Influence of Muscle Mass and Physical Activity on Serum and Urinary Creatinine and Serum Cystatin C

Abstract

Background and objectives: For addressing the influence of muscle mass on serum and urinary creatinine and serum cystatin C, body composition was assessed by skinfold thickness measurement and bioelectrical impedance analyses.Design, setting, participants, & measurements: A total of 170 healthy individuals (92 women, 78 men) were classified as sedentary or with mild or moderate/intense physical activity. Blood, 24-h urine samples, and 24-h food recall were obtained from all individuals.Results: Serum and urinary creatinine correlated significantly with body weight, but the level of correlation with lean mass was even greater. There was no significant correlation between body weight and lean mass with cystatin C. Individuals with moderate/intense physical activity presented significantly lower mean body mass index (23.1 ؎ 2.5 versus 25.7 ؎ 3.9 kg/m 2 ) and higher lean mass (55. A multivariate analysis of covariance showed that lean mass was significantly related to serum and urinary creatinine but not with cystatin, even after adjustment for protein/meat intake and physical activity.3Conclusions: Cystatin C may represent a more adequate alternative to assess renal function in individuals with higher muscle mass when mild kidney impairment is suspected.Clin J Am Soc Nephrol 3: 348-354, 2008348-354, . doi: 10.2215 A ccurate renal function measurements are important in the diagnosis and treatment of kidney diseases, adjustment of drug dosages, and decision-making regarding when to initiate renal replacement therapy. Serum creatinine is the most commonly used indicator of renal function, but its measurement suffers from a variety of analytical interferences and significant standardization problems (1,2).Serum creatinine can be affected by age, gender, ethnicity, dietary protein intake, and lean mass and may remain within the reference range despite marked renal impairment in patients with low muscle mass. Consequently, the sensitivity of serum creatinine for the early detection of kidney disease is poor and not a good predictor when analyzing the elderly (3,4). Conversely, theoretically, serum creatinine may be falsely increased in individuals with higher muscle mass and normal renal function.The GFR represents the best overall assessment of kidney function, but the gold standard techniques for the measurement of GFR, such as inulin clearance, [125 I]iothalamate, 51 Cr-EDTA, 99m Tc-diethylenetriaminepentaacetic acid, and iohexol are too labor-intensive and costly for routine clinical use (5,6), so creatinine clearance is used instead.To rid the need of 24-h urine collections, several serum creatinine-based prediction formulas have been proposed to predict GFR (7-16). The equations of Cockcroft and Gault (7,8) and the one derived from the Modification of Diet in Renal Disease (MDRD) study (10) are the most widely accepted; however, the competence of such formulas to predict GFR in patients with normal values of serum creatinine is debated.Despite the important influence of muscle mass on serum creatinine, the differ...

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