Divergent Changes in Peak Fat Oxidation and Fat(max) Following 3-Day Dietary Interventions Are Related to Muscle Glycogen Availability in Men.

Dietary intake has an important influence on rates of fuel use during exercise, but the extent to which short-term diet changes affect peak fat oxidation (PFO) and the intensity at which this occurs (Fat(max)) is unknown. This study examined the impact of diet-induced changes in substrate availability on PFO and Fat(max) and the expression of key lipid-regulatory genes and proteins in skeletal muscle. Forty moderately to well-trained males (27 ± 5 years, V̇O(2peak) 56.3 ± 4.8 mL/kg/min) were randomly allocated to either a low-carbohydrate, high-fat (LCHF, n = 20) (65E% fat, 20E% CHO, 15E% protein) or high-carbohydrate, low-fat diet (HCLF, n = 20) (70E% CHO, 15E% fat, 15E% protein) for 3 days. Fasting blood samples, muscle biopsies, and incremental exercise tests to determine PFO and maximal oxygen uptake were conducted before and after the diet. PFO increased after LCHF (pre: 0.39 ± 0.11; post: 0.59 ± 0.18 g/min, post hoc: p < 0.0001), and decreased following HCLF (pre: 0.36 ± 0.08; post: 0.28 ± 0.10 g/min, post hoc: p = 0.0065). Divergent shifts in Fat(max) of ~5% points were also observed (LCHF: pre: 38% ± 6%; post: 44% ± 7%; post hoc: p < 0.0001, HCLF: pre: 37% ± 4%; post: 33% ± 7%; post hoc: p = 0.0004). Intramuscular triacylglycerol (IMTG) stores were similar after the diets, but muscle glycogen was significantly reduced in LCHF (pre: 439 ± 98; post: 358 ± 117 nmol/mg, post hoc: p = 0.0019), and increased in HCLF (pre: 407 ± 107; post: 498 ± 139 nmol/mg, post hoc: p = 0.0101). Skeletal muscle gene and protein expression remained unchanged. PFO and Fat(max) are amenable to short-term changes in dietary macronutrient composition and are coupled to changes in muscle glycogen.