Sometimes high in saturated fat
- Sometimes high in cholesterol and trans fats
Recently, nutritionists have said that it’s not the type of carbohydrate, but rather the carb’s glycemic index, that’s important. The glycemic index measures how quickly and how much a carbohydrate raises blood sugar.
High-glycemic foods like pastries raise blood sugar highly and rapidly; low-glycemic foods raise it gently and to a lesser degree. Some research has linked high-glycemic foods with diabetes, obesity, heart disease and certain cancers, according to Harvard Medical School.
On the other hand, recent research suggests that following a low-glycemic diet may not actually be helpful. A 2014 study published in JAMA found that overweight adults eating a balanced diet did not see much additional improvement on a low-calorie, low-glycemic index diet. Scientists measured insulin sensitivity, systolic blood pressure, LDL cholesterol and HDL cholesterol and saw that the low-glycemic diet did not improve them. It did lower triglycerides.
The right kind of carbs can be incredibly good for you. Not only are they necessary for your health, but they carry a variety of added benefits.
Carbohydrates may be important to mental health. A study published in 2009 in the journal JAMA Internal Medicine found that people on a high-fat, low-carb diet for a year had more anxiety, depression and anger than people on a low-fat, high-carb diet. Scientists suspect that carbohydrates help with the production of serotonin in the brain.
Carbs may help memory, too. A 2008 study at Tufts University had overweight women cut carbs entirely from their diets for one week. Then, they tested the women’s cognitive skills, visual attention and spatial memory. The women on no-carb diets did worse than overweight women on low-calorie diets that contained a healthy amount of carbohydrates.
Though carbs are often blamed for weight gain, the right kind of carbs can actually help you lose and maintain a healthy weight. This happens because many good carbohydrates, especially whole grains and vegetables with skin, contain fiber. It is difficult to get sufficient fiber on a low-carb diet. Dietary fiber helps you to feel full, and generally comes in relatively low-calorie foods.
A study published in the Journal of Nutrition in 2009 followed middle-age women for 20 months and found that participants who ate more fiber lost weight, while those who decreased their fiber intake gained weight. Another recent study linked fat loss with low-fat diets, not low-carb ones.
While some studies have found that low-carb diets do help people lose weight, a meta analysis conducted in 2015 and published in The Lancet found that when viewed long term, low-fat and low-carb diets had similar success rates. People lost more weight early on while on low-carb diets but after a year they were all in similar places.
Good source of nutrients
Whole, unprocessed fruits and vegetables are well known for their nutrient content. Some are even considered superfoods because of it — and all of these leafy greens, bright sweet potatoes, juicy berries, tangy citruses and crunchy apples contain carbs.
One important, plentiful source of good carbs is whole grains. A large study published in 2010 in the Journal of the American Dietetic Association found that those eating the most whole grains had significantly higher amounts of fiber, energy and polyunsaturated fats, as well as all micronutrients (except vitamin B12 and sodium). An additional study, published in 2014 in the journal Critical Reviews in Food Science and Nutrition, found that whole grains contain antioxidants, which were previously thought to exist almost exclusively in fruits and vegetables.
Fiber also helps to lower cholesterol, said Kelly Toups, a registered dietitian with the Whole Grains Council. The digestive process requires bile acids, which are made partly with cholesterol. As your digestion improves, the liver pulls cholesterol from the blood to create more bile acid, thereby reducing the amount of LDL, the “bad” cholesterol.
Toups referenced a study in the American Journal of Clinical Nutrition that looked at the effect of whole grains on patients taking cholesterol-lowering medications called statins. Those who ate more than 16 grams of whole grains daily had lower bad-cholesterol levels than those who took the statins without eating the whole grains.
Not getting enough carbs can cause problems. Without sufficient fuel, the body gets no energy. Additionally, without sufficient glucose, the central nervous system suffers, which may cause dizziness or mental and physical weakness, according to Iowa State University. A deficiency of glucose, or low blood sugar, is called hypoglycemia.
If the body has insufficient carbohydrate intake or stores, it will consume protein for fuel. This is problematic because the body needs protein to make muscles. Using protein for fuel instead of carbohydrates also puts stress on the kidneys, leading to the passage of painful byproducts in the urine, according to the University of Cincinnati.
People who don’t consume enough carbohydrates may also suffer from insufficient fiber, which can cause digestive problems and constipation.
- American Diabetes Association: Understanding Carbohydrates
- U.S. National Library of Medicine: Counting Carbohydrates
Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we eat. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often “carb-load” before important competitions to ensure that they have enough energy to compete at a high level. Carbohydrates are, in fact, an essential part of our diet; grains, fruits, and vegetables are all natural sources of carbohydrates. Carbohydrates provide energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient in many staple foods. Carbohydrates also have other important functions in humans, animals, and plants.
Carbohydrates can be represented by the stoichiometric formula (CH2O)n, where n is the number of carbons in the molecule. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. This formula also explains the origin of the term “carbohydrate”: the components are carbon (“carbo”) and the components of water (hence, “hydrate”). Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.
Monosaccharides (mono– = “one”; sacchar– = “sweet”) are simple sugars, the most common of which is glucose. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide names end with the suffix –ose. If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is known as an aldose, and if it has a ketone group (the functional group with the structure RC(=O)R′), it is known as a ketose. Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons). See Figure 1 for an illustration of the monosaccharides.
Figure 1. Monosaccharides are classified based on the position of their carbonyl group and the number of carbons in the backbone. Aldoses have a carbonyl group (indicated in green) at the end of the carbon chain, and ketoses have a carbonyl group in the middle of the carbon chain. Trioses, pentoses, and hexoses have three, five, and six carbon backbones, respectively.
The chemical formula for glucose is C6H12O6. In humans, glucose is an important source of energy. During cellular respiration, energy is released from glucose, and that energy is used to help make adenosine triphosphate (ATP). Plants synthesize glucose using carbon dioxide and water, and glucose in turn is used for energy requirements for the plant. Excess glucose is often stored as starch that is catabolized (the breakdown of larger molecules by cells) by humans and other animals that feed on plants.
Galactose and fructose are other common monosaccharides — galactose is found in milk sugars and fructose is found in fruit sugars. Although glucose, galactose, and fructose all have the same chemical formula (C6H12O6), they differ structurally and chemically (and are known as isomers) because of the different arrangement of functional groups around the asymmetric carbon; all of these monosaccharides have more than one asymmetric carbon (Figure 2).
Figure 2. Glucose, galactose, and fructose are all hexoses. They are structural isomers, meaning they have the same chemical formula (C6H12O6) but a different arrangement of atoms.
What kind of sugars are these, aldose or ketose?
Show Answer Glucose and galactose are aldoses. Fructose is a ketose.
Monosaccharides can exist as a linear chain or as ring-shaped molecules; in aqueous solutions they are usually found in ring forms (Figure 3). Glucose in a ring form can have two different arrangements of the hydroxyl group (−OH) around the anomeric carbon (carbon 1 that becomes asymmetric in the process of ring formation). If the hydroxyl group is below carbon number 1 in the sugar, it is said to be in the alpha (α) position, and if it is above the plane, it is said to be in the beta (β) position.
Figure 3. Five and six carbon monosaccharides exist in equilibrium between linear and ring forms. When the ring forms, the side chain it closes on is locked into an α or β position. Fructose and ribose also form rings, although they form five-membered rings as opposed to the six-membered ring of glucose.
Disaccharides (di– = “two”) form when two monosaccharides undergo a dehydration reaction (also known as a condensation reaction or dehydration synthesis). During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond. A covalent bond formed between a carbohydrate molecule and another molecule (in this case, between two monosaccharides) is known as a glycosidic bond (Figure 4). Glycosidic bonds (also called glycosidic linkages) can be of the alpha or the beta type.
Figure 4. Sucrose is formed when a monomer of glucose and a monomer of fructose are joined in a dehydration reaction to form a glycosidic bond. In the process, a water molecule is lost. By convention, the carbon atoms in a monosaccharide are numbered from the terminal carbon closest to the carbonyl group. In sucrose, a glycosidic linkage is formed between carbon 1 in glucose and carbon 2 in fructose.
Common disaccharides include lactose, maltose, and sucrose (Figure 5). Lactose is a disaccharide consisting of the monomers glucose and galactose. It is found naturally in milk. Maltose, or malt sugar, is a disaccharide formed by a dehydration reaction between two glucose molecules. The most common disaccharide is sucrose, or table sugar, which is composed of the monomers glucose and fructose.
Figure 5. Common disaccharides include maltose (grain sugar), lactose (milk sugar), and sucrose (table sugar).
A long chain of monosaccharides linked by glycosidic bonds is known as a polysaccharide (poly– = “many”). The chain may be branched or unbranched, and it may contain different types of monosaccharides. The molecular weight may be 100,000 daltons or more depending on the number of monomers joined. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides.
Starch is the stored form of sugars in plants and is made up of a mixture of amylose and amylopectin (both polymers of glucose). Plants are able to synthesize glucose, and the excess glucose, beyond the plant’s immediate energy needs, is stored as starch in different plant parts, including roots and seeds. The starch in the seeds provides food for the embryo as it germinates and can also act as a source of food for humans and animals. The starch that is consumed by humans is broken down by enzymes, such as salivary amylases, into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose.
Starch is made up of glucose monomers that are joined by α 1-4 or α 1-6 glycosidic bonds. The numbers 1-4 and 1-6 refer to the carbon number of the two residues that have joined to form the bond. As illustrated in Figure 6, amylose is starch formed by unbranched chains of glucose monomers (only α 1-4 linkages), whereas amylopectin is a branched polysaccharide (α 1-6 linkages at the branch points).
Figure 6. Amylose and amylopectin are two different forms of starch. Amylose is composed of unbranched chains of glucose monomers connected by α 1,4 glycosidic linkages. Amylopectin is composed of branched chains of glucose monomers connected by α 1,4 and α 1,6 glycosidic linkages. Because of the way the subunits are joined, the glucose chains have a helical structure. Glycogen (not shown) is similar in structure to amylopectin but more highly branched.
Glycogen is the storage form of glucose in humans and other vertebrates and is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis.
Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose; this provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Cellulose is made up of glucose monomers that are linked by β 1-4 glycosidic bonds (Figure 7).
Figure 7. In cellulose, glucose monomers are linked in unbranched chains by β 1-4 glycosidic linkages. Because of the way the glucose subunits are joined, every glucose monomer is flipped relative to the next one resulting in a linear, fibrous structure.
As shown in Figure 7, every other glucose monomer in cellulose is flipped over, and the monomers are packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells. While the β 1-4 linkage cannot be broken down by human digestive enzymes, herbivores such as cows, koalas, buffalos, and horses are able, with the help of the specialized flora in their stomach, to digest plant material that is rich in cellulose and use it as a food source. In these animals, certain species of bacteria and protists reside in the rumen (part of the digestive system of herbivores) and secrete the enzyme cellulase. The appendix of grazing animals also contains bacteria that digest cellulose, giving it an important role in the digestive systems of ruminants. Cellulases can break down cellulose into glucose monomers that can be used as an energy source by the animal. Termites are also able to break down cellulose because of the presence of other organisms in their bodies that secrete cellulases.
Figure 8. Insects have a hard outer exoskeleton made of chitin, a type of polysaccharide.
Carbohydrates serve various functions in different animals. Arthropods (insects, crustaceans, and others) have an outer skeleton, called the exoskeleton, which protects their internal body parts (as seen in the bee in Figure 8).
This exoskeleton is made of the biological macromolecule chitin, which is a polysaccharide-containing nitrogen. It is made of repeating units of N-acetyl-β-d-glucosamine, a modified sugar. Chitin is also a major component of fungal cell walls; fungi are neither animals nor plants and form a kingdom of their own in the domain Eukarya.
In Summary: Structure and Function of Carbohydrates
Carbohydrates are a group of macromolecules that are a vital energy source for the cell and provide structural support to plant cells, fungi, and all of the arthropods that include lobsters, crabs, shrimp, insects, and spiders. Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides depending on the number of monomers in the molecule. Monosaccharides are linked by glycosidic bonds that are formed as a result of dehydration reactions, forming disaccharides and polysaccharides with the elimination of a water molecule for each bond formed. Glucose, galactose, and fructose are common monosaccharides, whereas common disaccharides include lactose, maltose, and sucrose. Starch and glycogen, examples of polysaccharides, are the storage forms of glucose in plants and animals, respectively. The long polysaccharide chains may be branched or unbranched. Cellulose is an example of an unbranched polysaccharide, whereas amylopectin, a constituent of starch, is a highly branched molecule. Storage of glucose, in the form of polymers like starch of glycogen, makes it slightly less accessible for metabolism; however, this prevents it from leaking out of the cell or creating a high osmotic pressure that could cause excessive water uptake by the cell.