CASEIN AND SOYBEAN PROTEIN HAVE DIFFERENT EFFECTS ON WHOLE-BODY PROTEIN-TURNOVER AT THE SAME NITROGEN-BALANCE

The present study examined whether different proteins have different e ffects on whole-body protein turnover in adult rats. The rats were eit her starved, given a protein-free but energy-sufficient diet (1 MJ/kg body weight (BW) per d) or a diet containing intact casein, hydrolysed casein, or hydrolysed soya-bean protein at a level of 9.1 g/kg BW per d. The diets, which were isoenergetic with the same carbohydrate:fat ratio, were given as a continuous intragastric infusion for at least 4 d. During the last 19 h N-15-glycine (a primed continuous infusion) w as given intragastrically and N-15 was recovered from urinary ammonia and urea during isotope steady state for measurement of protein synthe sis and protein degradation. Compared with starvation the protein-free diet decreased N excretion by 75%, probably by increasing the rate of reutilization of amino acids from endogenous proteins for protein syn thesis. The protein diets produced a positive N balance which was inde pendent of the protein source. Intact and hydrolysed casein increased protein synthesis 2.6- and 2.0-fold respectively, compared with the pr otein-free diet. Protein degradation increased 1.4- and 1.2-fold respe ctively. Hydrolysed soya-bean protein did not increase protein synthes is but decreased protein degradation by 35 % compared with the protein -free diet. Compared with the hydrolysed soya-bean protein, intact cas ein resulted in 2.2- and 2.8-fold higher rates of protein synthesis an d degradation respectively. These results are not easily explained by known sources of misinterpretation associated with the N-15-glycine me thod. Hydrolysed casein and hydrolysed soya-bean protein produced simi lar concentrations of insulin-like growth factor-1, insulin, glucagon, and corticosterone. The difference in amino acid composition between the dietary proteins was reflected in plasma amino acid composition an d this is suggested to be responsible for the different effect on prot ein turnover. Preliminary results from this study have previously been published in abstract form (Nielsen et al. 1991).