Carbohydrates
Introduction
Carbohydrates are the main source of fuel for working muscles. The majority of adenosine triphosphate (ATP) production needed for contracting muscles depend on the availability of muscle glycogen and blood glucose. The body stores glycogen in skeletal muscles at a concentration of about 80-130 mmol/kg wet weight depending on the training of the individual. The liver is the primary source of blood glucose, and the concentration of glycogen in the liver is 270 mmol/kg wet weight.
During high intensity physical activity, muscle glycogen stores are the body's chief source of energy because the energy released from the breakdown of glycogen in muscle is faster than the energy released from the breakdown of fat. At normal concentrations of muscle glycogen, there is enough energy for about 90-120 minutes of activity. In the later stages of prolonged exercise, 75-90% of the body's carbohydrate metabolism may be from blood glucose. Athletes must intake adequate amounts of carbohydrates in order to have a sufficient amount of glycogen stored in the muscle and liver for work. Athletes undergoing daily hard training may need up to 70% of their total caloric intake to be from carbohydrates to meet the needs of their working muscles and to maintain adequate muscle glycogen stores. All carbohydrates must first be converted to glucose to be utilized as energy through anaerobic and/or aerobic metabolism. Carbohydrates contain 4 kcal/g carbohydrate of energy.
Carbohydrate Use
There are three types of carbohydrates:
simple
complex
indigestible (dietary fiber)
There have been many studies testing the physiological response to different starches in the blood and as a result suggested the use of a glycemic index to classify carbohydrates in food. The glycemic index is a percentage measure of the area under a blood glucose response curve relative to glucose consumption which is set at 100%. After digestion, carbohydrates enter the bloodstream, which stimulates the pancreas to secrete the hormone insulin. Insulin activates the hormone, glycogen synthetase which causes the storage uptake of glucose by the liver and muscle as glycogen, and adipose tissue as fat. Simple sugars enter the bloodstream quickly and such a high level of glucose characterizes a high glycemic index. The high influx of glucose stimulates the pancreas to secrete insulin which activates glycogen synthetase to remove the glucose quickly from the bloodstream. The complex carbohydrates have lower glycemic indexes and they are absorbed and digested by the small intestine more slowly than the simple sugars. Complex carbohydrates are released into the bloodstream at a lower rate, maintaining a more constant insulin release resulting in more glycogen stored than with simple carbohydrates. Athletes should consume complex carbohydrates to increase muscle glycogen stores and to improve performance.
Simple Carbohydrate
Simple carbohydrates are also known as the simple sugars, which are macromolecules of carbon, hydrogen and oxygen containing one (monosaccharides) or two (disaccharides) sugar units. The naturally occurring monosaccharides are glucose and fructose, and the naturally occurring disaccharides are sucrose, maltose and lactose. Glucose and fructose are commonly found in fruits and honey. Sucrose is common table sugar, and is made up of one unit of glucose and one unit of fructose. Maltose is made up of two glucose units and are found in sprouted grains, and malted cereals. Lactose is milk sugar which is made up of one unit each of glucose and galactose and is found in dairy products.
Complex (Polysaccharides)
Polysaccharides are molecules composed of many units of sugar. The most common and naturally occurring polysaccharides are the plant and animal starches (amylose, amylopectin and glycogen) which are polymers (branched chains) of glucose. Sources of complex carbohydrates are found in cereal grains, fruits (e.g. apples and bananas) and starchy vegetables (e.g. legumes and potatoes).
Maltodextrins are a source of complex carbohydrates shown to increase muscle glycogen stores when used as a supplement to a pasta and rice diet. Frequently found in processed foods, maltodextrin is a member of a family of corn sweeteners (including corn syrup, high fructose corn syrup, and dextrose monohydrate) derived from a starch-conversion process of corn. Maltodextrins are commonly found in carbohydrate supplements.
Indigestible Dietary Fibers
These are non-starch complex carbohydrates not enzymatically digested in the human intestinal tract. There are two classes of fiber:
water-soluble
non-water soluble
Sources of complex carbohydrates are also good sources of fiber. The water-soluble fibers are the gums, hemicellulose, mucilages and pectins, and they are found in fruits, oats and legumes. The non-water soluble fibers are components of the cell walls of plants such as cellulose and lignins and they are found in vegetables, wheat, and grains. Foods high in fiber are whole-grain breads, fruits, wheat brans, and legumes. Fiber helps to regulate bowel movements, and is believed to be associated with reduced incidences of colon and rectal cancers.
Glycemic Indexes of Common Foods
| 100% | 90-80% | 79-70% | |
| glucose | maltose | white rice | |
| honey | potatoes | ||
| cornflakes | millet | ||
| carrots | |||
| 69-60% | 59-50% | 49-40% | |
| bananas | potatoes | oranges | |
| brown rice | all-bran | navy beans | |
| white bread | sucrose | dried peas | |
| spaghetti | |||
| (white) | |||
| 39-30% | 29-20% | 19-10% | |
| milk | lentils | soya beans | |
| yogurt | fructose | peanuts | |
| beans |
References
Buchbinder, J.C. et al. Manipulation of Muscle Glycogen Concentrations Using High and Low Carbohydrate Diets and Exercise. May, 1987. U.S. Army Research Institute of Environmental Medicine, Natick, MA. pp. 1-4.
Costill, D.L. Nutrition and Dietetics. The Olympic Book of Sports Medicine. Vol.1. (Dirix, Knuttgen, and Tittel. Eds.) Blackwell Scientific Publications, London. 1988. 603-635.
Crapo, P. Simple versus complex carbohydrate use in the diabetic diet. Ann. Rev. of Nutr. (Darby, Broquist and Olson, Eds.). 1985. 5: 95-114.
Diet and Health: Implications for Reducing Chronic Disease Risk. Committee on Diet and Health Food and Nutrition Board and Commission on Life Sciences and National Research Council. National Academic Press, Washington D.C. 1989. pp. 139-142; 159;259; 273-276; 291-2.
Jenkins, D.J. et al. Glycemic index of foods: a physiological basis for carbohydrate exchange. Am. J. Clin. Nutr. 1981. 34:362-66.
Lamb, D.R., A.C. Snyder, and T.S.Baur. Muscle glycogen loading with a liquid carbohydrate supplement. 1991. 1:52-60.
Lemon, Peter. Influence of dietary protein and total energy intake on strength improvement. Sports Science Exchange, Sports Nutrition Vol.2, Gatorade Sports Science Institute, Chicago, Ill. April 1989.
Pomeranz, Y. Functional properties of food components. Academic Press, Inc., Orlando, Fl. 1985.
Sherman, William. Carbohydrates, Muscle Glycogen, and Muscle Glycogen Supercompensation. Ergogenic Aids in Sport (Williams, Ed.) 1983. Human Kinetics Publishers, Inc. Champaign, Illinois. pp. 3-27.
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