The Effects of Protein and Calorie Restriction on Tissue Nitrogen Content and Protein Catabolism

Abstract

Young adult female rats, previously labeled with L-(¹⁴ C-guanido) arginine and L-(3-³ H) glutamic acid were fed ad libitum diets containing either 10- or 0-percent lactalbumin supplemented with arginine and glutamic acid, 0-percent protein or 10-percent lactalbumin plus amino acids restricted to 5 g diet/ day. Retention of radioactivity in liver and muscle was greater in the restricted groups but was restored to normal by feeding cold amino acid, indicating amino acid recycling. It is concluded that the apparent changes in rates of protein synthesis and degradation during protein and calorie restriction are due to changes in amino acid recycling rather than changes in metabolic rate, and that amino acid reutilization can be minimized by providing dietary amino acids.

Protein-Energy Interrelationships during Dietary Restriction: Effects on Tissue Nitrogen and Protein Turnover

“…Rates of protein catabolism, as estimated from rates of loss o f 3 H-glutamic acid from various tissues, were not statistically affected by moderate or severe protein or energy restriction. These findings support prior conclusions from this laboratory (14,15) and are contrary to conclusions from other studies (9,10,22,25) in human infants and young and mature rats.…”

“…Assuming that this amino acid is only recycled to a minimal degree (15), the data would imply that the rates of protein catabolism were not appreciably increased by energy restriction, even when the restriction was severe enough to cause a very rapid loss of body weight and tissue nitrogen. In confirmation of prior studies (9,10,14,15,22,25), protein deficiency yielded somewhat prolonged half-lives of the tissue protein investigated.…”

“…Assuming that this amino acid is only recycled to a minimal degree (15), the data would imply that the rates of protein catabolism were not appreciably increased by energy restriction, even when the restriction was severe enough to cause a very rapid loss of body weight and tissue nitrogen. In confirmation of prior studies (9,10,14,15,22,25), protein deficiency yielded somewhat prolonged half-lives of the tissue protein investigated. Since it has been shown (15) that under such conditions the provi sion of glutamic acid in the diet will reduce the observed half-lives, presumably by dilution of the glutamate pool and a reduction in the degree of glutamate recycling, we assume that the data do not demonstrate a significant effect on the rates of protein catabolism by either protein or energy restriction.…”

“…Since it has been shown (15) that under such conditions the provi sion of glutamic acid in the diet will reduce the observed half-lives, presumably by dilution of the glutamate pool and a reduction in the degree of glutamate recycling, we assume that the data do not demonstrate a significant effect on the rates of protein catabolism by either protein or energy restriction. However, it was emphasized in a previous paper (15) that in studies of this kind in which the tissue proteins are labeled by a single injection of an isotopic amino acid and mixed tissue proteins are evaluated, only those tissue proteins which turn over rapidly will be appreciably labeled. One cannot assume that the data obtained are representative of tissue proteins in general.…”

“…Nitrogen was determined on aliquots of the homogenates by the micro-Kjeldahl procedure. Samples were also prepared for scintillation counting as previously described (15). Half-lives for the loss of the isotope from the muscles were calculated as (In 2)/k where k is the slope of the semilog linear decay of total activity with time.…”