During ageing, skeletal muscle develops anabolic resistance towards the stimulation of protein synthesis induced by dietary amino acids. The stimulation of muscle protein synthesis after food intake remains insufficient, even with a protein intake recommended for healthy adults. This alteration is one of the mechanisms known to be responsible for the decrease of muscle mass and function during ageing, namely sarcopenia. Increasing dietary protein intake above the current RDA(0⋅83 g/kg/d) has been strongly suggested to overcome the anabolic resistance observed. It is also specified that the dietary protein ingested should be of good quality. A protein of good quality is a protein whose amino acid (AA) composition covers the requirement of each AA when ingested at the RDA. However, the biological value of proteins may vary among dietary sources in which AA composition could be unbalanced. In the present review, we suggest that the quality of a dietary protein is also related to several other determinants. These determinants include the speed of digestion of dietary proteins, the presence of specific AA, the food matrix in which the dietary proteins are included, the processes involved in the production of food products (milk gelation and cooking temperature), the energy supply and its nature, and the interaction between nutrients before ingestion. Particular attention is given to plant proteins for nutrition of the elderly. Finally, the timing of protein intake and its association with the desynchronized intake of energetic nutrients are discussed.

The mink is a seasonal breeder with a propensity for seasonal fatness, and it is very responsive to changes in energy supply. The objectives of the present study were first, to validate a multi-species leptin assay for mink (Mustela vison) plasma, and second, to evaluate how plasma leptin and insulin concentrations responded to energy restriction and body-weight loss and refeeding with restoration of body reserves. The study was performed with six very fat yearling females (initial mean body weight 1451 (SD 119) g, i.e. approximately 300 g more than for a female in normal body condition). The animals were fed in restricted amounts (about 35 % metabolizable energy requirement for maintenance) in order to reach a very lean body condition. The target weight of 800 g was reached after about 1 month of restriction. The animals were then refed ad libitum until almost complete weight recovery. Blood samples were taken on days 1, 24, 34 (end of restriction), 44, 55 and 71 (end of experiment) and analysed for plasma concentrations of leptin and insulin. Three females were mated on day 44. Leptin and insulin concentrations mirrored each other and clearly reflected changes in body weight. Significant (P<0·001) Pearson correlation coefficients of 0·75 (leptin–insulin), 0·72 (leptin–body weight) and 0·59 (insulin–body weight) were found. Two of the three females that were mated gave birth to normal litters. It was concluded that the leptin assay yielded acceptable results for animals with body weight:fat content within the range investigated here, and that plasma leptin reflected body fat mass.