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To The Editor: Dr Volpi and colleagues¹ found that differences in basal muscle protein turnover in older and younger men do not explain age-related sarcopenia. This conclusion contradicts our collective results, which have found age-related differences in muscle protein synthesis.^2-5 We suggest several reasons for these differences.

First, differences in subject characteristics may have introduced measurement variability in the study by Volpi et al that prohibited detection of an age-associated lower rate of muscle protein synthesis. For instance, the authors did not control physical activity or diet in the days preceding the measurement of muscle amino acid metabolism, and they enrolled physically active young and elderly men. In contrast, we excluded regular exercisers (>2d/wk), and studied subjects who received similar diets and activity on the days prior to study. Even a modest amount of exercise or differences in dietary protein content on the preceding day can alter muscle amino acid . . . [Full Text of this Article]

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