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.