Body protein mass
A major and fundamental quantitative function of the dietary α-amino acid nitrogen and of the indis-pensable amino acids is to furnish substrate required for the support of organ protein synthesis and the maintenance of cell and organ protein content. Therefore, in the first instance the body protein mass is a factor that will influence the total daily require-ment for protein. Adult individuals of differing size but who are otherwise similar in age, body composi-tion, gender, and physiological state would be expected to require proportionately differing amounts of nitrogen and indispensable amino acids. Changes in the distribution and amount of body protein that occur during growth and development and later on during aging may be considered, therefore, as an initial approach for understanding the metabolic basis of the dietary protein and amino acid needs.
Direct measures of total body protein cannot yet be made in living subjects, although there are various indirect measures from which it is possible to obtain a picture of the body nitrogen (protein) content at various stages of life. From these approaches it is clear that body nitrogen increases rapidly from birth during childhood and early maturity, reaching a maximum by about the third decade. Thereafter, body nitrogen decreases gradually during the later years, with the decline occurring more rapidly in men than in women. A major contributor to this age-related erosion of body nitrogen is the skeletal musculature. Strength training during later life can attenuate or partially reverse this decline in the amount of protein in skeletal muscles and improve overall function.
The protein requirement of adults is usually considered to be the continuing dietary intake that is just sufficient to achieve a “maintenance” of body nitro-gen, often measured only over relatively short experimental periods. For infants and growing children and pregnant women an additional requirement is needed for protein deposition in tissues. However, this concept is oversimplified since the chemical com-position of the body is in a dynamic state and changes occur in the nitrogen content of individual tissues and organs in response to factors such as diet, hor-monal balance, activity patterns, and disease. Thus, proteins are being continually synthesized and degraded in an overall process referred to as turnover. The rate of turnover and the balance of synthesis and degradation of proteins, in addition to the mass of protein, are also important determinants of the requirements for nitrogen and amino acids, and these aspects will be discussed in the following section.