Author(s): TARRY, E., PRODANOV, T.1, DAO, G.1, MCEWEN, K.A.1, KOWALSKI, G.M.1, HOWLETT, K.F.1, HELGE, J.W.2, SHAW, C.S.1, Institution: UNIVERSITY OF COPENHAGEN, Country: DENMARK, Abstract-ID: 1868

INTRODUCTION:
Peak fat oxidation (PFO) rates derived from standardised incremental exercise (1) are frequently extrapolated to competition settings, despite carbohydrate feeding strategies that suppress fat oxidation. Moreover, variability in PFO rates within relatively homogeneous groups remains largely unexplained (2). To date, the relationship between PFO and substrate utilisation during prolonged steady-state exercise is unclear. This study examined whether PFO predicts substrate utilisation during prolonged steady-state exercise in fed and overnight-fasted states, hypothesising greater fat oxidation in individuals with higher versus lower PFO.
METHODS:
20 male cyclists (29 ± 6 yrs, BMI 23.0 ± 1.8, VO2peak 56.8 ± 5.7 ml/kg/min) underwent incremental exercise testing to determine PFO and maximal oxygen uptake. On separate visits, they completed 3h cycling at 60%VO2peak in both a fed and overnight fasted state. Blood samples were collected at baseline, with subsequent blood samples and substrate utilisation via indirect calorimetry taken throughout exercise. During the fed trial, participants consumed an 8% CHO-electrolyte solution every 20 min (70 g/hr), whereas an electrolyte-only drink was provided during the Fasted trial. Participants were separated into Low PFO (0.38 ± 0.06 g/min) and High PFO groups (0.57 ± 0.08 g/min) based on median PFO values. Two-way repeated measures ANOVA assessed group differences, and correlations between substrate use and PFO were examined using simple linear regression.
RESULTS:
PFO ranged from (0.26 to 0.70 g/min) across both groups (n=20). Fat oxidation was higher in the Fasted versus Fed trial (p<0.001), with concomitant increases in glycerol, NEFA, leucine and ketones observed (p<0.001). Lactate was higher in the Fed trial at all timepoints except T160 and T180 (p<0.05). PFO correlated with total fat oxidation in the Fed (r2=0.27, p=0.02) and Fasted trials (r2=0.46, p<0.01). Average RER was lower in the High PFO group during the first hour regardless of feeding status (p=0.02), with no differences between High PFO and Low PFO during the third hour (p=0.34). The correlation between PFO and fasted state RER was strongest at T20 (r2=0.53, p<0.001) and progressively weakened over time, becoming non-significant from T120 onward. No RER differences were observed between High and Low PFO groups in the Fed trial (p>0.05).
CONCLUSION:
As hypothesised, individuals with higher versus lower PFO exhibited greater fat oxidation during the first 1–2h of exercise, with differences diminishing as exercise duration increased. PFO explained 46% of the variance in total fat oxidation over 3h in the fasted state and 27% in the fed state. This study is the first to validate PFO across exercise durations >2.5h, showing the predictive value of PFO decreases as exercise duration ensues.

REFERENCES:
1. Achten, J. et al. Med. Sci. Sports Exerc. 34, 92-97 (2002).
2. Randell, R. K. et al. Med. Sci. Sports Exerc. 49, 133–140 (2017).