Fit-Fat Index is better associated with heart rate variability compared to fitness and fatness alone as indicators of cardiometabolic human health.

Objectives: Cardiorespiratory fitness and fatness indicators have been related to heart rate variability (HRV) parameters. The Fit-Fat Index (FFI) is a single index combining cardiorespiratory fitness and fatness indicators. To the best of our knowledge, no studies have previously analyzed whether FFI are related to cardiac autonomic nervous system activity assessed through HRV parameters. This study aimed (i) to examine the association of cardiorespiratory fitness, fatness indicators, and FFI with HRV parameters; and (ii) to report what of the different fatness indicators included in FFI is better associated with HRV parameters in sedentary adults.

Methods: One hundred and fifty healthy adults (74 women; 76 men), aged between 18 and 65 years old, participated in this cross-sectional study. We measured cardiorespiratory fitness (maximal oxygen consumption) and fatness indicators (waist-to-height ratio [WHR], fat mass percentage [FM%] and visceral adipose tissue [VAT]). Three FFIs were calculated as the quotient between cardiorespiratory fitness and one out of three possible fatness indicators: Fit-Fat Index calculated waist-to-height ratio (FFI ), Fit-Fat Index calculated with FM% (FFI ), and Fit-Fat Index calculated with VAT (FFI ). HRV parameters were measured in resting conditions using a Polar RS800CX.

Results: FFI , FFI and FFI were related to different HRV parameters (β ranges between -0.507 and 0.529; R ranges between 0.096 and 0.275; all p < .001) and the association was stronger with HRV parameters than the isolated fitness or fatness indicators (β ranges between -0.483 and 0.518; R ranges between 0.071 and 0.263; all p < .001). FFI was the index more consistently associated with HRV parameters (β ranges between -0.507 and 0.529; R ranges between 0.235 and 0.275; all p < .001).

Conclusion: Our study suggests that compound FFIs are better predictors of HRV parameters than either cardiorespiratory fitness or fatness indicators alone. The FFI was the best index in terms of its association to HRV.