Studies on gastric digestion of protein and carbohydrate, gastric secretion and exocrine pancreatic secretion in the growing pig

Abstract

1. We have investigated the relations between changes in plasma insulin and 3,5,3'-triiodothyronine (T,), and muscle growth and protein turnover in the rat in response to diets of varying protein concentrations.2. Young rats were fed ad lib. on a control (180 g casein/kg) diet or low-protein diets containing 80, 45 and 0 g casein/kg in four separate experiments. Measurements were made of food intakes, muscle and body-weight growth rates, muscle protein turnover in vivo, plasma insulin, and plasma free and total T,.3. The food intakes of the 80 and 45 g casein/kg diet groups were variable, with the 80 g casein/kg diet group consuming either the same or more than the controls, and the 45 g casein/kg diet group consuming less or more than the controls. Body-weight and skeletal-muscle growth rates varied with the protein but not energy intakes, which in turn reflected both dietary composition and the food intake, with the hyperphagic 80 g casein/kg diet group of rats growing almost normally and the 0 g caseinlkg diet group losing body-weight and muscle mass. 4. Changes in rates of muscle growth were accompanied by parallel changes in rates of protein synthesis and degradation, as well as parallel changes in concentrations of plasma insulin and free T,, to the extent that all these variables were highly correlated with each other.5. Partial correlation analysis was used to separate interactions between variables. This indicated that dietary energy had no identifiable influence on muscle growth. In contrast dietary protein appeared to stimulate muscle growth directly by increasing muscle RNA content and inhibiting proteolysis, as well as increasing insulin and free T, levels. Insulin and free T, stimulated each other as well as muscle protein turnover; insulin stimulating the RNA activity particularly at low insulin levels, free T, stimulating the RNA content and both hormones stimulating proteolysis.6 . These apparent relations are shown to be consistent in the main part with previous studies of the mechanism of action of insulin and T,, but the possibility cannot be discounted that other anabolic hormones not measured in these studies are involved, particularly in the apparent direct influence of dietary protein on muscle.

The nutritional value of plant-based diets in relation to human amino acid and protein requirements

“…Protein turnover in rats varies markedly in response to the adequacy of protein intake (Waterlow et al 1978;Jepson et al 1988). However, careful studies in human subjects indicate that protein turnover is in fact insensitive to protein or amino acid intake over a wide range (Marchini et al 1993;Zello et al 1993).…”

Maintenance protein requirements: the need for conceptual re-evaluation

“…9 the anabolic drive can be conceived in terms of exerting homeostatic and homeorhetic influences as well as exerting some functional regulation. Dietary protein is well known to regulate the growth of skeletal muscle (Jepson et al 1988) and in children growth in height may be another indicator of homeorhetic control by protein, at least according to the evidence assembled by Golden (1985).…”

Comparison of the Effects of Ornithine and Arginine on the Brain Protein Synthesis Rate in Young Rats

“…Several studies indicated that RNA activity represented the changes in the translational phase of tissue protein synthesis (26). Little information on the mechanism by which dietary ornithine or arginine regulates RNA activity in the brain of young rats is available.…”