The day-to-day reliability of peak fat oxidation and FATMAX

Abstract

Purpose Prior studies exploring the reliability of peak fat oxidation (PFO) and the intensity that elicits PFO (FATMAX) are often limited by small samples. This study characterised the reliability of PFO and FATMAX in a large cohort of healthy men and women. Methods Ninety-nine adults [49 women; age: 35 (11) years; $$\dot{V}$$ V ˙ O2peak: 42.2 (10.3) mL·kg BM−1·min−1; mean (SD)] completed two identical exercise tests (7–28 days apart) to determine PFO (g·min−1) and FATMAX (%$$\dot{V}$$ V ˙ O2peak) by indirect calorimetry. Systematic bias and the absolute and relative reliability of PFO and FATMAX were explored in the whole sample and sub-categories of: cardiorespiratory fitness, biological sex, objectively measured physical activity levels, fat mass index (derived by dual-energy X-ray absorptiometry) and menstrual cycle status. Results No systematic bias in PFO or FATMAX was found between exercise tests in the entire sample (− 0.01 g·min−1 and 0%$$\dot{V}$$ V ˙ O2peak, respectively; p > 0.05). Absolute reliability was poor [within-subject coefficient of variation: 21% and 26%; typical errors: ± 0.06 g·min−1 and × / ÷ 1.26%$$\dot{V}$$ V ˙ O2peak; 95% limits of agreement: ± 0.17 g·min−1 and × / ÷ 1.90%$$\dot{V}$$ V ˙ O2peak, respectively), despite high (r = 0.75) and moderate (r = 0.45) relative reliability for PFO and FATMAX, respectively. These findings were consistent across all sub-groups. Conclusion Repeated assessments are required to more accurately determine PFO and FATMAX.

Contextualizing the biological relevance of standardized high‐resolution respirometry to assess mitochondrial function in permeabilized human skeletal muscle

“…There is a main effect of aerobic fitness on OCR (Kruskal-Wallis statistic ≥108.7, P < .0001), SOR (Kruskal-Wallis statistic ≥89.7, P < .0001) and APR (Kruskal-Wallis statistic ≥108.7, P < .0001). Now, TEMP FAO p (g min −1 ) appears comparable to representative and fitnessmatched measures of MFO obtained with IC [53][54][55][56] (Figure 3B). Specifically, a group of untrained young men (n = 8, 24 years and VO 2max of 48 mL kg −1 min −1 ) and endurance-trained male cross-country skiers (n = 8, 21 years and VO 2max of 71 mL kg −1 min −1 ) presented with average MFO rates of 0.32 and 0.60 g min −1 respectively.…”

“…There is a main effect of aerobic fitness on OCR (Kruskal‐Wallis statistic ≥106.6, P < .0001), SOR (Kruskal‐Wallis statistic ≥87.9, P < .0001) and APR (Kruskal‐Wallis statistic ≥106.5, P < .0001). The group mean as well as fitness‐matched measures of fat oxidation (g min −1 ) fall below respective measures of IC‐derived maximal rates of whole‐body fat oxidation (MFO) 53‐56 . Additionally, all but one estimated APR are lower than the purported maximal rate of ATP production derived from FAO, 0.30 mmol kg −1 s −1 57,58 .…”

“…As per the third study aim, correcting respiratory measures to reflect approximate mitochondrial temperatures 10°C above skeletal muscle temperature at maximal exercise efforts, ~50°C, 82 transforms standardized HRR-derived values to those that closely resemble certain corresponding in vivo measures. Temperature-corrected FAO p ( TEMP FAO p ) SOR compare favourably to fitness-matched maximal rates fat oxidation (MFO; g min −1 ), as assessed with IC methodologies [53][54][55][56] (Figure 3B) and the collective group APR mean (Table S1) corresponds to traditionally reported rates of fat-specific ATP production (Figure 3C). 57,58 Temperaturecorrected OXPHOS p ( TEMP OXPHOS p ) values are statistically comparable to fitness-matched measures determined from a-vO 2 diff during maximal efforts of normoxic one-legged knee extension exercise (KE MAX ; Figures 3D and 4A-C,F, & Table S2).…”

“…The group mean as well as fitness-matched measures of fat oxidation (g min −1 ) fall below respective measures of IC-derived maximal rates of whole-body fat oxidation (MFO). [53][54][55][56] Additionally, all but one estimated APR are lower than the purported maximal rate of ATP production derived from FAO, 0.30 mmol kg −1 s −1 . 57,58 The slope of paired in vivo (IC) and ex vivo (HRR) correlates differ significantly (F = 42.6, R 2 = 0.29, P < .0001) from a perfect relationship (r = 1.0; Figure 2D).…”

Resting skeletal muscle PNPLA2 (ATGL) and CPT1B are associated with peak fat oxidation rates in men and women but do not explain observed sex differences

“…Trials were scheduled to be completed after an overnight fast (12 ± 1 h) and at a similar time of day (between 06.30 and 12.30 h, and ±1 h within participants). Additionally, for 48 h preceding each trial, participants were asked to avoid vigorous physical activity and to replicate their diet and physical activity patterns for 48 h. Chrzanowski-Smith et al (2020, 2021.…”