Analysis of the Relationship between Body Mass Index and Kidney Function Decline in a Middle-Aged Japanese Population

Background: The number of dialysis patients in Japan is increasing every year. In patients with chronic kidney disease, both obesity and underweight are reported to be risks for renal function decline. On the other hand, in the general population, obesity is a risk factor, but the relationship between underweight and renal function decline has not been well studied. Therefore, we aimed to analyze the relationship between body mass index (BMI), especially underweight, and renal function decline using health checkup data from Gunma prefecture, Japan. Methods: We studied National Health Insurance beneficiaries aged 40 to 74 in Gunma who underwent health checkups in FY 2018 and 2020 and had usable estimated glomerular filtration rate (eGFR) and BMI data. We excluded those who self-reported receiving dialysis or had a BMI < 14 kg/m2 or ≥ 40 kg/m2. The outcome was defined as an eGFR decline rate of ≥ 30% over 2 years. Baseline BMI was classified into 7 categories, and odds ratios (OR) and 95% confidence intervals (95% CI) were calculated by logistic regression analysis. Results: 64,970 individuals were included in the analysis. In a model adjusted for age, sex, smoking, alcohol consumption, blood pressure, hemoglobin A1c, and lipid profile, BMI and renal function decline showed an inverse J-shaped relationship, with underweight (BMI = 14.0-18.9 kg/m2, OR [95% CI] 2.27 [1.55-3.31]) posing a higher risk of renal function decline compared to obesity (BMI = 30.0-39.9 kg/m2, OR [95% CI] 1.95 [1.24-3.09]). Similar trends were seen across subgroups by sex, age, urinary protein presence, baseline eGFR, and diabetes, especially among those with negative urinary protein and non-diabetic status. Conclusion: In the middle-aged and elderly general population, both obesity and underweight were shown to be risk factors for renal function decline.To prevent the need for dialysis, intervention is necessary for underweight groups who are not targeted for health guidance.