Decline in muscle mass, protein synthesis and mitochondrial function occur with age and amino acids are reported to enhance both muscle protein synthesis and mitochondrial function. It is unclear whether increasing dietary protein intake corrects postabsorptive muscle changes in aging. We determined whether a 10-day diet of high (HP, 3.0 g protein/kg FFM*d) versus usual protein intake (UP, 1.5 g protein/kg FFM*d) favourably affects mitochondrial function, protein metabolism and nitrogen balance or adversely affects insulin sensitivity and glomerular filtration rate (GFR) in 10 healthy younger (24 ± 1 years) and 9 older (70 ± 2 years) participants in a randomized crossover study. Net daily nitrogen balance increased equally in young and older but postabsorptive catabolic state also increased as indicated by higher whole body protein turnover and leucine oxidation with no change in protein synthesis. Maximal muscle mitochondrial ATP production rate was lower in older people, with no change occurred with diet. GFR was lower in older people and response to HP was significantly different between the two groups with a significant increase occurring only in younger people thus, widening the differences in GFR between the young and older. In conclusion a short-term high protein diet increased net daily nitrogen balance but increased the postabsorptive use of protein as a fuel. HP did not enhance protein synthesis or muscle mitochondrial function in either young or older. Additionally, widening differences in GFR between young and older is potential cause of concern in using HP diet in older people.