Abstract
Purpose: In this study, we determined ketone oxidation rates in athletes under metabolic conditions of high and low carbohydrate (CHO) and fat availability.Methods: Six healthy male athletes completed 1 h of bicycle ergometer exercise at 75% maximal power (WMax) on three occasions. Prior to exercise, participants consumed 573 mg·kg bw−1 of a ketone ester (KE) containing a 13C label. To manipulate CHO availability, athletes undertook glycogen depleting exercise followed by isocaloric high-CHO or very-low-CHO diets. To manipulate fat availability, participants were given a continuous infusion of lipid during two visits. Using stable isotope methodology, β-hydroxybutyrate (βHB) oxidation rates were therefore investigated under the following metabolic conditions: (i) high CHO + normal fat (KE+CHO); (ii) high CHO + high fat KE+CHO+FAT); and (iii) low CHO + high fat (KE+FAT).Results: Pre-exercise intramuscular glycogen (IMGLY) was approximately halved in the KE+FAT vs. KE+CHO and KE+CHO+FAT conditions (both p < 0.05). Blood free fatty acids (FFA) and intramuscular long-chain acylcarnitines were significantly greater in the KE+FAT vs. other conditions and in the KE+CHO+FAT vs. KE+CHO conditions before exercise. Following ingestion of the 13C labeled KE, blood βHB levels increased to ≈4.5 mM before exercise in all conditions. βHB oxidation was modestly greater in the KE+CHO vs. KE+FAT conditions (mean diff. = 0.09 g·min−1, p = 0.03; d = 0.3), tended to be greater in the KE+CHO+FAT vs. KE+FAT conditions (mean diff. = 0.07 g·min−1; p = 0.1; d = 0.3) and were the same in the KE+CHO vs. KE+CHO+FAT conditions (p < 0.05; d < 0.1). A moderate positive correlation between pre-exercise IMGLY and βHB oxidation rates during exercise was present (p = 0.04; r = 0.5). Post-exercise intramuscular βHB abundance was markedly elevated in the KE+FAT vs. KE+CHO and KE+CHO+FAT conditions (both, p < 0.001; d = 2.3).Conclusion: βHB oxidation rates during exercise are modestly impaired by low CHO availability, independent of circulating βHB levels.
Highlights
Ketogenesis is a vital metabolic adaptation to starvation [1]
Blood free fatty acids (FFA) and intramuscular long-chain acylcarnitines were significantly greater in the ketone ester (KE)+FAT vs. other conditions and in the KE + high-CHO diet + normal fat (KE+CHO)+FAT vs. KE+CHO conditions before exercise
Following ingestion of the 13C labeled KE, blood βHB levels increased to ≈4.5 mM before exercise in all conditions. βHB oxidation was modestly greater in the KE+CHO vs. KE+FAT conditions, tended to be greater in the KE+CHO+FAT vs. KE+FAT conditions and were the same in the KE+CHO vs. KE+CHO+FAT conditions (p < 0.05; d < 0.1)
Summary
Ketogenesis is a vital metabolic adaptation to starvation [1]. Whilst the teleological purpose of ketogenesis is to provide a supplementary fuel for the brain [1, 2], most oxidative tissues can metabolize ketones [3]. Given its substantial contribution to overall body mass, skeletal muscle is the largest site of ketone uptake and oxidation [3], during exercise [4,5,6] when metabolic rate is markedly elevated [7]. The contribution of exogenous ketone oxidation to whole-body energy expenditure appears to be low in exercising humans, with estimates ranging from 0 to 10% [4, 6, 10, 11]
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