MUSCLE MASS AND ESTIMATED RENAL FUNCTION IN ATHLETES

Abstract

Objective: Serum creatinine level is the most common marker used to assess renal function. Because creatinine is a derivative of creatine phosphate, an energy source present in the skeletal muscles, serum creatinine levels are influenced by muscle mass. Elderly people with less muscle mass have low creatinine levels, which may lead to an overestimation of their true renal function. Creatinine levels also tend to be lower in females than males, as females tend to have less muscle mass. Serum cystatin C, a cysteine proteinase inhibitor, is another renal function marker that is produced by every nucleated cell in the body, and thus, unlike creatinine, is thought to be little influenced by muscle mass. Cystatin C level is a sensitive marker of reduction in glomerular filtration rate and suggested to be a better predictive factor for cardiovascular events and death. Design and method: Study subjects were 130 male athletes belonging to the National Institute of Fitness and Sports in Kanoya, Japan. A Smedley-type Hand Dynamometer was used to assess grip strength. Serum creatinine and cystatin C levels were measured by enzyme and latex coagulation methods, respectively. Results: Muscle mass was significantly correlated with both right and left grip strength, indicating that muscle mass measured in the present study represents skeletal muscle mass. Serum creatinine levels and estimated glomerular filtration rates calculated from creatinine (eGFRcre) showed significant correlations with muscle mass, suggesting that creatinine is influenced by muscle mass and that true renal function may be underestimated in athletes. Cystatin C and estimated glomerular filtration rates calculated from cystatin C (eGFRcys) were also significantly correlated with muscle mass. Serum creatinine levels were significantly correlated with serum cystatin C levels. Consistent with this, eGFRcre was significantly correlated with eGFRcys. This suggests that muscle mass influences the assessment of renal function in athletes when cystatin C is used as a marker. Conclusions: The findings in this study suggest that muscle mass of the subject must be considered prior to assessment of renal function using cystatin C in athletes.