The absorbed glucose, fructose, and galactose ultimately travel to the liver, where the fructose and galactose are converted to glucose. The studies of Katz and McGarry (1984) suggest that most of the ingested glucose must first be metabolized to lactate by either skeletal muscle or possibly by the liver itself (D.W. Foster, 1984).
The lactate is then reconverted to glucose by the liverthe so-called “glucose paradox.” Glucose may then be stored as glycogen in the liver or may be exported to the skeletal muscles and heart, where the glucose is stored as muscle glycogen if the glycogen stores in those muscles are reduced. Alternatively, some of the glucose may be burned by certain tissues, such as the brain and the kidney, and by the red blood cells, which all depend on blood glucose for their normal function.
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Once the liver and muscle glycogen stores are filled, any excess carbohydrate is stored as fat (triglyceride) in the adipose tissue. It seems, however, that little of the triglyceride stored in adipose tissue originates from carbohydrate; almost all is derived from ingested fat.
The absorbed protein digestion productsthe amino acidsgo to the liver, where they may serve as precursors for glucose and glycogen production. More importantly, all body tissues, in particular muscle, use amino acids to replace proteins that are continually being broken down.
Mobilization and Utilization of Stored Fuels
Once exercise begins, the fuels that have been stored during the resting period must be mobilized to provide the ATP necessary for muscular contraction.
Glycogen stored with water in the liver and muscle is hydrolyzed in a reaction controlled by the enzyme phosphorylase to form glucose 1-phosphate and then glucose 6-phosphate. In the liver another enzyme, glucose 6-phosphatase, acts on the glucose 6-phosphate to produce glucose. This glucose then enters the bloodstream, from which it is later extracted by the muscles, brain, kidney,