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The developmental patterns of two amino acid-catabolizing enzymes, serine-threonine dehydratase (STD) and branched-chain α-keto acid dehydrogenase (BCKAD), were investigated in growing rats. At 10, 15, 20, 30, and 60 days of age, STD and BCKAD activities were determined in pup tissues. Hepatic STD activity increased more than 3-fold between 10 and 20 days of age; after this peak, activity decreased by 30 days of age. Threonine dosing did not affect STD activity at any age. In the liver, kidney, brain and skeletal muscle, total BCKAD activity increased 2- to 4-fold between 10 and 30 days of age. Percentages of BCKAD active in all tissues decreased between 10 and 15 days of age, associated with a large increase in total activity. The percentages of BCKAD activity were unchanged between 20 and 30 days of age but increased from 30 to 60 days of age, associated with a decrease in total activity. Leucine dosing did not affect total BCKAD activity at any age. These results demonstrate that both STD and BCKAD develop late during the suckling period and total activities are unresponsive to excess dietary amino acids.

Athletes associate performance with diet. Meat became a staple of ancient Greek and Roman athletes as they attempted to achieve the strength and endurance of carnivorous members of the animal kingdom. As knowledge of nutrition and muscle physiology increased, athletes became convinced that to increase muscle mass and strength required increased dietary protein. However, nutrition textbooks (1,2) and the Recommended Dietary Allowances (RDA's) established by the National Academy of Sciences (3) state that there is little or no need for extra protein for exercise.Review of the nutrition and exercise literature indicates that physical activity produces changes in protein metabolism. A few of these changes are increased urinary nitrogen, increased nitrogen in sweat, and increased protein mass of muscles. These physiological changes, which suggest an increased need for dietary protein, together with the renewed popular interest in exercise have led to a reevaluation of protein utilization during exercise. Aspects of this topic have been addressed in other recent reviews (4,5,6).To examine the influence of exercise on protein metabolism, it is important to consider the differences between types of exercise.The previous chapters by Terjung and Goodman have examined the effects of exercise intensity and duration on different muscles and the primary fuels required for specific activities.For the purpose of this chapter, exercise will be classified as anaerobic or aerobic. These terms indicate metabolic differences and imply differences in intensity and duration. Anaerobic activities are of brief duration and at, or approaching, maximum exertion.Anaerobic training emphasizes strength and frequently results in muscle hypertrophy.Aerobic exercise features prolonged activities at less than maximum exertion.Training emphasizes endurance work and results in increased oxidative capacity of the muscles with little or no change in muscle mass. Thus, the type of exercise can influence muscle mass and the amount of muscle proteins and presumably the needs for dietary protein.To conceptualize amino acid metabolism, it is useful to consider a model which describes the flux of amino acids through

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