Rest and Maximal Fat Oxidation: Independent and combined role of body composition, cardiorespiratory fitness and strength in adults

Resumen

INTRODUCTION:Fat oxidation capacity might be modulated positively with physical fitness improvements, which is related to health benefits. However, itremains unclear what physical fitness components (Body Composition (BC), Cardiorespiratory fitness (CRF) or Muscular Strength (MS)) havea higher influence on fat oxidation capacity at rest (RFO) and during exercise (MFO). The aim of this study was to analyse the independentand combined effect of BC, CRF and MS on RFO and MFO.METHODS:One hundred adults (66 males; age 24.8±7.9y) were included. Gas exchange was recorded for 30min at rest for RFO. An incremental exercise protocol in cycle ergometer with two consecutive phases was performed for MFO and VO2max using indirect calorimetry. The firstphase aimed to determine MFO, it consisted of 3 min steps of 15/30W (depending on weight status) increments with a cadence of 60-80rpm. The test was stopped when RQ ≥1. After 3-5 min rest, the second phase to detect VO2max began with steps of 1 min and the sameincremental load until exhaustion. Bioimpedance analysis was used to assess BC (body fat index (BFI) and fat-free mass index (FFMI)). MSwas assessed by handgrip dynamometry (HG) and the standing long jump test (LJ). A strength cluster (SC) was made with HG and LG. T-testwas used to assess differences between males and females (p<0.05). Linear regression analyses were performed between RFO and MFOwith BC, CRF, MS variables. To test the combined influences of CRF with BC and MS on RFO and MFO, groups of high/low were createdusing p50 of normative values adjusted by sex. The differences were assessed by ANOVA (p<0.05).RESULTS:Males have higher CRF, FFMI, MS and females have higher BFI, RFO/FFM and MFO/FFM (p<0.05). CRF and HG levels were correlated withRFO (r=0.255 p=0.01 and r=0.253 p=0.027; respectively). Likewise, BFI, CRF and SC were correlated with MFO (r=0.311 p=0.002; r=0.59p<0.001 and r=0.280 p=0.015; respectively). The best regression model for the prediction of RFO included BFI and CRF (r2=0.107) and forMFO, Sex(men) and CRF (r2=0.487). Differences in RFO/FFM, MFO and MFO/FFM were observed between groups combining low and highCRF/BFI and CRF/FFMI, in which high CRF groups presented higher fat oxidation rates regardless of their body composition status. Similarly,there were differences in MFO/FFM between combined groups of HighCRF with Low or High HG.CONCLUSION:MFO is mainly affected by CRF and MS and to a lesser extent by FFMI and BFI. RFO is poorly influenced by physical fitness. Optimal strategies for increasing fat oxidation rates should include exercise intervention in order to increase CRF and MS jointly.