Age-related changes in body composition, hydroxyproline, and creatinine excretion in normal women

We have made a cross-sectional study of relationships among age, whole body bone mineral content (WBBMC), and non-bone lean body mass (NBLBM) measured by dual energy X-ray absorptiometry (DXA), and daily excretion of hydroxyproline (OHP) and creatinine (Cr) in a group of normal women. WBBMC fell with age from the 6th decade, whereas NBLBM was almost constant. Creatinine excretion fell with age from the 5th decade until the 9th, to a much greater degree than NBLBM, reaching a nadir in the 8th decade. Daily excretion of hydroxyproline showed a peak in the 6th decade and fell moderately thereafter. The greater fall of creatinine compared with hydroxyproline resulted in rising OHP/Cr ratios with advancing age, in contrast to the pattern of hydroxyproline excretion. The use of creatinine as a correction for urine dilution or for lean body mass (LBM) in assays for markers of bone turnover must therefore be viewed with caution.