Burning fat instead of sugar — how can you get your body to use its reserves?

Aim of the study: Ketogenic diets (KDs) have received increasing attention among athletes and physically active individuals.

Results: For a more stratified and focused analysis, these articles were further categorized according to the outcomes examined, including 1) body mass (BM) and % fat, 2) substrate utilization, 3) blood substrate and hormonal responses, 4) aerobic capacity and endurance performance, and 5) strength, power, and anaerobic capacity. Results: Our review indicates that a non-calorie-restricted KD followed for ≥3 weeks can produce a modest reduction in BM and % fat while maintaining fat-free mass. This diet leads to increased use of fat as fuel, but this adaptation does not seem to improve endurance performance.

Aim of the study: Many successful ultra-endurance athletes have switched from a high-carbohydrate to a low-carbohydrate diet, but they have not previously been studied to determine the extent of metabolic adaptations.

Results: Peak fat oxidation was 2.3-fold higher in the LC group (1.54±0.18 vs 0.67±0.14 g/min; P=0.000) and occurred at a higher percentage of VO2max (70.3±6.3 vs 54.9±7.8%; P=0.000). Mean fat oxidation during submaximal exercise was 59% higher in the LC group (1.21±0.02 vs 0.76±0.11 g/min; P=0.000), corresponding to a greater relative contribution of fat (88±2 vs 56±8%; P=0.000). Despite these marked differences in fuel use between LC and HC athletes, there were no significant differences in resting muscle glycogen or in the level of depletion after 180 min of running (-64% from pre-exercise) and 120 min of recovery (-36% from pre-exercise). Compared with highly trained ultra-endurance athletes consuming an HC diet, long-term keto-adaptation results in extraordinarily high rates of fat oxidation, whereas muscle glycogen use and replenishment patterns during and after a 3-hour run are similar.

Aim of the study: 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 an LCHF diet during exercise training increases the availability of fat (i.e., intramuscular triglyceride stores; plasma free fatty acids) and decreases muscle glycogen stores.

Results: 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.

Aim of the study: In exercise science, the crossover effect means that fat oxidation is the primary fuel at rest and during low-intensity exercise, with a shift toward greater reliance on carbohydrate oxidation at moderate to high exercise intensities. This model makes four predictions: First, >50% of energy comes from carbohydrate oxidation at ≥60% of maximum oxygen consumption (VO2max), a point called the crossover point.

Results: These studies challenge the prevailing view that carbohydrates are the main fuel oxidized during high-intensity exercise. We recently found that 30% of middle-aged competitive athletes had pre-diabetic blood glucose values while on an HCLF diet, which were reversed on an LCHF diet. We speculate that these rapid changes between diet, insulin, glucose regulation, and fat oxidation may be linked by diet-induced changes in mitochondrial function and insulin action. Together, we present evidence that challenges the current crossover concept and shows that an LCHF diet may also reverse features of pre-diabetes and future metabolic disease risk, demonstrating that the impact of dietary choices extends beyond physical performance, even in athletic populations.

Aim of the study: The role of an athlete's dietary intake (both timing and food type) goes beyond simply providing fuel to support the body's vital processes. Nutritional choices also affect metabolic adaptations to training.

Results: Nutritional choices also affect metabolic adaptations to training. Over the past 20 years, research has suggested that strategically reducing carbohydrate (CHO) availability during an athlete's training can modify metabolic responses instead of simply maintaining a high-CHO diet. Several methods have been explored to manipulate CHO availability, including: low-carb, high-fat (LCHF) diets; doing two training sessions a day without restoring glycogen between sessions; and a “sleep-low” approach, which involves a glycogen-depleting session in the evening and avoiding CHO until after a morning training session performed after an overnight fast. Each of these methods can produce beneficial metabolic adaptations for endurance athletes, including increases in mitochondrial enzyme activity, mitochondrial content, and rates of fat oxidation, although evidence of a direct performance benefit is still unclear.

Aim of the study: Training with low carbohydrate availability (LCA) has been proposed as an independent determinant of physiological changes commonly attributed to low energy availability (LEA), and as a way to increase skeletal muscle oxidative capacity. However, the effects of LCA separate from LEA remain unclear.

Results: In contrast, β-CTX increased and IGF-1 decreased relative to NORM. Muscle glycogen concentration decreased only in LOW (40% ± 14%). Proteomic analysis identified 671 proteins; 57 that were differentially expressed in LOW relative to NORM were limited to fatty acid metabolism pathways and the suppression of ribosomal, sarcomeric, and extracellular matrix proteins. These findings indicate that isolated LCA causes limited endocrine disruption but may selectively compromise bone turnover and the muscle anabolic response, suggesting that without acute LEA, LCA has limited influence on the muscle oxidative phenotype.

Aim of the study: A ketogenic diet (KD) is a low-carbohydrate, high-fat dietary approach. Beyond treating neurological disorders, KDs have attracted significant media attention for their potential to improve obesity and diabetes.

Results: We explore the metabolic effects of KDs in obesity and diabetes, drawing on preclinical and clinical studies, and suggest that combining KDs with antidiabetic agents may provide synergistic benefits. However, combining KDs with these medicines, particularly SGLT-2 inhibitors, requires careful clinical supervision because of potential risks, including euglycemic diabetic ketoacidosis. We explore how a KD alters the composition of the gut microbiota, thereby affecting host health. We conclude by highlighting challenges and future directions for optimizing KD-based therapies and outlining the limitations of the current review.