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

Abstract

Whole body protein turnover was studied in growing rats fed restrictively on isoenergetic (GE 17.6MJ/kg DM) and isonitrogenous (104g CP/kg DM) diets based on soy protein isolate or casein supplemented with D,L‐methionine. During each of the three separate experiments six male Fischer rats per group were housed individually in metabolic cages at 24°C. Prefeeding of both dietary groups up to similar body weights at the start of the main experimental 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‐product method using a single tracer dose of a mixture of ¹⁵N‐labelled amino acids. Fractional protein accretion rate [% of the protein pool accreted per day] was significantly lower in soy protein‐rats than in casein‐fed rats in all three experiments whereas fractional synthesis rate was not significantly lower. Therefore, protein breakdown subsequently calculated as the difference between synthesis and accretion showed a tendency towards higher values in this group. In soy protein‐fed rats also a tendency towards higher excretion of 3‐methylhistidine as a marker of myofibrillar protein breakdown was observed. It is concluded that increase in lean tissue growth resulting 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.

Key words:

• Casein
• Soy protein
• Protein synthesis
• Nitrogen 15
• IGF‐I
• Rat