Several studies have examined the acute and chronic effects of protein ingestion/supplementation on muscle protein anabolism in elders and the results have been well documented [4
]. The majority of studies suggest that a moderate-to-large serving of protein or amino acids increases muscle protein synthesis similarly in both young and elderly [6
]. Conversely, one recent study with a nice mix of signaling and metabolic measures but limited cohort (n
= 4 per group) reported that 10 and 20g servings of essential amino acids were unable to stimulate muscle protein synthesis in older individuals to the same level as noted in the young [2
Historically, the majority of protein metabolism studies have focused on the anabolic response to ingestion of protein or amino acids alone. However, when protein and carbohydrate are coingested, elderly respond with a diminished anabolic response compared with their younger counterparts. Volpi et al.
] demonstrated that after ingestion of an amino acid glucose mixture, muscle protein synthesis increased in the young, but remained unchanged in the elderly. A subsequent study confirmed these results [19
]. Although there is no evidence that coingestion of protein and fat negatively or differentially effects protein anabolism in young or elderly [9••
], it appears that ageing may be associated with reduced anabolic efficiency in response to a carbohydrate-containing mixed nutrient meal. Fortunately, a modest bout of physical activity may be able to sensitize ageing muscle to subsequent nutritional stimuli. We have recently shown that 45min of treadmill walking in older individuals, the evening prior to an hyperinsulinemic–euglycemic clamp, restores the ability of insulin to stimulate muscle protein synthesis [4
]. More recently, we demonstrated that ingestion of leucine-enriched essential amino acids stimulates muscle protein synthesis to a similar extent in young and elderly (although the response was delayed in the elderly) during the first 6 hours following exercise [21
]. These data provide further supporting evidence that regular physical activity plays an important role in restoring or maintaining the normal protein anabolic response in the skeletal muscle of older individuals.
As mentioned, numerous studies support the notion that ingestion of a sufficient amount of amino acids or protein results in a similar increase in muscle protein synthesis in young and elderly [7
]. However, several recent studies have adopted a more practical approach and sought to examine the ability of protein-rich foods (e.g., milk) to stimulate protein anabolism. Although many of these studies have focused on a younger population [23
], they are important as they more closely reflect responses to actual dietary practices and hence provide information on how meal choices may influence accrual of muscle mass and ultimately functional capacity. In one study directly comparing young and elderly, Symons et al.
] reported that a moderate 113g serving of an intact protein (that is, lean beef) contains sufficient amino acids (30g total; ~12g essential amino acids) to increase mixed muscle protein synthesis by 50% in both young and elderly men and women.