CONTEXT: An impaired muscle protein synthetic response to feeding likely contributes to muscle loss with aging. There are few data available on the effect of the macronutrient composition of clinical supplements on the postprandial muscle protein synthetic response in older subjects. OBJECTIVE: To determine the impact of the macronutrient composition of a nutritional supplement on the postprandial muscle protein synthetic response in older men. METHODS: A total of 45 non-sarcopenic older men (age: 69+/-1 y; BMI: 25.7+/-0.3 kg/m2) were randomly assigned to ingest 21 g of leucine-enriched whey protein with carbohydrate (9 g) and fat (3 g) (Pro-En), an isonitrogenous amount of 21 g of leucine-enriched whey protein without carbohydrate and fat (Pro), or an isocaloric mixture (628 kJ) containing carbohydrate and fat only (En). Stable isotope tracer methodology was applied to assess basal as well as postprandial muscle protein synthesis rates in the three groups. RESULTS: Ingestion of protein in the Pro-En and Pro groups significantly increased muscle protein synthesis rates when compared with basal rates (from 0.032+/-0.003 to 0.053+/-0.004 and 0.040+/-0.003 to 0.049+/-0.003 %/h, respectively; P<0.05), whereas ingestion of carbohydrate and fat did not increase muscle protein synthesis rates in the En group (from 0.039+/-0.004 to 0.040+/-0.003 %/h; P=0.60). Despite the greater postprandial rise in circulating insulin concentration in the Pro-En group, no significant differences were observed in postprandial muscle protein synthesis rates between the Pro-En and Pro groups (P=0.32). Postprandial muscle protein synthesis rates were higher in the Pro-En vs En group (P=0.01). CONCLUSION: The ingestion of a nutritional supplement containing 21 g of leucine-enriched whey protein significantly raises muscle protein synthesis rates in non-sarcopenic older men, but co-ingestion of carbohydrate and fat does not modulate the postprandial muscle protein synthetic response to protein ingestion in older men.