Carbohydrates

What they do

Digestible carbohydrate is one of the body’s major energy sources. Starches and sugars are broken down into simple sugars that can be absorbed, with glucose providing much of the carbohydrate-derived energy used by cells.

Red blood cells depend on glucose because they have no mitochondria. The brain normally uses large amounts of glucose but can shift part of its energy demand to ketones when carbohydrate availability stays very low.

Glucose can be burned immediately or stored as glycogen. Liver glycogen helps maintain blood glucose between meals. Muscle glycogen is stored locally and can be broken down rapidly during exercise.

Carbohydrate also includes fiber, which is covered separately because it is not digested and used like sugars and starches.

There is no essential carbohydrate, and there is still an RDA

Protein contains essential amino acids and fat contains essential fatty acids. There is no individual sugar or starch that healthy adults must obtain directly from food.

The liver can make glucose from lactate, glycerol, and certain amino acids. When carbohydrate intake remains very low, the liver also produces more ketone bodies from fat, allowing the brain to use less glucose.

The adult carbohydrate RDA is still 130 grams per day. It was established around the amount of glucose normally used by the brain, not as the lowest carbohydrate intake compatible with life.

Pregnancy raises the RDA to 175 g/day, and breastfeeding raises it to 210 g/day.

How much do adults need?

The adult Acceptable Macronutrient Distribution Range is 45 to 65 percent of total calories.

Lower-carbohydrate diets can also be nutritionally adequate when they are planned carefully. Reducing carbohydrate often removes foods that supplied fiber, potassium, magnesium, folate, thiamin, and other nutrients, so the rest of the diet determines what the reduction actually means.

The types of carbohydrate

No RDA exists for glucose, fructose, sucrose, lactose, starch, or any other individual digestible carbohydrate. The differences below describe how they are digested and used, not separate targets to hit.

Single sugars

Glucose is the main sugar circulating in blood. Starch digestion produces large amounts of it, and it is absorbed in the intestine primarily through the SGLT1 transporter.

Fructose occurs naturally in fruit and honey and is also part of sucrose and high-fructose corn syrup. It is absorbed primarily through GLUT5 and is processed extensively by the intestine and liver before its carbon reaches the rest of the body as glucose, lactate, or other metabolites.

Galactose comes mainly from the digestion of lactose in dairy foods and is largely converted into other metabolites by the liver.

Paired sugars

Sucrose, or table sugar, contains one glucose and one fructose molecule. It occurs naturally in many plants and is also widely used as an added sweetener.

Lactose contains glucose and galactose and is the main sugar in milk. Digestion requires the enzyme lactase. Lactase activity falls substantially after childhood in much of the world’s population, which is why lactose tolerance varies so widely between adults.

Maltose contains two glucose molecules. It appears during starch digestion and in malted grains.

Starches

Starch consists of long chains of glucose.

Amylose has relatively straight chains that pack tightly and are often digested more slowly.

Amylopectin is highly branched, giving digestive enzymes many points of access. Most food starch contains both amylose and amylopectin in different proportions.

This is why the phrase “complex carbohydrate” does not reliably mean slowly digested. A finely milled, highly processed starch can release glucose very quickly even though its molecules began as long chains.

Maltodextrin is starch that has already been broken into shorter glucose chains. It is rapidly digested and is common in sports drinks, gels, and other products designed to deliver carbohydrate quickly.

Resistant starch escapes digestion in the small intestine and behaves more like fiber. Legumes, green bananas, and some cooked-and-cooled potatoes, rice, and pasta contain resistant starch.

Sugar alcohols

Erythritol, xylitol, sorbitol, maltitol, and related compounds are neither ordinary sugars nor alcohol in the beverage sense. They are absorbed incompletely or handled differently from glucose and generally provide less energy and a smaller blood-glucose response than conventional sugars.

The amount reaching the colon varies by compound. Sorbitol and maltitol are more likely to cause gas, bloating, or diarrhea at larger doses. Most erythritol is absorbed in the small intestine and excreted in urine, which is why it is generally better tolerated gastrointestinally.

When the type of sugar actually matters

For ordinary meals, there is little reason to choose a specific isolated sugar for its absorption pathway. The distinctions become useful when someone is trying to move carbohydrate through the intestine quickly during prolonged exercise.

During exercise, glucose and glucose polymers such as maltodextrin rely heavily on the SGLT1 transport pathway. With glucose-based carbohydrate alone, exogenous carbohydrate oxidation generally levels off around 60 g per hour.

Fructose uses a different intestinal transporter. Combining fructose with glucose or maltodextrin allows both transport pathways to contribute, increasing the amount of ingested carbohydrate that can be absorbed and oxidized during prolonged exercise.

Established endurance guidance has commonly used multiple-transportable carbohydrate mixtures at rates up to about 90 g/hour for long events. Older formulations often used roughly twice as much glucose or maltodextrin as fructose. More recent studies have tested ratios closer to 1:0.8 and intakes approaching 120 g/hour in highly trained endurance athletes. Those higher rates are an active area of research rather than a general requirement.

For most exercise below those high fueling rates, glucose, maltodextrin, sucrose, ordinary foods, or mixtures of them can supply adequate carbohydrate without deliberately maximizing two transport systems.

Fructose is therefore most useful as a partner to glucose-based carbohydrate when very high carbohydrate delivery is needed. Large amounts of fructose by itself are more likely to exceed intestinal tolerance and cause gastrointestinal symptoms.

Sucrose already delivers glucose and fructose together after digestion, so it can also function as a multiple-transportable carbohydrate source.

After glycogen-depleting endurance exercise, adding fructose to glucose-based carbohydrate can speed the restoration of liver glycogen, which can matter when another long session or event follows within a short recovery period. Muscle-glycogen restoration depends more on getting enough total carbohydrate.

Carbohydrate quality is more than digestion speed

Beans, oats, potatoes, fruit, white bread, candy, and a sports drink can all provide carbohydrate. The number of carbohydrate grams does not tell you what else came with them.

Beans and lentils bring fiber, protein, potassium, magnesium, and folate. Whole fruit provides water, fiber, vitamins, minerals, and intact plant structure. Whole grains generally retain more fiber and micronutrients than refined versions.

Processing changes how quickly digestive enzymes can reach starch and sugar. Milling grain into fine flour, juicing fruit, cooking starch, and breaking food into smaller particles can all change digestion.

Rapid digestion is not automatically undesirable. A sports drink, white rice, bread, or maltodextrin can be useful when someone needs carbohydrate that is easy to digest and becomes available quickly.

The same properties provide less advantage when rapidly digested, low-fiber foods dominate an ordinary sedentary diet and displace foods that would have supplied fiber and micronutrients.

Blood glucose and glycemic index

The amount of digestible carbohydrate strongly affects the glucose response to a meal, but food structure, fiber, protein, fat, cooking, ripeness, processing, and portion size also matter.

The glycemic index compares the blood-glucose response to foods containing a standardized amount of available carbohydrate, measured under controlled conditions. Glycemic load combines that ranking with the amount of available carbohydrate in a serving, which makes portion size part of the calculation.

Glycemic index describes one property of a food rather than its overall nutritional quality. Mixed meals can produce different responses from a food eaten by itself, and individuals can respond differently to the same meal.

Total sugar and added sugar are different

The Nutrition Facts label separates Total Sugars from Added Sugars.

Total Sugars includes sugars naturally present in foods such as fruit and milk as well as sugars added during manufacturing or preparation. There is no FDA Daily Value for Total Sugars.

Added Sugars includes caloric sweeteners added during preparation or processing. The FDA Daily Value remains 50 g on a 2,000-calorie diet.

The current 2025–2030 Dietary Guidelines no longer use the previous limit of less than 10% of calories per day as their main added-sugar recommendation. That edition states that one meal should contain no more than 10 g of added sugars, and it recommends avoiding sugar-sweetened beverages and limiting foods high in added sugars.

The main nutritional problem with a high added-sugar intake is not that sucrose or fructose becomes a different molecule when it is added to food. It is that added sugars can supply substantial calories without fiber, protein, or many micronutrients, displacing foods that would have supplied them. Sugar-sweetened beverages make that particularly easy because a large dose of sugar can be consumed quickly.

The difference between whole fruit and a sugar-sweetened drink is the food around them: fruit brings water, fiber, micronutrients, and intact structure, while a sweetened drink can deliver a large amount of sugar with little else.

High-fructose corn syrup is not pure fructose. The common U.S. forms contain about 42% or 55% fructose, with most of the remainder as glucose. Sucrose supplies glucose and fructose in a 50:50 ratio after digestion. The FDA does not consider HFCS-42 or HFCS-55 uniquely less safe than similar amounts of other glucose-fructose sweeteners.

Which carbohydrates to get more of, and less of

Fiber-rich carbohydrate foods are the clearest place most U.S. adults need more. Beans, lentils, fruit, vegetables, whole grains, nuts, and seeds contribute fiber along with varying amounts of vitamins and minerals. Fiber has its own Nutrition Foundations page because typical intake remains well below recommendations.

Added sugars are the clearest place to reduce intake. Sugar-sweetened beverages are particularly easy to consume in large amounts without bringing fiber or much nutritional value.

Between those ends, which one fits depends on the situation. Whole and minimally processed carbohydrate foods generally bring more fiber and micronutrients. Refined or rapidly digested carbohydrate can be useful when quick energy, easy digestion, or rapid glycogen replacement is the goal.

No dietary target exists for fructose, glucose, sucrose, lactose, amylose, or amylopectin individually. For ordinary eating, there is no reason to construct the diet around reaching a particular amount of any one of them.

Carbohydrate and training

Muscle and liver glycogen make carbohydrate especially important during repeated hard or prolonged exercise. Glycogen demand rises with training volume and intensity.

Sports-nutrition guidance scales daily carbohydrate intake to training demand:

During shorter recreational or resistance-training sessions, carbohydrate consumed during the workout is often unnecessary when normal meals have supplied adequate energy beforehand.

Longer endurance sessions are different because carbohydrate can become limiting during the event itself, which is where the absorption limits described earlier begin to matter.

Where carbohydrates come from

The fiber column shows why two foods with similar carbohydrate totals can have very different nutritional profiles.

Who may need to pay closer attention

• people with diabetes or impaired glucose regulation
• people doing high-volume endurance training or repeated hard sessions
• people following very-low-carbohydrate or ketogenic diets
• people whose carbohydrate intake comes mainly from refined grains, sweets, and sugar-sweetened beverages
• people who reduce carbohydrate enough to remove major sources of fiber and micronutrients without replacing them
• people with lactose intolerance or other specific carbohydrate malabsorption
• people with irritable bowel syndrome, where particular fermentable carbohydrates may trigger symptoms
• pregnant or breastfeeding women, whose carbohydrate RDAs are higher

Diabetes treated with insulin, hereditary fructose intolerance, galactosemia, glycogen-storage diseases, epilepsy treated with a prescribed ketogenic diet, and other metabolic conditions require individualized carbohydrate guidance.

What to check in your routine

Start with the amount that fits the person’s activity level, then look at the foods supplying it.

For ordinary meals, prioritize carbohydrate foods that also bring fiber, vitamins, minerals, or useful food structure: legumes, fruit, vegetables, potatoes, oats, and whole grains.

Check how much added sugar comes from drinks and highly processed foods, and whether those foods are replacing more nutrient-dense options.

For long endurance training or competition, rapidly digested carbohydrate has a different job. Glucose, maltodextrin, sucrose, and glucose-fructose mixtures can be useful because fast absorption is the point.

For very-low-carbohydrate diets, check what disappeared along with the carbohydrate. Fiber, potassium, magnesium, folate, thiamin, fruit, legumes, and whole grains may need deliberate replacement.

Related: Fiber · Vitamin B1 (Thiamine) · Fat

The information on this page is educational and does not constitute medical advice. Talk to a qualified healthcare provider or dietitian before making major changes to carbohydrate intake if you have diabetes, take glucose-lowering medication, are pregnant, or follow a medically prescribed low-carbohydrate or ketogenic diet.