Fit-Fat Index and the risk of sudden cardiac death in Finnish men

Background Emerging evidence suggests that the Fit-Fat Index (FFI), which combines measures of fitness and fatness, may offer a more accurate assessment of cardiometabolic risk than either component alone. We aimed to investigate the prospective associations of cardiorespiratory fitness (CRF), fatness indices, and FFI variants with the risk of sudden cardiac death (SCD), and to evaluate their utility in SCD risk prediction. Methods Baseline assessments of CRF (measured using a respiratory gas exchange analyzer during exercise testing) and fatness indices (body mass index (BMI), waist-to-hip ratio (WHR), and waist-to-height ratio (WHtR)) were conducted in 1662 men aged 42–61 years. FFI variants (FFI BMI , FFI WHR , and FFI WHtR ) were calculated by dividing CRF by each corresponding fatness measure. Hazard ratios (HRs) with 95 % confidence intervals (CIs) were estimated using Cox regression, and improvements in risk prediction were assessed using measures of discrimination and reclassification. Results Over a median follow-up of 28.3 years, 172 SCDs occurred. After adjustment for confounders and potential mediators, each 1 SD increase in CRF and FFI variants was associated with a significantly lower risk of SCD: HRs (95 % CI) were 0.68 (0.56–0.82) for CRF, 0.65 (0.53–0.79) for FFI BMI , 0.66 (0.54–0.81) for FFI WHR , and 0.65 (0.53–0.80) for FFI WHtR . BMI and WHtR, but not WHR, remained associated with SCD after full adjustment. CRF and all FFI variants significantly improved model fit, net reclassification, and discrimination ( p < .001 for all). Conclusions CRF and FFI variants were robustly associated with lower SCD risk and offered comparable improvements in prediction. These findings support their potential utility in clinical and research settings for SCD risk stratification.