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Everything You Need to Know About Carbohydrates

By Shereen Lehman, MS 

Medically reviewed by Richard Fogoros, MD Updated on May 27, 2021

Carbohydrates are the primary source of energy for the body and include both simple sugars and larger complex carbohydrates. Your body can use carbohydrates right away or convert them into a storage form called glycogen. Excess carbohydrates can also be converted into fat.1

Carbohydrate Chemistry

No matter how big they are, all carbohydrates are made of carbon, hydrogen, and oxygen with the general formula of Cx(H2O)y.2 For example, a simple sugar molecule like glucose is made up of six carbon atoms, 12 hydrogen atoms, and six oxygen atoms. It has the formula C6H12O6.3 A large starch molecule can be made of many little sugar molecules connected to form a long chain. The small x and y in the general formula, Cx(H2O)y, can run into the hundreds.

Simple Sugars

Simple sugars are made up of one or two sugar units. Glucose is a common simple sugar that our bodies and brains use for energy every day. Glucose is called a monosaccharide, which means “single sugar.” Other monosaccharides include fructose, galactose, and ribose.

Fructose is found in fruits and vegetables; galactose is found in milk; and ribose is best known as a part of ribonucleic acid, which is a part of the genetic material in our cells.4

Rather than go deeper into the chemistry of simple sugars, it is important to know that the single sugars glucose, fructose, and galactose can form different combinations to become disaccharides, a term that means “two sugars.” These sugars include:5

Simple sugars are water-soluble and easy for your body to digest into the individual glucose and fructose molecules. They’re also quickly absorbed through the intestinal walls and into the bloodstream.

Complex Carbohydrates

Complex carbohydrates are long chains of single sugar units. For example, the complex carbohydrate we know as starch is made up of many glucose units.6 These complex carbohydrates can be in the form of long chains, or the chains can form branches. Complex carbohydrates include:

Dietary starch and cellulose are the complex carbohydrates that are essential for good health. Potatoes, dry beans, grains, rice, corn, squash, and peas contain significant amounts of starch.

Vegetables like broccoli, cauliflower, asparagus, lettuce, and other greens are not starchy. That’s because the stems and leafy parts of plants don’t contain much starch, but they do provide a great deal of cellulose. Since we can’t digest cellulose, that means that the green and leafy vegetables contain fewer calories than the starchy vegetables.7

Carbohydrate Metabolism

The body begins the process of breaking carbohydrates down into their individual monosaccharides almost before we start to eat them. When you smell the delicious aroma of fresh-baked bread or think about that tasty chocolate that you’re about to consume, your mouth begins to water.

Since table sugar is water-soluble, it starts to dissolve in your mouth. Your saliva also contains a small amount of amylase, which is an enzyme that begins to break starch down into glucose while you are chewing.8

Carbohydrate digestion continues in the small intestine with the help of pancreatic amylase. Amylase breaks carbohydrates down into monosaccharides that can be absorbed into the bloodstream. Once in the blood, the monosaccharides are either used for energy, stored in the liver and muscles as glycogen, or converted into fat and stored in adipose tissue.9

Your body needs insulin to use and store glucose. Insulin “unlocks” cells to allow glucose to get in. People with diabetes or metabolic syndrome either can’t produce enough insulin, or they are not sensitive enough to the insulin they produce and need to regulate their blood sugar with medications, insulin, or dietary changes.10

Your body prefers to use glucose as the primary source of fuel for all your daily activity. Muscles need glucose to move, and organs need glucose to function. While your body can make glucose from any extra dietary protein by a process called gluconeogenesis, it’s best if you consume carbohydrates.

Carb Requirements and Sources

Carbohydrates should contribute 45% to 65% of your daily calories.11 One gram of carbohydrate contains 4 calories whether it is sugar or starch. One slice of bread has about 12 grams of carbohydrates. One typical chocolate bar may have about 50 grams of carbohydrates. A medium potato has about 35 grams of carbohydrates.

Although all carbohydrates have 4 calories per gram, some sources provide more beneficial micronutrients per calorie, thereby making them more health-promoting. Fruits, vegetables, legumes, nuts, seeds, and grains provide more nutrients than candy, sodas, and pastries.

The healthy carbohydrate sources also have significant amounts of vitamins, minerals, phytochemicals, and fiber, all of which are vital to good health. Candy, sodas, pastries, and other convenience foods usually are inadequate sources of nutrients, and sometimes we refer to these foods as having “empty calories.” That means the foods are high in energy derived from carbohydrates but with little to no vitamins, minerals, phytochemicals, or fiber.

Since about half your calories should come from carbohydrates, it’s easy to calculate how many grams of carbohydrates you need per day. For example, let’s say a person needs 2,000 calories per day. That means that 1,000 calories should come from carbohydrates (2,000 X 0.5). Since each gram of carbohydrate has 4 calories, then you divide 1,000 by four (1,000/4) to get 250.

That person who needs 2,000 calories each day needs about 250 grams of carbohydrates per day. Of those 250 grams, less than 10% should come from added table sugar and sweeteners.11 That would be about 25 grams for a 2,000 calorie per day diet. That would equal about half of a candy bar or less than one can of sugary soda.

Carb Counts for Common Foods

Once you know how many grams of carbs you need every day, you can select your foods based on their carb counts and fit them into your daily calorie and carb budget. It really is impossible to list every carbohydrate-containing food here, however, here are some approximate amounts from typical examples:12

Nutrition facts labels on packaged foods will also list the amount of carbohydrates per serving. It takes a little extra time and effort to look up the carbohydrate counts for all of the foods you eat, but with experience, you will begin to have a good idea of approximate calorie counts and carbohydrate counts.

  1. Schwarz J-M, Clearfield M, Mulligan K. Conversion of sugar to fat: is hepatic de novo lipogenesis leading to metabolic syndrome and associated chronic diseases? J Am Osteopath Assoc. 2017;117(8):520-527. doi:10.7556/jaoa.2017.102
  2. Davidson EA. Encyclopedia Britannica, Inc. Carbohydrate.
  3. Encyclopedia Britannica, Inc. Glucose.
  4. National Human Genome Research Institute. Ribonucleic Acid (RNA).
  5. Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. Section 11.2, Complex Carbohydrates Are Formed by Linkage of Monosaccharides. New York: W H Freeman; 2002.
  6. National Center for Biotechnology Information. PubChem Compound Summary for CID 145864875.
  7. Yan L. U.S. Department of Agriculture. Agricultural Research Service. Dark Green Leafy Vegetables. Updated August 13, 2016.
  8. Peyrot des Gachons C, Breslin PAS. Salivary amylase: digestion and metabolic syndrome. Curr Diab Rep. 2016;16(10):102. doi:10.1007/s11892-016-0794-7
  9. University of Michigan Medicine. Carbohydrates, Protein, Fats, and Blood Sugar. Updated August 31, 2020.
  10. Centers for Disease Control and Prevention. Insulin Resistance and Diabetes. Updated August 12, 2019.
  11. U.S. Department of Health and Human Services and U.S. Department of Agriculture. 2020–2025 Dietary Guidelines for Americans. Ninth Edition. December 2020.
  12. U.S. Department of Agriculture. Agricultural Research Service. FoodData Central.

*Very Well Fit, Everything You Need to Know About Carbohydrates, By Shereen Lehman, MS,, 05/27/2021

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