Abstract
Interest in low-carbohydrate, high-fat (LCHF) diets has increased over recent decades given the theorized benefit of associated intramuscular adaptations and shifts in fuel utilization on endurance exercise performance. Consuming a LCHF diet during exercise training increases the availability of fat (i.e., intramuscular triglyceride stores; plasma free fatty acids) and decreases muscle glycogen stores. These changes in substrate availability increase reliance on fat oxidation for energy production while simultaneously decreasing reliance on carbohydrate oxidation for fuel during submaximal exercise. LCHF diet-mediated changes in substrate oxidation remain even after endogenous or exogenous carbohydrate availability is increased, suggesting that the adaptive response driving changes in fat and carbohydrate oxidation lies within the muscle and persists even when the macronutrient content of the diet is altered. This narrative review explores the intramuscular adaptations underlying increases in fat oxidation and decreases in carbohydrate oxidation with LCHF feeding. The possible effects of LCHF diets on protein metabolism and post-exercise muscle remodeling are also considered.
Highlights
Interest in implementing low-carbohydrate, high-fat (LCHF) diets during exercise training has persisted over recent decades given the associated increase in fat oxidation and reduction in carbohydrate oxidation during exercise [1,2,3]
Endogenous sources of oxidized carbohydrate include muscle glycogen and blood glucose, while fat oxidation is primarily maintained by intramuscular triglycerides (IMTGs) and adipose tissue-derived plasma free fatty acids (FFAs)
Restoration and the provision of carbohydrate before exercise testing [37]. These findings suggest that it is the high-fat, not the low-carbohydrate component, of the LCHF diet driving molecular adaptations and concomitant alterations in substrate utilization that persist with increases in endogenous and exogenous carbohydrate availability
Summary
Interest in implementing low-carbohydrate, high-fat (LCHF) diets during exercise training has persisted over recent decades given the associated increase in fat oxidation and reduction in carbohydrate oxidation during exercise [1,2,3]. Adaptations in whole-body substrate oxidation with LCHF feeding during exercise training may be attributed, in part, to alterations in substrate availability, as intramuscular triglyceride (IMTG) stores increase and muscle glycogen concentrations decrease [8,9,10]. LCHF feeding increases reliance on fat as a fuel source, and decreases muscle glycogenolysis during prolonged aerobic exercise [4,11] These changes in substrate utilization are robust and remain even after endogenous or exogenous carbohydrate availability is increased [12,13,14,15]. Nutrients 2020, 12, 2496 of these dietary strategies and an understanding of what individuals, if any, may benefit This narrative review considers the intramuscular adaptations to LCHF feeding during exercise training underlying associated decreases in carbohydrate oxidation and increases in fat oxidation
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