Whole body protein turnover of growing rats in response to different dietary proteins soy protein or casein

Whole body protein turnover was studied in growing rats fed restrictively o n isoenergetic (GE 17.6 MJ:kg DM) and isonitrogenous (104 g CP/kg DM) diets based on soy protein isolate or casein supplemented with D,L-methionine. D uring each of the three separate experiments six male Fischer rats per grou p were housed individually in metabolic cages at 24 degrees C. Prefeeding o f both dietary groups up to similar body wrights at the start of the main e xperimental periods (105 - 134 g) lasted up to 16 d for casein-fed rats and up to 30 d for the soy protein-fed rats. Following the energy and nitrogen balance periods whole-body protein synthesis was estimated by the end-prod uct method using a single tracer dose of a mixture of N-15-labelled amino a cids. Fractional protein accretion rate [% of the protein pool accreted pel day] was significantly lower in soy protein-rats than in casein-fed rats i n all three experiments whereas fractional synthesis rate was not significa ntly lower. Therefore, protein breakdown subsequently calculated as the dif ference between synthesis and accretion showed a tendency towards higher va lues in this group. In soy protein-fed rats also a tendency towards higher excretion of 3-methylhistidine as a marker of myofibrillar protein breakdow n was observed. It is concluded that increase in lean tissue growth resulti ng From improved protein quality is brought about by changes of both rates, by small increase of protein synthesis and by reduced rate of body protein breakdown.