Muscle mass is known to decline with age, but only limited longitudinal data exist to quantify the rate of loss. Using 24-hour urine creatinine, corrected for the contribution of dietary meat intake, we assessed the change with age prospectively in 107 women who provided a minimum of 3 sets of measurements spanning an average interval of 11.9 years, centered around age 55 years. The rate of change in 24-hour urine creatinine at that age averaged -0.94%/yr (95% confidence interval [CI], -1.24% to -0.64%/yr; P <.001). Change in creatinine excretion was directly correlated with change in weight (P <.01), with those gaining weight tending to gain both lean and fat mass. The rate of change in creatinine excretion with age in our subjects is similar to that described in published cross-sectional studies on age-related change in total body potassium and in longitudinal studies using (40)K and dual-energy xray absorptiometry. Our study is the largest longitudinal study reported to date and provides, perhaps, a more secure basis for estimating muscle mass change with age than has been available heretofore.
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