Independent and combined associations of body mass index and visceral fat area with kidney function decline in a healthy Japanese urban population: a longitudinal study

BACKGROUND: Obesity has been linked to progressive loss of kidney function, but the independent and combined associations of body mass index (BMI) and visceral fat accumulation remain uncertain in healthy individuals. This study aimed to clarify the longitudinal relationship between BMI, visceral fat area (VFA), and chronic kidney disease (CKD), defined as an estimated glomerular filtration rate based on serum cystatin C (eGFRcys) < 60 mL/min/1.73 m² among the Kobe Study participants targeting healthy urban residents in Japan. METHODS: A total of 897 participants (267 men, 630 women) without CKD and with a mean age of 61.4 years were analyzed. VFA was measured using the impedance method, and participants were followed for a median of 8.3 years. Participants were classified into four groups based on baseline BMI (kg/m²) and VFA (cm²): G1 (VFA < 100 and BMI < 25), G2 (VFA < 100 and BMI ≥ 25), G3 (VFA ≥ 100 and BMI < 25), G4 (VFA ≥ 100 and BMI ≥ 25). Multivariable linear regression models were used to estimate adjusted mean annual changes in eGFRcys, and Cox proportional hazards models were applied to calculate hazard ratios (HRs) for incident CKD, adjusting for potential confounders (age, sex, smoking status, alcohol consumption, dyslipidemia, diabetes, hypertension and eGFRcys at baseline). RESULTS: During follow-up, 85 participants (9.5%) developed CKD. The adjusted mean annual decline in eGFRcys was − 2.17% in G1, − 3.29% in G2, − 2.81% in G3, and − 3.81% in G4. Compared with G1, both G3 and G4 showed an elevated risks of CKD incidence (G3: HR 2.40, 95% CI 1.14–5.05; G4: HR 3.89, 95% CI 2.17–6.98), whereas the confidence interval crossed 1 for G2 (HR 0.88, 95% CI 0.12–6.47). Participants in G4 exhibited the steepest decline in kidney function and the highest CKD incidence rate (49.2 per 1,000 person-years). CONCLUSIONS: In healthy adults, the combination of elevated BMI and VFA was associated with accelerated kidney function decline. These findings underscore the importance of evaluating visceral fat in addition to BMI for early identification of individuals at increased risk of CKD and enable preventive strategies. CLINICAL TRIAL NUMBER: Not applicable.