Bile acid required for fat digestion and absorption.
Glucose polymer formed in mammalian liver and muscle. A storage form of glucose. Glycogen, when stimulated to release its glucose by the catabolic hormones, glucagon, epinephrine, the glucocorticoids, and thyroxine, and/or by the absence of food in the digestive tract, provides glucose to the body. Because the glycogen molecule has molecules of water as part of its structure, it is a very large molecule and cumbersome to store in large amounts. An average 70-kg man has only an 18-hour fuel supply stored as glycogen, while the same individual might have up to a 2-month supply of fuel stored as fat. Muscle and liver glycogen stores have very different functions. Muscle glycogen is used to synthesize ATP for muscle contraction, whereas hepatic glycogen is the glucose reserve for the entire body, particularly the CNS. The amount of glycogen in the muscle is dependent upon the physical activity of the individual. After bouts of strenuous exercise, glycogen store will be depleted only to be rebuilt during the resting period following exercise. Hepatic glycogen stores are dependent on nutritional status. They are virtually absent in the 24-hour starved animal while being replenished within hours of ad libitum feeding. Clusters of glycogen molecules with an average molecular weight of 2 x 10⁷ form quickly when an abundance of glucose is provided to the liver. The amount of glycogen in the liver is diet dependent. There is a 24-hour rhythmic change in hepatic glycogen that corresponds to the feeding pattern of the animal. In nocturnal animals such as the rat, the peak hepatic glycogen store will be found in the early morning hours, while the nadir will be found in the evening hours just before the nocturnal feeding begins. In humans accustomed to eating during the day, the reverse pattern will be observed.