Amino acid supplementation prevents the loss of appetite for casein in old Lou/Cjall rats

A deleterious reduction of casein intake occurring earlier in males than in females had been previously observed in old Lou/Cjall rats. On the contrary, protein intake was observed to be maintained in old males when they were offered whey protein. Present studies were designed to investigate the effect of dietary casein modification on protein decrease. In two lifelong studies, male and female Lou/Cjall rats were tested every four months in order to study protein intake depending on the protein available: casein, whey protein or casein supplemented with an amino acid mixture (SC). In subsequent cross-sectional studies, young, adult, middle-aged and old rats were successively fed with casein, casein supplemented either with leucine or with alanine or with glycine. Supplementing casein with an amino acid mixture both globally increased protein intake and allowed old males to maintain a high rate of protein intake. In cross-sectional experiments, no effect of supplementation was seen in the young group. In older animals, the greatest effect was seen when casein was supplemented with alanine or glycine, independently of sex and age. We therefore, concluded that supplemented casein is more beneficial for old rats than casein alone, probably by increasing amino acid availability. We hypothesize that alanine could act through its effect on gluconeogenesis.