Impact of Misclassification of Obesity by Body Mass Index on Mortality in Patients With CKD

Unlike the general population, a higher body mass index (BMI) is associated with greater survival among patients with chronic kidney disease (CKD). This “obesity paradox” may be due to limitations of BMI as a measure of adiposity in CKD. Both BMI and body fat percentage (BF%) are used to classify obesity, but outcomes may vary. Therefore, we investigated the 2 different cutoffs for diagnosing obesity (BMI ≥28 kg/m2 or BF% >25% for men and >35% for women) and the impact on all-cause mortality in CKD. A total of 326 patients with non–dialysis-dependent CKD were prospectively followed for a median of 4.9 years (range 2.9–5.3). BF% and lean body mass were determined using the Body Composition Monitor, a novel multifrequency bioimpedance spectroscopy device. Covariates included age, gender, diabetes, cardiovascular disease, estimated glomerular filtration rate, proteinuria, and high-sensitivity C-reactive protein. Per the BMI definition, 27.9% of patients were obese. However, 48.8% of patients were obese according to the BF% definition. A BMI ≥28 kg/m2 had a moderately high specificity of 83.2% but a low sensitivity of 39.6% for detecting BF%-defined obesity. In the fully adjusted models containing both BMI and BF%, obesity defined by BMI was associated with a significantly lower risk of death (hazard ratio [HR]: 0.23; 95% CI: 0.07–0.71; P = 0.011), whereas the result was reversed when obesity was defined by BF% (HR: 2.75; 95% CI: 1.28–5.89; P = 0.009). When patients were classified into 4 distinct groups based on both the BMI and BF% cutoffs for obesity, a considerable proportion of patients (29.4%) had excess body fat in the context of a normal BMI. These patients were more likely to have lower lean body mass (i.e., sarcopenic obesity) and had higher mortality compared with patients with obesity defined by both BMI and BF% (HR: 5.11; 95% CI: 1.43–18.26; P = 0.012). Diagnostic discordance between BMI and BF% may partly explain the obesity paradox. Proper diagnosis of obesity in patients with CKD is required for both risk prediction and treatment.