What Are Carbohydrates? A Complete Scientific Guide

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 eating plans like the ketogenic diet, it’s understandable to wonder if carbs are the enemy.

The fact of the matter is, they aren’t. Carbs are one of the three macronutrients, the two others being protein and fat, that your body needs to survive.

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, the author with Mark Bittman of How to Eat and the CEO of Diet ID. This particular structure is found in everything from lentils to lollipops, he adds.

Still, you may be wondering: Should you be cutting carbs or going on a low-carb diet if you want to lose weight? Are you eating the right carbs? Can carbs increase your risk of chronic disease? We’re here to explore this much-debated macronutrient.

Carbohydrates play an essential role in the body. Here’s how they help you function at your best.

Manage Blood Sugar and Insulin Production

When you eat carbohydrate-containing foods, the body breaks down those carbohydrates into glucose, which then enters your bloodstream, raising your blood sugar, explains the American Diabetes Association. That signals your pancreas to release insulin, a hormone that pushes glucose into your cells, where it can be used for energy. And that takes us to our next point.

Provide Fuel

As mentioned, carbs provide the primary source of glucose (sugar) that the body can readily use for energy, says Carolyn Williams, PhD, RDN, an author and cohost of the podcast Happy Eating. You need fuel not only to power a walk or other exercise, but to run basic body functions. “It’s easy to forget about the energy we need to keep our heart beating, lungs breathing, create new blood cells, and more,” she says. What’s more, “glucose is the main thing the brain uses for energy, which is why it’s so vital,” Dr. Williams says.

Indirectly Aid in Hormonal Health

Outside of food insecurity, there are two instances where people tend to consume too few calories, and that’s on a low-calorie diet (consuming less than 1,200 calories per day) or on a low-carb diet, says Williams. When you don’t consume enough calories for your body’s metabolic needs, your hormonal heath may suffer, especially if you’re a woman. “A lack of adequate calories is associated with a decline in estrogen levels, which can cause changes in the menstrual cycle,” she says. In addition, eating less than your calorie needs can decrease thyroid hormones, as well as raise levels of the “stress hormone,” cortisol, Williams adds.

Improve Digestion and Microbiome Health

In certain carbohydrate-containing foods, such as fruits, vegetables, whole grains, nuts, seeds, and legumes, there lies an important nutrient: fiber. Fiber is an indigestible nutrient that’s fermented in the colon by the microbes in your microbiome, explains StatPearls. That fermentation creates healthful bacteria, and the fiber provides bulk to your stool that aids in digestive health.

Promote Healthy Lipid Levels

Sources of carbohydrates that contain soluble fiber, such as whole grains, avocado, beans, and certain fruits and veggies like apples and broccoli, these pack soluble fiber, notes the National Lipid Association. One role of soluble fiber is to bind to cholesterol and ferry it out of the body, which helps lower total and “bad” LDL cholesterol.

When it comes to weight loss, choosing high-quality complex carbs, particularly those high in fiber, is crucial.

A study published in BMJ tracked how eating habits affected weight changes in 137,000 adults with an average age of 50. They found that those who chose healthier carbs (like fruit, nonstarchy veggies, and whole grains) gained less weight than those who tended to reach for less healthy, refined carbs, such as soda, white bread, and starchy veggies.

If you’re looking to reduce belly fat, insoluble fiber may be your best bet, per two studies conducted in youths, one published in the Journal of Clinical Endocrinology & Metabolism and the other in the American Journal of Clinical Nutrition.

Find soluble fiber in foods like beans, oats, apples, and carrots, per the Mayo Clinic.

In general, fiber helps promote a healthy weight by helping you feel more full, which can prevent overeating, according to the aforementioned review.

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 gram (g) serving of a given food spikes your blood sugar, according to the Glycemic Index Foundation. 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, notes Harvard Medical School.

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 Harvard. 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 by those following a low-carb diet and often by those with diabetes. To find out how many net carbs a food has, take 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), notes Atkins. 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, says the American Diabetes Association. 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.

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, notes the Endocrine Society.

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. A low-carb, high-fat diet, like keto, can lead to ketosis, says Cleveland Clinic.

Ketones This is a by-product of burning fat in the absence of enough carbohydrates.

When people discuss carbs, they often differentiate them with the terms “simple” and “complex,” but what does this mean? Here’s a 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, 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 include sugar, like fructose, sucrose, and lactose. 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 — 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.

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

Monosaccharide

This is also known as a simple sugar, according to Britannica.

Disaccharide

A “double sugar,” this is one that contains two simple sugars.

Polysaccharide

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

The 2020–2025 U.S. Dietary Guidelines for Americans recommends that most people get 45 to 65 percent of their daily calories from carbs. That means if you eat a standard 2,000-calorie diet, 900 to 1,300 of your calories should come from carbs. One gram of carbohydrate contains four calories, which means between 225 and 325 g of carbs would be your target intake daily.

But you don’t eat “carbs”; you eat food. “One of the places people go wrong is thinking that carbohydrates are indicative of some particular kind of food. On the other hand, 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.

Experts advise limiting certain 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 warn 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 past research, people who got more than 10 percent of their daily calories from sugar were 30 percent more likely to die of heart disease than those who ate less. (Those who ate up to 25 percent of their daily calories in the form of sugar increased that risk 2.75-fold.)

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

Here’s a cheat sheet to use next time you’re at the grocery store trying to make healthy carb choices.

Healthy Sources of Carbohydrates

• 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

Unhealthy Sources of Carbohydrates

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

Ready to make-over your carb game? Follow these pointers to add more healthy carbs to your eating plan.

Pack Produce Into Each Meal

When we think of carbs, says Williams, we often think about grains, like bread and cereal. But don’t forget that you get carbohydrates from veggies and fruits. “What works for me is focusing on eating one or two vegetables or fruits with every meal, aiming for a total of at least five per day,” says Williams. “And when I focus on that goal, all other aspects of my diet tend to fall in place,” she says. Doing so helps you diversify your carbs from grain to these other wonderful sources. And speaking of fruit …

Don’t Fear Fruit

Fruit provides an important source of vitamins, minerals, and antioxidants. And although fruit does contain sugar, it also has fiber, which slows digestion to blunt your blood sugar response. Williams recommends eating lower-GI (glycemic index) fruits, such as berries, apples, and pears. Add one to your snack (such as a pear and walnuts) or in your breakfast (such as berries smashed on nut butter toast).

Sneak in Beans

Are you eating beans every week? “Legumes like beans and lentils are hidden gems. They are easy, affordable, and packed with powerhouse nutrients like fiber and protein that has a low impact on your blood sugar,” says Williams. Add them to your plate when you can. For example, if you’re ordering a burrito bowl, ask for a double serving of beans instead of rice. At home, top a salad with chickpeas or black beans. Snack on packages of Lupini beans or roasted chickpeas, which are found in prepackaged flavored varieties at the store.

Go for Whole Grains

When you’re eating a grain, make it whole. That means choosing whole-grain bread or tortillas instead of white ones, brown rice over white, or trying other interesting grains, like farro or bulgur as a side with dinner. If you’re not yet into the taste of whole-grain versions of your favorite carbs, Williams recommends looking for a blend of both whole and refined grains, which can help you ease in.

Sprinkle on Nuts and Seeds

Nuts, like almonds, walnuts, and pistachios, along with seeds like sunflower and hemp, contain both fiber-rich carbohydrates and protein, making them a multitasking source of carbohydrates. They are also lower in carbohydrates than other foods on this list, so they can fit into lower-carb diets, too. Sprinkle these on yogurt bowls, eat a handful with a snack, or blend them into smoothies. Don’t forget them on salads, too.

If you eat fewer carbs than recommended, you may not 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.

While some people may find success on a low-carb diet, you won’t want to completely eliminate them from your diet. “You need carbs on a healthy diet. I don’t advise totally deleting them from your eating plan,” says Kathy Chauncey, PhD, RDN, a professor emerita at Texas Tech School of Medicine in Lubbock, and the 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 (at 15 g of carbs per serving). That equates to 45 to 75 g per day and would qualify as a moderately low-carb plan.

Research indicates that when it comes to type 2 diabetes, getting more than 70 percent of your calories from carbohydrates increases your risk of the disease by 18 percent —and staying in the recommended 45 to 65 percent range likely does not affect your risk.

On the other hand, 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 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 death from cancer by 15 percent.

As for fruit, research has found that those who ate fruit daily had a 12 percent lower risk of diabetes than those who avoided it.

Some people might do well to eat more carbs than what’s typically recommended by the U.S. Dietary Guidelines. Namely, athletes may benefit from higher carb intake, says Farrell Allen. “Breaking down muscle for energy [as you would if you limited carbs] is not favorable, in fact, carbs spare proteins from making glucose for the necessary energy,” she says.

According to one study that 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 than those who ate plant foods. Translation? The type of food counts more than the macronutrient content.

Katz advises eating wholesome foods in a sensible, balanced combination and appropriate portions. He recommends filling your plate with minimally processed foods such as 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.)

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 manage your 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. In general, 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 compared this low-carb plant-based diet — “eco-Atkins” — with a high-carb, low-fat diet, and found that eco-Atkins was better at reducing total and “bad” LDL cholesterol levels.
• 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.
• Ketogenic diet High fat (70 to 80 percent of calories), moderate protein, and very low carb (20 to 50 g of net carbs daily)