The ultimate guide to carbohydrates

Confused about carbs? Here's everything you need to know about carbohydrates.
Published September 25, 2020

Carbs may be the world’s most confounding macronutrient. On the one hand, most health experts say carbohydrates help form the foundation of a healthy diet. On the other hand, some proponents of low-carb diets suggest we’d be slimmer, stronger, more focused, and more energized if we just quit eating stuff like crackers and cake. 

In reality, no carb-containing food is inherently good or bad. Carbohydrates come in many forms, with various implications for health. Once you have a little know-how on the science of carbohydrates—starting with what they are!—you’ll be empowered to make informed food choices that support your wellness goals. Not to worry: There's no pressure to swear off spaghetti here!

What are carbohydrates? 

Carbs are essentially sugar molecules—groupings of carbon, hydrogen, and oxygen—that occur either alone or linked together in different combinations. Most of the carbs we eat are digested and broken down into glucose to provide energy for our muscles and brain; in fact, carbohydrates are the body’s preferred source of fuel, says Silvia Klinger, MS, RDN, a nutritionist based in Hinsdale, Illinois. Some carb types support digestion, gut health, and other functions. Taken together, carbohydrates are classified as macronutrients, meaning the body needs relatively high amounts in the diet for energy.

Definition of carbohydrate types 

Some of us hear the word “carbs” and immediately think “bread.” In fact, the world of carbs is diverse, Klinger says. Carbohydrates fall into one of three main groups, with many foods containing more than one type of carbohydrate. 

  • Sugars. It’s not just the granulated white stuff you spoon into your coffee. This carb group also includes sugars that occur naturally in whole foods, such as the fructose in fruits and lactose in yogurt. Also in the sugar category: caloric sweeteners found in commercially made foods and drinks, from candy and soda to salad dressing and pasta sauce. On ingredient labels, sugar goes by a number of names, including corn syrup, dextrose, and molasses.
  • Starches. For most adults in the U.S., starches are the most widely consumed type of carbohydrate. Food sources include grains such as rice, oats, and barley, as well as grain-based products such as bread, cereal, and pasta. Potatoes, beans, peas, and corn are major sources of starch, too.
  • Fibers. Found only in plant-based foods like fruits, veggies, and grains, fiber adds bulk to your diet. It has a number of subtypes—for example, there’s pectin in citrus fruits, inulin in onions, and cellulose in kale—which are classified as either soluble or insoluble depending on how they interact with water. Different types of dietary fiber support health in different ways.

Simple and complex carbohydrates

Not enough categories for ya? Nutrition experts further classify carbs as simple or complex depending on the number of sugar molecules the carbs contain. Sugars fall into the simple category because they’re made up of just one or two molecules. Starches and fibers, meanwhile, comprise at least three sugar molecules and are therefore classified as complex carbs. Here’s a little more about what that means for the body at mealtimes.

  • Simple carbohydrates: sugars. Thanks to their basic molecular structure, sugars require little or no digestive breakdown. This means their glucose enters the bloodstream soon after a person eats. “Simple carbs can be beneficial, especially for people who are very active physically and need quick bursts of energy during exercise,” says Mindy Hermann, MBA, RDN, a nutritionist based in Somers, New York.

The presence of fiber in a food can slow the release of simple sugars to result in a more sustained energy release and greater satiety, Hermann notes. That’s one reason she recommends fiber-rich whole foods such as apples over sweetened fare like cookies, which tend to lack fiber. “Unlike fruit, processed foods with added sugar really aren’t that filling,” Hermann says. “Some people find that [foods with added sugar] can lead to overeating.” 

  • Complex carbohydrates: starches and fiber. As noted above, ​complex carbs are larger chains of sugar molecules linked by chemical bonds. For starches, digestive enzymes in the gut need time to break the chains into single units, so starches tend to provide a steadier and more gradual source of glucose energy than simple sugars do, says Susan Mitchell, PhD, RDN, creator of the podcast Breaking Down Nutrition.

Dietary fiber, meanwhile, works a little differently from starches and sugars. Our bodies lack the enzymes needed to divvy up fiber chains into simple sugars, Mitchell explains. As a result, fiber passes through the gut without yielding glucose energy. Fiber supports health in other ways depending on its form. Soluble fiber, found in foods like oat bran and beans, combines with water and forms a gel for a stabilizing effect on blood sugar, for example. Insoluble fiber, found in foods like whole grains and veggies, helps move contents through the digestive tract.

Function of carbohydrates

Carbohydrates play a complex and integral role in human health. Here’s a closer look at how carbs affect the body.

Carbs and energy 

After you eat a carb-containing meal or snack, it travels to the small intestine for digestion and absorption into the bloodstream. Any complex carbs are broken down into single units of glucose, which then enter the bloodstream. Once glucose makes its way into cells, it’s synthesized into a molecule called ATP (adenosine triphosphate), a cell’s main carrier of chemical energy. Although cells can also make ATP from the building blocks of protein- and fat-based foods, glucose from carbs is the body’s preferred and most widely used source.

Not all glucose ends up being used for ATP production. The liver and muscles also stash some glucose away in the form of glycogen to protect against future shortages. Any glucose not needed after that is stored as fat.

Carbs, blood sugar, and insulin

Glucose can’t do its job without a major assist from insulin. In healthy people, the pancreas secretes this hormone in response to rising blood levels of glucose after a carb-containing meal or snack. Insulin then “unlocks” the body’s cells so glucose can enter. In some people, however, the body’s cells don’t respond well to insulin, a condition known as insulin resistance. This can lead to chronically high blood sugar—and health impacts such as prediabetes and type 2 diabetes. You can learn more about prediabetes and insulin resistance in WeightWatchers®’s comprehensive guide. Many people are able to improve their insulin response with lifestyle measures.

Carbs and digestion 

It’s not glamorous, but fiber is probably most famous for preventing constipation. In taking on water, the soluble type helps form soft, bulky stools. Insoluble fiber adds further bulk and, with a broom-like effect, keeps waste material moving along.  

That’s not all fiber does during the digestive process. When some types of fiber (such as pectin) reach the colon, they help feed the resident population of beneficial bacteria—aka probiotics—through a process of fermentation. As those bacteria feast on the fiber, some produce short-chain fatty acids believed to play a role in immunity, inflammation control, and more. Other gut bacteria may use their fiber fill-up to influence hormones that regulate appetite. 

Many adults in the U.S. fall short of getting the recommended 22g to 38g of fiber per day, depending on age and sex. You can boost your fiber intake by eating whole grains and whole-grain foods, as well as legumes, fruits, vegetables, nuts, and seeds. 

Carbs and weight control 

“It can be hard not to get swept up in the carb confusion,” Klinger says. “The truth is, carbs can still be a part of your weight-loss plan.” After all, even diets billed as low-carb contain some carbs.

Hermann agrees. For weight management, she recommends choosing foods that contain complex carbs because they tend to be digested and absorbed more slowly than foods that contain little fiber or just simple sugars (white bread and sweets being classic examples). “Much of the time, simple carbs provide a lot of calories but not a lot of nutrition,” Hermann says.  

Carbs, brain function, and mood

Despite accounting for just 2% of body weight, the brain uses about 20% of the body’s glucose, making it a major consumer. That energy goes toward maintaining neurons, generating neurotransmitters, and performing multitude of, well, brainy tasks. The brain’s reliance on glucose likely explains why lower-than-normal blood levels of glucose, a condition known as hypoglycemia, can make people feel confused, tired, or irritable. (Hypoglycemia warrants prompt medical attention; contact your doctor if you experience symptoms.) 

“Good” vs. “bad” carbohydrate foods 

At WW, we steer clear of value judgments concerning individual foods. After all, no food is eaten in isolation! A healthy pattern of eating includes selections from a variety of food groups while allowing you the flexibility to plan for any food you love.

That said, if you’re looking to boost your overall nutrient intake while getting your carbs, consider putting these selections on your plate.

Healthier carbohydrate-containing foods 

  • Vegetables.​ From Brussels sprouts to beets, all veggies contain some measure of carbs. Eating a variety will serve you well. In addition to being good sources of many vitamins and minerals, “veggies provide an A-to-Z mix of naturally occurring plant compounds called phytonutrients, which help protect us from heart disease, cancer, and other disease,” Mitchell says.
  • Whole fruits. The fiber in any given fruit slows the release of its simple sugars, making fruit a nutritious option for those of us with a sweet tooth. And though some people may worry that fruits’ naturally occurring simple sugars will add pounds on the scale, research following U.S. men and women for 24 years found that the more fruit (and vegetables) people ate, the less weight they gained over time.
  • Pulses.​ The edible seeds of beans, peas, and lentils boast a unique combination of starch, fiber, and protein, which helps satisfy appetite and keep post-meal munchies at bay. A meta-analysis of 11 clinical trials found that people who regularly ate pulses had a reduced risk of developing cardiovascular disease, possibly due to beneficial effects on blood pressure, cholesterol, blood sugar, and more.
  • Whole grains. There’s a reason this article has mentioned them multiple times—whole grains are pretty great! In addition to delivering complex carbs, including fiber, whole grains are rich in magnesium, antioxidants, and vitamin E . Research suggests that a diet high in whole grains can help lower the risk of developing type 2 diabetes and cardiovascular disease.
  • Tubers. ​Skeptics may scoff that potatoesyams, and sweet potatoes are starchy, but again, that’s not an inherently negative quality. (Remember, starch is a complex carb!)  Depending on the variety, a tuber might be a good source of potassium, vitamin C, vitamin B6, and more.

Foods with added sugars 

It’s no surprise that sugar is a big reason cookies, cakes, candy, ice cream, and chocolate are so delicious. But you might be taken aback to learn that food manufacturers commonly add sweeteners to savory items, too, including crackers, non-dairy milk, marinades, and jerky.

As mentioned above, added sugar goes by many names on ingredient labels, including corn sugar, dextrose, fructose, glucose, high-fructose corn syrup, honey, maple syrup, agave syrup, isoglucose, maltose, and sucrose. 

Though some added sugars, such as maple syrup and honey, contain trace amounts of minerals and nutrients, most don’t offer much beyond their main job of making food taste sweeter, Mitchell says. 

As of 2020, food manufacturers are required to list grams of added sugar and the percentage of Daily Value (DV) on their products’ Nutrition Facts labels. Percentages are based on a recommended daily limit of 50g of added sugar per day for a 2,000-calorie diet. (Sugars that occur naturally in food don’t count.) Foods that contain 5% DV or less are considered low in added sugar, according to guidelines set by the U.S. Food and Drug Administration. Foods that contain 20% DV or more are considered high in added sugar.

Low-carbohydrate diets 

From keto to paleo, low-carb diets are having a moment. Low-carb diets often result in short-term weight loss but are tough to maintain because they’re so restrictive, Hermann says. If dieters return to their former eating habits, any weight that was lost tends to come back, she notes.

How a low-carbohydrate diet works 

Some proponents of low-carb diets believe that limiting carbs accelerates weight loss by setting off metabolic changes that optimize fat burn and keep appetite in check. While research suggests that medically supervised carb restriction might be helpful to some people living with certain health conditions, there’s little scientific evidence that low-carb diets impart special weight-loss powers. A 2019 research review concluded that low-carb diets are no more effective than other dietary approaches to weight loss, building on previous research—like this small study of adults in 2009—that uncovered no low-carb weight-loss advantage beyond the first few months. Other studies have raised questions about the long-term safety of strictly limiting carbs. 

Low-carb diets vary in their degree of carb restriction. The popular keto diet generally caps carbs at 20g to 50g a day, whereas a more moderate low-carb diet might allow for up to 130g a day (roughly the amount you’d get in 3 cups of cooked rice). 

Hermann says the most sensible approach to a low-carb weight-loss diet would encourage people to limit foods and drinks that contain added sugars, which are often high in calories and low in nutrients, while still encouraging consumption of nutritious whole foods.

Low-carb diet foods 

Diets that limit carbs generally require an increase in fat and protein intake. Just like with carbs, however, all fats are not created equal. For example, bacon contains zero carbs but is high in saturated fat, which is linked to coronary heart disease. On the other hand, unsaturated fats found in nuts, seeds, and vegetable oils are considered heart-protective. 

A healthier low-carb diet takes these factors into account, emphasizing lean sources of protein such as chicken, fish, legumes, eggs, and low-fat dairy—plus plenty of plant-based foods.

Examples of low-carbohydrate diets 

  • Standard low-carb diet. A basic low-carbohydrate plan encourages dieters to get around 25% of calories from carbohydrates (up to 130g a day), with the rest coming from fat and protein.
  • Ketogenic diet. Considered a very low-carb eating plan, the keto diet restricts carbohydrates to 5% to 10% of total calorie intake, and protein is limited to about 20%. For better or worse, the remainder of daily caloric intake is devoted to high-fat foods like butter and nuts.
  • Paleo diet. Also known as the Stone Age diet or caveman diet, the paleo approach is designed to mimic the meals of prehistoric human ancestors. It restricts grains, legumes, dairy, certain vegetable oils, added sugar, and highly processed products. Instead, dieters are encouraged to eat lean meats, fish, fruits, vegetables, nuts, and seeds.
  • Atkins diet. The Atkins diet uses a value known as net carbs—grams of total carbs minus grams of fiber. The Atkins diet generally allows 20g to 100g of net carbs a day. Because fiber doesn’t count toward carbs on the Atkins program, its various plans steer dieters toward high-fiber, lower-sugar produce. Fat and protein fill out the rest of a typical Atkins plan.
  • Low-carb Mediterranean diet. This approach emphasizes low-carb foods commonly eaten in the Mediterranean region of the world, including fish, nuts, seeds, and plant oils. Carbohydrates are generally capped at 40g to 100g a day from sources such as fruits, vegetables, legumes, whole grains, and yogurt.
  • Zero-carb diet. More of a fringe Internet fad than anything else, this weight-loss diet prohibits all carbs, even those from whole fruits and vegetables. WW’s experts say this ultra-restrictive eating approach is neither advisable nor sustainable.

Potential side effects and risks of low-carb diets 

Low-carbohydrate diets vary in their intensity and affect people in different ways. It’s best to consult with your doctor before switching to a low-carb eating plan (or making any radical change to your diet). Below are some potential pros and cons to consider.

Potential side effects of a low-carb diet:

  • Vitamin and mineral deficiencies
  • Digestive difficulties, particularly constipation
  • Headache
  • Halitosis
  • Muscle cramps
  • Difficulty with adherence

Potential benefits of a low-carb diet:

What should your daily carb intake be?

The latest Dietary Guidelines for Americans recommend that carbs compose between 45% and 65% of total calories. The optimal proportion of carbs for you depends on your lifestyle and personal health picture, Mitchell says. For example, if you are living with type 2 diabetes, your optimal carb intake might be on the lower end of that bracket to help keep blood glucose level within a healthy range, she explains. Lifestyle factors such as physical activity can increase the need for carbs. Your doctor can advise you on the carb intake that makes sense for you.

The upshot: Should you eat carbohydrates? 

Given that they are the preferred energy source for the muscles and brain and support other important functions, carbohydrates are key in a healthy pattern of eating, a large body of research has determined. Carbs come in several forms and are abundant in many foods. (Not just bread!) 

Health experts generally recommend limiting intake of carbs from added sugars and refined grains. Instead, consider centering your diet on whole foods that deliver good-for-you vitamins, minerals, and phytonutrients along with carbs. Examples of nutritious, carb-rich foods include fruits, veggies, whole grains, and legumes. 

Despite the popularity of low-carb diets, there’s scant evidence to suggest that people have more long-term weight-loss success when they severely cut back on this macronutrient group. In fact, whole foods that contain carbohydrates often promote satiety while being relatively low in calories. “Carbs can and should be part of a diet for weight management,” Hermann says. 

A healthy approach to eating—whether or not you’re looking to lose weight—is sustainable in the long term, includes a variety of whole foods, and allows for personal choice in what to eat. With carbohydrates, nutritious options abound.


Karen Ansel, MS, RDN is a journalist and author specializing in nutrition, health, and wellness. Her latest book is Healing Superfoods for Anti-Aging: Stay Younger, Live Longer.