Performance Nutrition Blog

  • What You Need to Know About Carbohydrates

    A carbohydrate is a macronutrient that primarily serves as an energy source for all of our bodily functions. Carbohydrates are found mostly in grains, fruits, vegetables, legumes and dairy foods. A common lay name for carbohydrates is “sugars,” which is a bit of a misnomer as discussed below.


    Carbohydrates are necessary to optimize health, training and performance. They are a major source of energy for our bodies and are needed to fuel the brain, other parts of the central nervous system and muscles. Portions of our central nervous systems rely exclusively on carbohydrates for fuel, including the brain itself.1

    Carbohydrates are the only substrates that can allow the sustained moderate- to high-level efforts required in most sports. A portion of the carbohydrates we consume are converted and stored as glycogen in our muscles and liver. Our muscles depend on glycogen to provide energy during higher intensity anaerobic activity. In addition, carbohydrates stored as muscle glycogen also provide approximately 50% of energy needed for aerobic endurance exercise of moderate intensity and nearly all of the energy needed during high intensity aerobic exercise.1

    Carbohydrates are molecules composed of carbon, hydrogen and oxygen. They are classified according to the number of saccharide or “sugar” units they contain, which is why they are commonly called “sugars.” Carbohydrates should not be confused with simple table sugars, since carbohydrates include a wide variety of molecules with unique chemical structures. The following is a chart of carbohydrate classifications and a list of the types of carbohydrates in each class.

    Class # of Sugar Units Types
    Monosaccharides Single-sugar molecule Glucose - blood sugar
    Fructose - sugar in fruits and vegetables
    Disaccharides Two simple sugar units joined together Sucrose - table sugar (glucose + fructose)
    Lactose - milk sugar (glucose + galactose)
    Maltose - (glucose + glucose)
    Oligosaccharides Three to ten monosaccharides Maltodextrins
    Corn syrup
    High-fructose corn syrup
    Polysaccharides Complex carbohydrates, up to thousands of glucose units Starch


    Disaccharides and polysaccharides are ultimately converted into simple sugars (i.e. glucose or fructose) through the processes of digestion and absorption . The fewer the number of sugar molecules, the easier the carbohydrate is to digest (in fact, some complex carbohydrates can’t always be fully digested).

    After digestion, some of the glucose (blood sugar) is used as fuel by tissues of the brain, nervous system and muscles. A portion of the glucose is converted to glycogen after a meal and stored within the liver and muscles. Any excess glucose is converted to fat and stored throughout the body as a reserve source of energy.1

    Inadequate carbohydrate intake
    If you don’t consume enough carbohydrates, you run the risk of running low on energy.  Without the proper intake of carbohydrates throughout your day, you may feel tired, weak and not think well.

    Fiber, a major carbohydrate, provides bulk in the diet and increases the satiety value of foods (how full you feel). It also helps maintain good intestinal mobility and regulates the body’s absorption of glucose. Diets that are low in fiber have been associated with several medical conditions including diverticulosis, constipation, heart disease, cancer of the colon and diabetes mellitus type 2.2

    Carbohydrates also help regulate the digestion and utilization of protein and fat.1 So, if you don’t consume adequate amounts of carbohydrate you may metabolize protein instead and not have enough protein to maintain your muscle mass. This is called muscle wasting, something that active individuals and high performance individuals can’t afford.

    Excess carbohydrate intake
    On the other hand, if you consume too much carbohydrate, you run the risk of increasing your body weight not by adding muscle, but by adding fat. As mentioned, excess carbohydrates, beyond that which are used for fuel and stored as glycogen, are converted to fat and stored throughout the body. This is the body’s way of storing up energy for later use. In this day and age however, our lifestyles don’t often allow us to use all of that stored up energy and we end up holding onto energy in the form of excess body weight.

    Digging deeper

    Glucose is the most common monosaccharide. It is present as sugar in the blood stream and is the primary energy source for our cells. Glucose is usually combined with other monosaccharides when it’s present in food. For example, sucrose (table sugar) is glucose combined with fructose. Isolated glucose, referred to as dextrose, is used in intravenous fluids and sometimes in sports drinks.

    Fructose occurs naturally in fruits and vegetables and gives honey its sweet taste. Fructose in the body causes less insulin secretion than other sugars. Though, large doses of fructose have been shown to increase the risk of gastric cramping and diarrhea. So, applications of fructose as a primary source of carbohydrates during exercise are limited.2

    Sucrose is the most common disaccharide. Sucrose occurs naturally in most fruits and is crystallized from the syrup of sugar cane and sugar beets to make brown, powdered or white sugar.2

    Maltose occurs primarily when polysaccharides are broken down during digestion. It also occurs in the fermentation process of alcohol and is the main carbohydrate in beer.

    Starch is the storage form of glucose in plants. Grains, nuts, legumes and vegetables are good sources of starch. Starch must be broken down into glucose components, which requires prolonged enzymatic action,  before it can used as a source of energy.

    Dietary fiber, a constituent of the plant cell wall, is also a form of carbohydrate. It is generally resistant to human digestive enzymes and therefore increases bulk and water content and decreases transit time of feces. 

    Glycogen, not present to any large extent in the foods we eat, is found in small amounts in human and animal tissue as a temporary source of stored energy. When glucose enters the muscles and liver and is not metabolized for energy it is synthesized into its storage form: glycogen. Two-thirds of the glycogen in our body is stored in skeletal muscle and the remaining third in the liver.

    1National Academy of Sports Medicine. (2008). NASM Essentials of personal fitness training (3rd ed.). (M. A. Clark, S. C. Lucett, & R. J. Corn, Eds.) Baltimore, MD: Lippincott Williams & Wilkins.
    2Reimers, K. (2008). Nutritional factors in health and performance. In National Strength and Conditioning Association, T. R. Baechle, & R. W. Earle (Eds.), NSCA Essentials of strength training and conditioning (3rd ed., pp. 201-233). Champaign, IL: Human Kinetics.


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