Moderate adiposity levels counteract protein metabolism modifications associated with aging in rats

Purpose Physiological parameters such as adiposity and age are likely to influence protein digestion and utilization. The aim of this study was to evaluate the combined effects of age and adiposity on casein protein and amino acid true digestibility and its postprandial utilization in rats. Methods Four groups were included ( n = 7/8): 2 months/normal adiposity, 2 months/high adiposity, 11 months/normal adiposity and 11 months/high adiposity. Rats were given a calibrated meal containing 15 N-labeled casein (Ingredia, Arras, France) and were euthanized 6 h later. Digestive contents were collected to assess protein and amino acid digestibilities. 15 N enrichments were measured in plasma and urine to determine total body deamination. Fractional protein synthesis rate (FSR) was determined in different organs using a flooding dose of 13 C valine. Results Nitrogen and amino acid true digestibility of casein was around 95–96% depending on the group and was increased by 1% in high adiposity rats ( P = 0.04). Higher adiposity levels counteracted the increase in total body deamination ( P = 0.03) that was associated with older age. Significant effects of age ( P = 0.006) and adiposity ( P = 0.002) were observed in the muscle FSR, with age decreasing it and adiposity increasing it. Conclusion This study revealed that a higher level of adiposity resulted in a slight increase in protein and individual amino acid true digestibility values and seemed to compensate for the metabolic postprandial protein alterations observed at older age.