What Are Carbohydrates? Benefits, Functions, Best Sources, Low-Carb Diets, and More

Been giving the stink eye to carbohydrates lately? You and the rest of America. In a time when it seems as if everyone you know has been on, is on, or is talking about going on very-low-carb diets like the ketogenic diet, it’s understandable to wonder if carbs are the enemy. Should you be cutting carbs or going on a low-carb diet? Are you eating the right carbs? Can eating carbs increase your risk for chronic disease? We’re here to explore this much-debated macronutrient.

What Are Carbohydrates Exactly?

There are three macronutrients: carbohydrates, protein, and fat. If you want to get scientific, a “carb” “refers to a particular molecular structure. It is a string of carbons with a water molecule attached to each of them,” says David Katz, MD, MPH, founder and president of the TrueHealth Initiative in Chesterfield, Missouri, and author of The Truth About Food. And that’s all it says. This particular structure is found in everything from lentils to lollypops, he adds.

Carbohydrates provide energy for our bodies to run on. When you eat a food that contains carbs, the carbs are broken down by the body into glucose. “Glucose is the primary fuel that circulates in our blood at all times. It’s also the principle fuel for the brain,” says Dr. Katz.

The 2015–2020 U.S. Dietary Guidelines for Americans recommends getting 45 to 65 percent of your daily calories from carbs. That means if you were to eat a standard 2,000-calorie diet, 900 to 1,300 of your calories would come from carbs. One gram (g) of carbohydrate contains four calories, which means between 225 and 325 g of carbs would be your target intake daily. (1)

But you don’t eat “carbs” alone. You eat food. “One of the places people go wrong is thinking that carbohydrates are indicative of some particular kind of food. However, all plants are made up of carbohydrates,” says Katz. These foods — whole grains, legumes, seeds, nuts, fruits, dairy, and vegetables — also contain essential nutrients, like fiber, vitamins, antioxidants, and minerals. “Since plant foods are always a carb source, you cannot get any of those essential nutrients [from foods] without eating carbs. It’s valid to say then that carb foods are supporting almost every aspect of human physiology,” he adds.

Below you’ll find a sampling of some of the common terms we’ll be using as we dissect this macronutrient.

Glycemic Index (GI) This is a score from 0 to 100 for how sharply a 100 g serving of a given food spikes your blood sugar. Foods that score low on the index are known to cause your blood sugar levels to rise gradually, while the highest score, 100, is assigned to pure glucose, which is a sugar. (2)

Glycemic Load This is a measure of “how quickly [a food] makes glucose enter the bloodstream and how much glucose per serving it can deliver,” explains the Harvard Health blog. (2) Glycemic load considers a typical serving (rather than 100 g) of a given food. One food may be high GI, but if it contains few carbs, its glycemic load will be lower.

Net Carb This is a carbohydrate counting method used when following a low-carb diet and often for those with diabetes. This number is calculated by taking the total number of carbs minus the grams of fiber and sugar alcohols (which are forms of carbohydrates that are not digested or not fully digested by the body). (3) Although people on low-carb plans, like the keto diet and the Atkins diet, commonly use net carbs to regulate their carb intake, the federal government doesn’t recognize this term as an official way to track the macronutrient.

Insulin Insulin is a hormone secreted by the pancreas. (4) After you eat, the pancreas releases insulin to help drive blood sugar (glucose) into cells, where it can be used by the body; if not used, it will be stored. (5)

Glucagon When blood sugar levels dip, this hormone converts stored glucose (in the form of glycogen) into energy in order to regulate blood sugar levels. (6)

Ketogenic Diet This is a high-fat, moderate-protein, very-low-carb diet.

Ketosis This metabolic state occurs when the body has transitioned from burning glucose from carbs to ketones from fat digestion for fuel. Eating a low-carb, high-fat diet, like keto, can lead to ketosis. (7)

Ketones This is a byproduct created after burning fat in the absence of enough carbohydrates. (7)

You may see the following terms used to describe the chemical makeup of carbohydrates:

Monosaccharide This is also known as a simple sugar. (8)

Disaccharide A “double sugar,” this is one that contains two simple sugars. (9)

Polysaccharide You may also hear this called glycan. This is the most common form of natural carbs, according to Encyclopedia Britannica. (10)

What's the Difference Between Simple and Complex Carbs?

When people discuss carbs, they often differentiate them with the terms “simple” and “complex,” but what does this mean? Here’s a simple way to remember the difference: “I like to explain to my patients that simple carbs are like a short-beaded bracelet, and complex carbs are like a long-beaded necklace. One is a short chain, the other is a long chain. This is just like carbs,” says Nancy Farrell Allen, RDN, who is an Academy of Nutrition and Dietetics spokesperson based in Fredericksburg, Virginia.

A long chain takes longer to digest, and it enters the bloodstream slowly. A simple — or short chain — on the other hand, is rapidly broken down and sent into the bloodstream, she says. Simple carbs refer to sugar, like fructose, sucrose, and lactose. (1) Complex carbs include starches (vegetables, legumes, grains) and fiber. “Complex carbs put less strain on the pancreas and provide satiety (a feeling of fullness) for a longer period of time,” she says. Of course, some foods are made up of both — like fruit contains both fructose and fiber. It’s the fiber in fruit that slows digestion, which means that fruit is not equivalent to candy in the body.

If you wiped out all the carbs from your diet, as some diets (like the carnivore diet, which is essentially an all-meat diet) instruct, you probably wouldn’t feel your best. “Carbs provide all cells of the body with energy needed for mental and physical tasks and activity. They provide about half of all the energy the body needs,” says Farrell Allen. Cut it short and you may feel tired and foggy-headed. That said, there are many success stories from people on low-carb diets, though while you can reduce them, experts say that you should have some carbs in your diet.

“You need carbs on a healthy diet. I don’t advise totally deleting them from your eating plan,” says Kathy Chauncey, PhD, RD, professor of nutrition at Texas Tech School of Medicine in Lubbock, and author of Low-Carb Dieting for Dummies. When she advises people on a low-carb eating plan, she recommends they generally eat three to five servings of carbs per day (each being 15 grams of carbs per serving). That equates to 45 to 75 g per day and would qualify as a moderately low-carb plan.

That said, there are various low-carb plans at your disposal (from Atkins and keto, to low-carb paleo and Whole30). No low-carb diet is recommended across the board from experts, and each person has different carb needs, so be sure to ask your healthcare team which plan (if any) is a good fit for you.

Unless your doctor tells you to go on a low- or high-carb diet, the healthiest diet doesn’t actually count carbohydrates and it doesn’t aim to stay under a certain number, says Katz. “There are so many ways to go bad when focusing on one nutrient. Let’s talk about foods instead of macronutrients,” he says. Your aim should be to eat the right foods rather than count grams of carbs.

Katz advises eating wholesome foods in a sensible, balanced combination and appropriate portions. He recommends filling your plate with minimally processed vegetables, whole grains, fruits, nuts, seeds, beans, legumes, and plain water for thirst. Add in some eggs, fish, and seafood, and meat like poultry on occasion if desired. If you want to eat dairy, do so in moderation. A sprinkle of cheese on top of a meal here or there won’t hurt you, but know that it’s a major source of saturated fat, which may be harmful to your heart if you overdo it. (The American Heart Association recommends eating no more than 13 g of saturated fat per day.) (11)

If the idea of winging your carb intake scares you, consider a study published in August 2018 in the journal The Lancet. (12) The research included more than 1,500 middle-aged adults who filled out a dietary questionnaire. Those who followed a high- or low-carbohydrate diet had a greater risk of premature death. It was those who ate a moderate amount of carbs — 50 to 55 percent of their daily calories — who had the lowest risk of death. More than that, those who ate a lot of animal protein were the ones who had higher mortality compared with those who ate plant foods. Translation? The type of food counts more than the macronutrient content.

That said, some people may want to follow a low-carb or moderate-carb diet, says Farrell Allen. That includes people with prediabetes and diabetes. If you take insulin to manage type 1 or type 2 diabetes, you may be advised by your doctor to count carbohydrate grams to help manage blood sugar levels. If you have diabetes, be sure to speak with your healthcare team before making changes to your diet, as they’ll want to make sure your carbohydrate level and medication are appropriately balanced. (13)

There may also be some people who would want to eat more carbs than what’s typically recommended by the U.S. Dietary Guidelines. Namely, athletes may benefit from increasing their carb intake, says Farrell Allen. “Breaking down muscle for energy [as you would limiting carbs] is not favorable, in fact, eating carbs helps spare proteins from making glucose for the necessary energy,” she says.

Across the board, experts advise limiting certain sources of foods that contain simple carbs. These include highly processed snack foods, white breads, desserts, chips, candy, fast foods, muffins, bagels, cookies, and more. Typically, these foods happen to be high in carbohydrates but low in fiber and other nutrients, like vitamins and minerals.

The problem is overconsumption of these foods, particularly those with added sugars. (And remember that these highly processed “high-carb” foods are often packed with excess sugar.) “The U.S. Dietary Guidelines warns that added sugars may increase the risk of certain chronic diseases. These sugars add unnecessary calories, which can mean body weight gain; some research indicates that added sugars can alter the fat-clearing abilities of the liver, leading to increased blood fat levels and increasing the risk of heart disease,” says Farrell Allen. Furthermore, it’s known that the body takes extra calories from sugar and converts them into triglycerides, or fats in the blood, directly.

For instance, according to a study published in April 2014 in the journal JAMA Internal Medicine, people who got more than 10 percent of their daily calories from sugar were 30 percent more likely to die from heart disease compared with those who ate less. (Eating up to 25 percent of daily calories from sugar increased that risk by 2.75-fold.) (14)

A meta-analysis and systematic review published in January 2019 in the journal The Lancet compiled data from 58 clinical trials and found that those who consumed at least 25 g of fiber — a level reached by eating those more complex carbohydrates — had up to 30 percent lower odds for death from any cause. In addition, those participants had a lower risk of developing heart disease, stroke, type 2 diabetes, and colorectal cancer. (15)

On the other hand, eating the right type of carbs notably lowers disease risk, and that includes two of the most commonly maligned carbs, whole grains and fruit. A meta-analysis of 45 studies, published in June 2016 in The BMJ, concluded that eating three servings of whole grains daily decreased coronary heart disease risk by 19 percent, lowered stroke risk by 12 percent, and lowered the risk of dying from cancer by 15 percent. (16)

As for fruit, a study of Chinese adults published in April 2017 in the journal PLoS Medicine found that those who ate fruit daily had a 12 percent lower risk of developing diabetes compared with those who avoided it. (17)

For now, if you are concerned about your carb intake, it’s much more important to pay attention to the individual foods you’re eating and make the most nutritious choice in each category.

All foods can fit in moderation, but some foods that are sources of carbohydrates are more nutritious than others. Here is a sampling of foods that fit in each category:

Best

• Leafy greens
• Sweet potatoes
• Broccoli
• Cauliflower
• Beans
• Chickpeas
• Lentils
• Nuts
• Seeds
• Plain, low-fat or fat-free yogurt
• Oatmeal (steel-cut or old-fashioned)
• Quinoa
• Whole-grain bread
• Farro
• Barley
• Apples
• Pears
• Berries
• Bananas
• Citrus

Worst

• Candy
• White bread
• Sugary cereal
• Sweetened juice
• Bagels
• Cakes
• Cookies
• Crackers
• Muffins
• Flavored yogurts
• Chips
• Soda
• Lemonade

For an idea of what a traditionally healthy diet that doesn’t restrict the number of carbs you eat looks like, here are three days of what you may be eating:

Day 1

Breakfast Steel-cut oatmeal topped with strawberries

Snack Reduced-fat cheese stick with pear

Lunch Sandwich with hummus and turkey on whole-wheat bread

Snack Small handful of raw almonds

Dinner Roasted chicken, sweet potato, and broccoli

Day 2

Breakfast Whole-grain toast with nut butter

Snack Orange

Lunch Three-bean chili

Snack Roasted chickpeas and baby carrots

Dinner Farro bowl topped with roasted vegetables and sliced chicken sausage

Day 3

Breakfast Yogurt bowl topped with walnuts with a side of watermelon

Snack Granola bar

Lunch Tuna salad with seed crackers

Snack Air-popped popcorn

Dinner Pizza made with whole-grain dough, reduced-fat cheese, and vegetables, with a side salad

If you’re curious about trying out a low-carb diet, it’s important that you talk with your doctor before making any drastic dietary change. It also helps to connect with a registered dietitian-nutritionist who is knowledgeable in your desired approach to ensure that you’re covering your nutritional bases.

Here are a few diets you may want to ask about:

• Atkins Diet High protein, low carb
• Eco-Atkins Diet This is like the Atkins diet but vegan. Research published in the journal Archives of Internal Medicine compared this low-carb plant-based diet — “eco-Atkins” — with a high-carb, low-fat diet, and found that eco-Atkins better reduced total and “bad” LDL cholesterol levels. (18)
• South Beach Diet The South Beach Diet is a lower-carbohydrate plan, meaning it reduces carbs but not to the level of Atkins or keto. (19)
• Keto Diet High fat (70 to 80 percent of calories), moderate protein, and very low carb (20 to 50 grams of net carbs daily)

A 3-Day Sample Menu on a Standard Low-Carb Diet

There are many different ways to eat low-carb, and much of that depends on your personal carb goals, but you certainly don’t have to be afraid of carbs. “The healthy sources of carbs are the ones that revolve around whole foods like fruit and whole grains,” says Dr. Chauncey.

Here’s an idea of what you may have on your plate:

Day 1

Breakfast Greek yogurt with nuts and raspberries

Snack Cheese stick

Lunch Large salad (leafy greens, cucumbers, peppers, and zucchini) with grilled chicken and avocado

Snack Beef jerky

Dinner Stuffed peppers made with ground beef, a small amount of brown rice, and veggies

Day 2

Breakfast Vegetable and sausage frittata

Snack Handful of mixed nuts

Lunch Chicken and vegetable soup

Snack Apple

Dinner Cauliflower crust pizza with side salad

Day 3

Breakfast Scrambled eggs with avocado

Snack Chia pudding

Lunch Broccoli cheese soup

Snack Pear and almond butter

Dinner Grilled shrimp in a red sauce over spaghetti squash noodles

Carbohydrates are one of three macronutrients that your body needs to function at its best. Their main job is to supply the body and brain with energy. More important than the number of carbs is the types of foods you’re eating. Stick to wholesome sources of carbs, like whole grains, legumes, fruit, vegetables, nuts, and seeds.