Carbohydrate Metabolism

Humans can store approximately 350 grams (1,400 kilocalories) in the form of muscle glycogen, an additional 90 grams (360 kilocalories) in the liver, and a small amount of circulating glucose in the blood (~5 grams, or about 20 kilocalories). The larger the muscle mass, the greater the potential glycogen storage but also the greater the potential need.

We have systems for maintaining blood glucose within a relatively narrow range (70 to 110 milligrams per deciliter) by recruiting insulin and glucagon. Insulin and glucagon are pancreatic hormones that work synergistically to control blood glucose. Excess production of insulin can result in hypoglycemia (low blood sugar), with a resultant excess production of fat; inadequate insulin production results in hyperglycemia (high blood sugar) and diabetes.

Insulin is secreted by the beta cells of the pancreas, whereas glucagon is secreted by the alpha cells of the pancreas. The stimulus for insulin secretion is high blood glucose (the higher the glucose, the higher the insulin response), but a small amount of insulin is constantly being secreted by the pancreas even when blood glucose is in the normal range, causing a steady flow of glucose to the cells of the brain and muscles. Insulin lowers blood glucose by affecting the cell membranes of muscle and fat cells, thereby allowing glucose from the blood to enter the cell. This action causes a transfer from blood glucose to cell glucose and explains the blood-glucose-lowering effect of insulin; it also enables cells to receive a needed source of energy.

With low blood glucose, as occurs between meals and during exercise, glucagon is secreted. Lower levels of blood glucose result in greater glucagon production. Glucagon causes catabolism of liver glycogen, which results in the release of some of its glucose molecules into the blood. Glucagon may also stimulate gluconeogenesis (the manufacture of glucose from nonglucose substances). The amino acid alanine, for instance, is derived from protein and is converted to glucose by the liver.

About 60 percent of the glucose released by the liver to sustain blood sugar is from liver glycogen stores, but the remainder is from glucose synthesized from lactate, pyruvate, glycerol, and amino acids.¹ The rate of liver glucose infused into the blood during exercise is a function of exercise intensity, with higher-intensity exercise causing a faster rate of liver glucose release.² The combination of lower blood insulin and higher epinephrine and glucagon during long-duration activity stimulates liver glucose release.²

Besides insulin and glucagon, two other hormones also influence blood glucose. Epinephrine (adrenaline) is a stress hormone that initiates an extremely rapid breakdown of liver glycogen to quickly increase blood glucose levels. Cortisol, which is secreted from the adrenal gland, is also a stress hormone that promotes protein catabolism. This protein breakdown makes certain glucogenic amino acids available for gluconeogenesis, ultimately resulting in an increase in blood glucose. Both epinephrine and cortisol are released as a result of exercise-related stress, and both can be mediated through maintenance of blood glucose. Controlling epinephrine production helps preserve liver glycogen, and controlling cortisol helps preserve muscle protein. This is a strong argument for consuming carbohydrate during exercise.

The glucose circulating in the blood is derived mainly from dietary carbohydrate, with starch constituting the major source. Complex carbohydrates (starches) are digested into monosaccharides (glucose, fructose, and galactose) for absorption into the blood. In some cases, individuals have inadequate lactase to break milk sugar (lactose) into its component monosaccharides (glucose and galactose), causing the lactose to go indigested in the gut. Referred to as lactose intolerance, this leads to bloating, abdominal pain, diarrhea, and dehydration.

Excess glucose in the liver and muscle is stored as glycogen, but only up to the glycogen saturation point. The liver has a maximum glycogen storage capacity of approximately 87 to 100 grams (348 to 400 kilocalories), while the muscles can store approximately 350 grams (1,400 kilocalories), or more in larger individuals. Providing additional glucose to cells when glycogen stores are saturated leads to the excess being stored as fat (in both muscle and fat cells). The liver glycogen is primarily responsible for stabilizing blood glucose, while the muscle glycogen is mainly responsible for providing an energy source to working muscles that can be used both aerobically and anaerobically. Blood sugar is not easily maintained when liver glycogen is depleted, even when muscle glycogen stores are full.

Blood glucose is the primary fuel source for the central nervous system. Low blood sugar results in depressed central nervous system activity, coupled with increased irritability and a lower capacity to concentrate. For athletes, low blood sugar may be related to mental fatigue, which is related to muscle fatigue. Because liver glycogen and blood glucose stores are easily depleted during even short-duration activities, the intake of carbohydrate during activity is a critical factor in maintaining mental function and, ultimately, muscle function.