Carbohydrate Supplementation and the Influence of Breakfast on Fuel Use in Hypoxia

Purpose: This study investigated the effect of carbohydrate supplementation on substrate oxidation during exercise in hypoxia after preexercise breakfast consumption and omission. Methods: Eleven men walked in normobaric hypoxia (FiO2 similar to 11.7%) for 90 min at 50% of hypoxic <(V)over dot>O-2max. Participants were supplemented with a carbohydrate beverage (1.2 g.min(-1) glucose) and a placebo beverage (both enriched with (UC6D)-C-13-glucose) after breakfast consumption and after omission. Indirect calorimetry and isotope ratio mass spectrometry were used to calculate carbohydrate (exogenous and endogenous [muscle and liver]) and fat oxidation. Results: In the first 60 min of exercise, there was no significant change in relative substrate oxidation in the carbohydrate compared with placebo trial after breakfast consumption or omission (both P = 0.99). In the last 30 min of exercise, increased relative carbohydrate oxidation occurred in the carbohydrate compared with placebo trial after breakfast omission (44.0 +/- 8.8 vs 28.0 +/- 12.3, P < 0.01) but not consumption (51.7 +/- 12.3 vs 44.2 +/- 10.4, P = 0.38). In the same period, a reduction in relative liver (but not muscle) glucose oxidation was observed in the carbohydrate compared with placebo trials after breakfast consumption (liver, 7.7% +/- 1.6% vs 14.8% +/- 2.3%, P < 0.01; muscle, 25.4% +/- 9.4% vs 29.4% +/- 11.1%, P = 0.99) and omission (liver, 3.8% +/- 0.8% vs 8.7% +/- 2.8%, P < 0.01; muscle, 19.4% +/- 7.5% vs 19.2% +/- 12.2%, P = 0.99). No significant difference in relative exogenous carbohydrate oxidation was observed between breakfast consumption and omission trials (P = 0.14). Conclusion: In acute normobaric hypoxia, carbohydrate supplementation increased relative carbohydrate oxidation during exercise (>60 min) after breakfast omission, but not consumption.