Reply to JD Stookey

University of Vermont, Department of Medicine, Given Building, Room C-247, Burlington, VT 05405, E-mail: epoehlma{at}zoo.uvm.edu

Dear Sir:

We appreciate the comments of Stookey. We want to reiterate the stated conclusion of our study that "maintaining physical activity and protein intake may not offset the age-related loss of appendicular skeletal muscle mass in older men" (1). Our cross-sectional data suggest that factors other than endurance activities and adequate protein intake contribute to the decline in muscle mass with aging in older men.

Our results should be viewed from a physiologic perspective. Our data showed that protein intake adjusted for body size or energy intake was not associated with appendicular skeletal muscle mass. Furthermore, age was inversely associated with appendicular skeletal muscle mass (after adjustment for appropriate confounders) when individuals with <0.80 g protein intake•kg body wt^-1•d^-1 were excluded, suggesting that preservation of muscle mass with aging may not be completely accounted for by protein intakes above the recommended dietary allowance (RDA). The findings in this article are concordant with those of previous studies that used intervention designs. For example, Fiatarone et al (2) showed that nutritional supplementation with resistance exercise had no additive effect on fat-free mass in elderly people. On a cellular level, Welle and Thorton (3) examined whether increasing the protein content of isoenergetic meals enhanced the rate of myofibrillar synthesis in muscle of older individuals after resistance exercise. They found that the protein content of meals exerted no detectable effect on myofibrillar synthesis. Protein synthesis is necessary to maintain protein mass and protein quality. Collectively, these data suggest that protein intake at or above the RDA in combination with exercise does not appear to potentiate growth of skeletal muscle mass. Our results should not be generalized to long-term changes in dietary protein intake or to individuals consuming inadequate amounts of energy. Longitudinal analyses are needed to confirm the cross-sectional data of our study.

REFERENCES

1. Starling RD, Ades PA, Poehlman ET. Physical activity, protein intake, and appendicular skeletal muscle mass in older men. Am J Clin Nutr 1999;70:91–6.
2. Fiatarone MA, O'Neill EF, Ryan ND, et al. Exercise training and nutritional supplementation for physical frailty in very elderly people. N Engl J Med 1994;330:1769–75.
3. Welle S, Thorton CA. High-protein meals do not enhance myofibrillar synthesis after resistance exercise in 62- to 75-yr-old men and women. Am J Physiol 1998;274:E677–83.