Associations Between Low Serum Urate, Body Composition, and Mortality.

Abstract

Controversy remains as to whether low serum urate or uric acid (UA) levels contribute to adverse outcomes. We evaluated the relation between low serum UA levels and sarcopenia and assessed whether sarcopenia confounds associations between these low levels and mortality. We utilized data from the National Health and Nutrition Examination Survey (1999-2006). Participants with available whole-body dual x-ray absorptiometry body composition measurements and serum UA concentrations were included. Body composition assessments included body mass index (BMI), waist circumference, maximum lifetime BMI, and age-, sex-, and race-specific appendicular lean mass index (ALMI) and fat mass index (FMI) Z scores. We also calculated Z scores for ALMI relative to FMI (ALMIFMI ). We evaluated associations between serum UA levels and body composition using logistic regression and assessed associations between serum UA levels and mortality before and after adjusting for differences in body composition using Cox proportional hazards regression. Among the 13,979 participants, low serum UA concentrations (<2.5 mg/dl in women, <3.5 mg/dl in men) were associated with low lean mass (ALMI and ALMIFMI Z scores), underweight BMI (<18.5 kg/m2 ), and higher rates of weight loss. The proportion of patients with low ALMI Z scores was 29% in the low serum UA group and 16% in the normal serum UA group (P=0.001). Low serum UA levels were associated with increased mortality before we adjusted for body composition (hazard ratio 1.61 [95% confidence interval 1.14-2.28]; P=0.008) but was attenuated and not significant after adjustment for body composition and weight loss (hazard ratio 1.30 [95% confidence interval 0.92-1.85], P=0.13). Sarcopenia and weight loss are more common among patients with low serum UA concentrations. Differences in body composition may help to explain associations between low levels of serum UA and higher mortality.