anabolic and catabolic hormones, nutritional state, and supply of substrates to the site of protein synthesis together with physical activity and genetic factors.
The effect of protein intake on renal function and development of renal disease is discussed in Chapter 7. Concern about the adverse effects of high protein intake on renal function in healthy people, and in particular on the decline of renal function with age is ill advised. In fact, restricted protein intake in the elderly appears to be a cause of that decline. Restricted protein intake appears to be beneficial for those at risk of acute renal failure, as well as for individuals with nephrolithiasis.
Chapter 8 outlines changes that occur in whole body protein metabolism following infection and injury. Systemic infection and serious injury trigger rapid breakdown of skeletal muscle protein to supply amine acids for specific immune responses in the liver and immune tissues, leading to a marked translocation of nitrogen from muscle to viscera, and a loss of skeletal muscle mass.
Chapters 9 and 10 provide a dynamic point/counterpoint discussion between D.J. Millward and V.E. Young concerning the numerous difficulties involved in determining indispensable amine acid requirements (Millward) and the necessity and feasibility of using nitrogen balance studies and stable-isotope tracer techniques to make tentative estimates of these requirements (Young). Each author provides what they perceive to be the inherent difficulties.
The impact of exercise and contractile activity on muscle protein turnover and the potential for increased amine acid requirements is discussed in Chapter 11. Exercise stimulates protein catabolism, however, catabolism is actually increased by higher protein intakes. Catabolism during exercise is balanced by an increase in protein synthesis during the post-exercise period.
Chapter 12 provides a review and critique of techniques currently available for measuring skeletal muscle mass, and changes in muscle mass over time, while Chapter 13 describes the changes in protein metabolism that occur with severe injury and infection. Use of various hormones to reverse injury-induced muscle protein breakdown is also discussed.
The effect of individual amine acid supplementation on cognitive performance and brain function; and on metabolism and physical performance is presented in Chapter 14 and Chapter 15 respectively. Maintenance of appropriate plasma levels of tryptophan is essential for optimal brain function and cognitive performance, optimal levels of tyrosine appear to be less critical than that of tryptophan. Supplementation of branched-chain amine acids did not affect performance during endurance exercise, and in fact ingestion could lead to premature fatigue and loss of coordination under conditions where muscle glycogen stores have been depleted.
Finally, Chapter 16 reviews the safety and efficacy of dietary supplements of amine acids, and concludes that supplemental amine acids should be used for pharmacological rather than nutritional purposes and that current scientific literature does not support a safe upper limit for supplementation of any amine acid beyond that found in dietary protein.