Changes In Fat And Carbohydrate Oxidation From Rest To Exercise After Different Fasting Lengths

Abstract

An overnight fast (10-12 hours) is a popular pre-exercise trial control in fuel use studies, but can lead to premature fatigue during exercise in participants. Measuring substrate oxidation at rest could be an alternative, but it is unclear how length of fasting effects fuel use responses from rest to during exercise. PURPOSE: The purpose was to examine the changes in fat (fatox) and carbohydrate (carbox) oxidation rates during rest and exercise after various fasting lengths. METHODS: Participants (24.5 ± 5.1 yrs) randomly performed 3 experimental trials. Trials were preceded by a standard meal (19.4 ± 1.8% of daily energy expenditure) followed by a fast for 12 hours (hrs), 3 hrs or 1 hr. Each trial consisted of 30 min of rest and 30 min of exercise at 55% of peak oxygen uptake (VO2peak). VO2 and carbon dioxide production (VCO2) were averaged over the final 10 minutes of rest and exercise. The equations fatox = 1.695 * VO2-1.70*VCO2 and carbox = 4.585*VCO2-3.226*VO2 were used to calculate oxidation rates (g*min-1). Two-way repeated measures (RM) ANOVAs and one-way RM ANOVAs analyzed differences. Significance was established if p<0.05. RESULTS: Participants exercised at a similar VO2 and % of VO2peak in the 12hr (1.58 ± 0.28 L·min-1; 56.8 ± 2.4 % of VO2peak), 3hr (1.60 ± 0.30 L·min-1; 57.7 ± 4.6 % of VO2peak), and 1hr (1.64 ± 0.30 L·min-1; 59.1 ± 1.7 % of VO2peak). There was no significant interactions for fatox and carbox, but there were significant main effects of time. Fatox increased from rest to exercise in the 12hr (0.30 ± 0.04 g*min-1 vs. 7.34 ± 3.00 g*min-1), 3hr (0.30 ± 0.10 g*min-1 vs. 7.56 ± 3.53 g*min-1) and 1hr (0.34 ± 0.10 g*min-1 vs. 8.43 ± 4.00 g*min-1) trials. Additionally, carbox increased from rest (12 hr = 0.30 ± 0.07 g*min-1; 3hr = 0.27 ± 0.09 g*min-1; 1hr = 0.34 ± 0.10 g*min-1) to exercise (12hr = 1.92 ± 0.55 g*min-1; 3hr = 1.90 ± 0.64 g*min-1; 1hr = 2.18 ± 0.81 g*min-1). Relative percent changes from rest to during exercise were not different between 1hr, 3 hrs and 12 hrs for fatox (2489.1 ± 1008.9 % vs. 2257.5 ± 853.3 % vs. 2200.8 ± 901.7 %) or carbox (660.2 ± 294.8 % vs. 579.6 ± 231.2 % vs. 492.5 ± 241.9 %). CONCLUSIONS: Fatox and carbox responses from rest to exercise were similar between trials. This suggests that a standard meal and resting oxidation rates could control for differences in substrate use during exercise, regardless of fasting length.