Background:
Obesity is an established risk factor for hypertension and end stage kidney disease. There is little information on how obesity relates to risk of impaired renal function in apparently healthy individuals. We investigated associations of body mass index (BMI) with timed 24-hour creatinine clearance (Ccr) and glomerular filtration rate (GFR) in the cross-sectional population-based INTERLIPID Study, ancillary study of the International Study on Macro- and Micro-Nutrients and Blood Pressure (INTERMAP).
Methods:
INTERLIPID participants ages 40-59 y from 5 INTERMAP research samples (4 Japanese and 1 Hawaiian) were investigated. Participants were ethnically all Japanese. Two timed 24-hour urine specimens were collected by each participant and analyses were performed in a central laboratory. Values of serum creatinine (Scr) were assayed by Jaffe’s method in a Japanese central laboratory. Data were analyzed from 1,338 Japanese, 570 men and 563 women in Japan, 100 men and 105 women in Hawaii. Measures of kidney function were 24-hour Ccr (ml/min) and GFR (ml/min/1.73m
2
). Ccr was calculated by equation (urine creatinine х urine volume)/(Scr х 1440); GFR was calculated as Ccr х (1.73/body surface area (BSA)). BSA was estimated by the DuBois formula. In addition, estimated GFR (eGFR) was calculated by use of the Modification of Diet in Renal Disease (MDRD) Study equation, 186 х Scr
-1.154
х age
-0.203
х 0.742 (if female). All statistical analyses were stratified by gender. Adjusted mean values of Ccr, GFR, and eGFR in quartiles of BMI were estimated using analysis of covariance. Model 1 was adjusted for age; model 2, for age and urine volume; model 3 for variables in model 2 plus blood pressure, pulse rate, physical activity, smoking habit, alcohol consumption, urinary sodium excretion, HbA1c, and total serum cholesterol.
Results:
Ccr (mean (SD)) were 107.5 (18.9) in men in Japan, 91.9 (17.0) in women in Japan, 123.4 (25.1) in men in Hawaii, and 98.5 (20.3) in women in Hawaii. BMI quartile ranges were in men Q1 16.8-22.3, Q2 22.3-24.1, Q3 24.1-26.1, and Q4 26.1-42.9; in women Q1 15.9-21.2, Q2 21.2-23.0, Q3 23.0-25.1, and Q4 25.1-47.0. In men, BMI was positively related to multivariate-adjusted Ccr and GFR (
P
for trend across BMI quartiles <0.001 and <0.001, respectively), however, multivariate-adjusted eGFR was inversely associated with BMI (
P
for trend 0.001). In women, BMI was positively related to multivariate-adjusted Ccr (
P
for trend across BMI quartiles <0.001); multivariate-adjusted GFR and eGFR were not significantly associated with BMI. Interactions between age and BMI were not significant in men or women.
Conclusions:
These results indicate that excess BMI may lead to higher Ccr, and that the MDRD equation may underestimate GFR in middle-aged overweight or obese Japanese men.
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