Researchers: a low carb diet to improve athletic performance and physical endurance

IN BRIEF

• Low carb diets promote a metabolic profile characterized by increased fat oxidation and ketone production, which can reduce glycemic variability and alter the use of energy substrates.
• Adapting to a low carb diet takes time (weeks) and can increase fat-burning capacity during exercise, but it does not always guarantee a performance advantage in all sports disciplines.
• Some evidence indicates improvements in body composition during training programs combined with carbohydrate restriction, but results depend on the context, exercise intensity, and appropriate management of protein, fat, and minerals.
• Evidence on inflammation, oxidative stress, and recovery is promising but not uniform: many results come from short-term studies, animal models, or specific cohorts.
• Dietary choices should be evaluated on a case-by-case basis; practical application requires attention to macronutrient dosage, hydration, sodium/magnesium, and overall energy balance.

Abstract: what does science say?

A low carb diet (including the ketogenic variant) shifts metabolism towards greater fat utilization and, under adapted conditions, increases ketone production. Systematic reviews and experimental studies show that this change can increase lipid oxidation during exercise and modify body composition in the presence of training. However, the effects on performance vary: for prolonged efforts at moderate intensity, some athletes increase caloric endurance, while for highly intense efforts, efficiency and power may be affected. The main uncertainties concern the duration of adaptation, individual variability, and long-term consequences on cardiovascular health and metabolism. The available data require cautious and contextualized interpretation.

Metabolism and adaptation to carbohydrate restriction

The human body has very different energy reserves for different substrates: body fat reserves are generally much larger than carbohydrate reserves. Reducing carbohydrate intake induces a series of metabolic adaptations: glycogen reserves decrease, lipolysis increases, and in later stages, the production of ketone bodies increases, which can be used by many tissues, including the brain. These adaptations do not occur instantaneously; full "keto-adaptation" typically requires days–weeks of exposure to the diet and sometimes adjustments in the training program to maintain performance during the transition. Recent reviews document that carbohydrate restriction reduces glucose oxidation and leads to a marked increase in lipid oxidation during exercise in adapted subjects, but also highlight individual variability in metabolic responses and dependence on effort intensity [1].

Molecular mechanisms and adaptation time

Metabolic adaptation involves enzymatic and gene expression changes in skeletal muscle and liver: for example, increased activity of enzymes favoring fatty acid oxidation and regulation of pyruvate dehydrogenase, which reduces pyruvate entry into the Krebs cycle, favoring lipid use as fuel [3]. Over time, those who adopt a low carb regimen can achieve much higher fat oxidation levels than those who eat many carbohydrates; however, concrete functional benefits emerge after the completion of metabolic adaptation and vary depending on the intensity and duration of physical activity [1][3].

Evidence on sports performance

The literature shows heterogeneous results: some controlled studies document that habituation to low carb diets drastically increases the ability to oxidize fats during moderate and prolonged endurance exercise, reaching higher oxidation rates compared to high-carbohydrate regimens [2][8]. These changes are consistent with the concept that, once glycogen stores are depleted, the ability to use fat as fuel allows for sustained prolonged efforts with less reliance on carbohydrate stores. However, research on elite athletes has also shown reductions in exercise economy and poorer performance in high-intensity trials when switching to a strongly ketogenic regimen: the increased fat oxidative capacity does not always compensate for the loss of efficiency in high-intensity phases [2][9]. The overall effect therefore depends on the type of sport, the duration of adaptation, and the overall energy balance.

Body composition and strength training

Controlled clinical studies indicate that combining resistance training with a low carb diet can lead to significant reductions in fat mass while maintaining or improving lean mass in some contexts [4][5]. Randomized literature shows positive results on body recomposition in trained subjects when energy control, protein intake, and the training program are adequately balanced. However, it remains crucial to avoid excessive or too low protein intake to preserve lean mass during low carb regimens [4][5].

Mechanisms related to inflammation and oxidative stress

The reduction in carbohydrate intake and the increase in lipid use appear to have effects on the biology of the inflammatory response and oxidative stress, at least in experimental models and observational studies. Narrative reviews and experimental studies report signs of reduction in some inflammatory markers and lower production of reactive oxygen species under particular experimental conditions [6][7]. These plausible biological effects can be explained by less glycemic oscillation, different substrate availability, and potential direct effects of ketone bodies on metabolic and immune pathways. However, the results are not uniform across human studies, which differ in duration, populations, and measures used; therefore, the practical implications for long-term health require further research.

What it means in practice

For those who practice sports or regular physical activity, evidence indicates that a low carb diet can increase the metabolic capacity to use fats and favorably modify body composition if placed in a correct context of training and caloric control. However, it is not a universal solution: for disciplines that require very high-intensity efforts or repeated fast sprints, glycogen availability remains a critical factor, and prolonged carbohydrate restriction can reduce efficiency and performance. In practical terms, those considering this option should evaluate: sports goal (endurance vs. power), duration of adaptation (weeks), adequate protein intake (to protect lean mass), and supplementation/minerals (sodium, magnesium) to manage the initial effects of the transition. Personalization and monitoring are essential; the decision should be based on concrete results (e.g., body composition, race performance) and any health indicators.

Key points to remember

• Carbohydrate restriction increases fat oxidation and ketone production after a period of adaptation.
• Benefits on sports results depend on the type of activity: possible advantages in prolonged endurance, limitations in high-intensity performance.
• A low-carb/ketogenic diet can promote fat mass loss during training if managed correctly.
• Effects on inflammation and oxidative stress are promising but not yet definitive for the general population.
• Pay attention to hydration, sodium, and magnesium in the initial phases; adequate protein regimens are fundamental for lean mass.

Limitations of the evidence

It is important to distinguish between observational results, controlled experimental studies, and meta-analyses: many available studies are short-term, with limited numbers of subjects, or conducted in specific populations (athletes, overweight/obese subjects, or animal models). Observational data show associations but cannot establish causality; randomized studies provide more causal strength, but often differ in definitions of "low-carb" and caloric control. Other limitations include variability in training protocols, individual differences in metabolic response, and lack of long-term data on safety and cardiovascular risk. For these reasons, interpretation must be cautious and contextual.

Editorial conclusion

Low carb diets and ketogenic strategies represent metabolic tools with potential advantages for fat oxidation and body recomposition in the presence of training. However, they are not an optimal choice in every sports situation and should be evaluated considering individual goals, type of effort, health status, and personal preferences. Informed decision-making requires critical reading of the evidence, monitoring of results, and, when appropriate, consultation with a nutrition professional or sports medicine specialist.

Editorial note

This article was previously published in an earlier version and updated according to scientific and divulgative criteria to improve clarity and transparency. The purpose is informative: it does not replace medical advice or personalized evaluation by healthcare professionals. For clinical or performance decisions, always consult a qualified doctor or dietitian.

SCIENTIFIC RESEARCH

1. Macedo RCO, Santos HO, Tinsley GM, Reischak-Oliveira A. Low-carbohydrate diets: Effects on metabolism and exercise - A comprehensive literature review. Clin Nutr ESPEN. 2020. https://doi.org/10.1016/j.clnesp.2020.07.022
2. Burke LM. Ketogenic low‑CHO, high‑fat diet: the future of elite endurance sport? J Physiol. 2021;599(3):819–843. https://doi.org/10.1113/JP278928
3. High-Fat/Low-Carbohydrate Diet Reduces Insulin-Stimulated Carbohydrate Oxidation but Stimulates Nonoxidative Glucose Disposal in Humans: An Important Role for Skeletal Muscle Pyruvate Dehydrogenase Kinase 4. J Clin Endocrinol Metab. 2007;92(1):284–292. https://doi.org/10.1210/jc.2006-1592
4. Vargas S, Romance R, Petro JL, et al. Efficacy of ketogenic diet on body composition during resistance training in trained men: a randomized controlled trial. J Int Soc Sports Nutr. 2018;15:31. https://doi.org/10.1186/s12970-018-0236-9
5. Wilson JM, Lowery RP, Roberts MD, et al. The effects of ketogenic dieting on body composition, strength, power, and hormonal profiles in resistance training males. J Strength Cond Res. 2017. https://doi.org/10.1519/JSC.0000000000001935
6. Investigating the Link between Ketogenic Diet, NAFLD, Mitochondria, and Oxidative Stress: A Narrative Review. Antioxidants (Basel). 2023;12(5):1065. https://doi.org/10.3390/antiox12051065
7. An 8-Week Ketogenic Low Carbohydrate, High Fat Diet Enhanced Exhaustive Exercise Capacity in Mice. Nutrients. 2018;10(6):673. https://doi.org/10.3390/nu10060673
8. Low and high carbohydrate isocaloric diets on performance, fat oxidation, glucose and cardiometabolic health in middle age males. Front Nutr. 2023; https://doi.org/10.3389/fnut.2023.1084021
9. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol / PLOS/PMC. 2017–2020. https://doi.org/10.1113/JP278928 [study on elite athletes highlighting performance limitations at high intensity]