L. J. Filer

RELATIONSHIP OF BODY COMPOSITION, CHEMICAL MATURATION, HOMEOSTASIS, AND DIET IN THE NEWBORN MAMMAL

Physiologic integration of growth, renal function, and diet in the achievement of homeostasis by the newborn mammal was elegantly described in 1957 by McCance and Widdowson. They pointed out that urea derived from dietary protein and presented for urinary excretion is diminished in proportion to the amount of protein incorporated into new tissue. Consequently, when renal function is still at an immature level, capacity for growth becomes an important factor in maintaining homeostasis. Specific organs such as the kidney and liver are known to hypertrophy in response to increase in dietary protein (Walter and Addis, 1939; Filer et al., 1960). Studies involving the transfusion of radioisotope-labeled blood plasma proteins into experimental animals ( Steinbock and Tarver, 1954; Jeffay and Winzler, 1958; Yuile et al., 1959; Vaughan et al., 1962) and humans (Blahd et al., 1955; Iber et al., 1958) show protein turnover in plasma to be more rapid when a high-protein diet is fed than when a diet low in protein is fed. These observations lead to the conclusion that protein catabolism increases as the leveI of protein in the diet is increased. Studies of composition of the 8-week-old pig carcass gave evidence of chemical maturation; variation in dietary rotein from 11 to 62 per cent of calories was without influence (Filer et a[ 1960). The level of dietary fat, on the other hand, effected considerable change in carcass fat content at age eight weeks. According to Widdowson and Dickerson (1960) skeletal muscle, skin, and skeleton contribute about 70 per cent of body weight and contain more than 70 per cent of the total nitrogen in the body. Thus, a potential for attaining homeostasis through growth would be readily detectable in hypertrophy or other deviation in composition of these compartments. I t therefore seemed plausible to study composition of carcass and skeletal muscle of the growing piglet as a function of age and level of dietary protein, to further delineate the role of growth in the achievement of homeostasis of the newborn. No attempt was made to study composition of skin or skeleton individually; their contribution to homeostasis is considered only in the gross concept of carcass.