Fuel substrate selection during exercise is influenced by several factors including nutritional status, sex and fitness (1). Compared to pre-exercise carbohydrate consumption, exercising in a fasted state results in higher fat oxidation rates. Therefore, fasted exercise is a popular training strategy for exercisers who aim to increase fat oxidative capacity. Less is known about the influence of pre-exercise protein consumption on fuel substrate selection. We and others have previously reported that pre-exercise protein consumption might not affect fat oxidation rates compared to fasted exercise (2,3,4). This study is a secondary analysis from previously published data (4) where we aimed to establish whether the effect of different pre-exercise protein doses on fat oxidation rates during exercise were influenced sex, fitness and body composition.

In a double-blinded randomised within-subject study design, fifteen healthy active individuals (9 males, 6 females, age: 25 ± 5 yr, height: 175 ± 10 cm, weight: 73.4 ± 13.1 kg, VO2max: 46.8 ± 9.0 ml/kg/min) performed one hour of cycling at 60% of their peak power, thirty minutes after having consumed either 0, 20 or 40 grams of whey protein hydrolysate. Indirect calorimetry was used to measure substrate oxidation every 15 min of exercise. Body composition, using bio-electrical impedance and fitness using a ramp-test protocol were completed during a screening visit. Two and three-way ANOVA were used for statistical analysis with time and treatment as within-subject factors and sex as between-subject factor. ANCOVA was deployed to adjust fat oxidation rates for fitness (VO2max). Outcomes are Mean ± SE.

Overall, fat oxidation increased over time during exercise (0.15 ± 0.04, 0.23 ± 0.03, 0.30 ± 0.03, 0.32 ± 0.03 g/min at 15, 30, 45 and 60 min respectively; p = 0.000), but no treatment (0 gr = 0.23 ± 0.03, 20 gr = 0.25 ± 0.04, 40 gr = 0.27 ± 0.04, g/min p = 0.16) or interaction effect (p = 0.32) was observed. Including VO2max as a co-variate had no effect on the outcomes (all p-values > 0.05). There was no overall effect of sex (F: 0.21 ± 0.05, M: 0.29 ± 0.4 g/min; p = 0.24) or treatment*sex (p = 0.73). No significant effects were observed when fat oxidation was expressed relative to body weight (g/min/kg BW; treatment-effect: p = 0.15) or fat-free mass (g/min/kg FFM; treatment-effect: p = 0.17).

Consumption of different doses of protein before exercise seems to maintain similar fat oxidation rates compared to fasted exercise and was not affected by sex, fitness and body composition. Therefore, pre-exercise protein ingestion might be considered as a feeding strategy for exercisers who aim to enhance fat oxidative capacity but struggle to incorporate fasted training into their program.