Association between extracellular-to-total body water ratio and eGFR decline in diabetic kidney disease: a longitudinal cohort study

Background Diabetic kidney disease (DKD) can progress even in the absence of overt albuminuria, highlighting the need for markers for evaluating nonglomerular pathways. The extracellular-to-total body water ratio (ECW/TBW), measured via bioelectrical impedance analysis (BIA), reflects systemic fluid status; however, its longitudinal relevance to DKD progression remains unclear to date. Methods We conducted a retrospective observational cohort study using longitudinal outpatient data from October 2020 to April 2025 at the Diabetic Kidney Disease Prevention Clinic, Oita University Hospital. A total of 40 adults with type 2 diabetes and clinically diagnosed DKD contributed 355 outpatient visits. The primary outcome was the annual rate of decline in estimated glomerular filtration rate (eGFR slope, mL/min/1.73 m 2 /year). Results Both baseline and time-averaged ECW/TBW values were negatively correlated with eGFR slope in the univariate analysis ( p = 0.014 and 0.025, respectively) and remained significant in the multivariate models. Log-transformed urinary albumin-to-creatinine ratio (UACR) was also significantly associated with faster eGFR decline. Treatment with sodium–glucose cotransporter 2 inhibitors (SGLT2is) was associated with lower ECW/TBW values, whereas other renoprotective medications exerted more modest individual effects, with greater reductions observed with the concurrent use of multiple agents. Conclusions Higher ECW/TBW values were associated with faster eGFR decline in DKD, suggesting that subclinical volume overload may represent a potential nonglomerular mechanism of disease progression. This volume-related burden may be attenuated by SGLT2is therapy.