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Does a Low-Carb, High Fat (Keto) Diet Benefit Endurance Exercise performance?

The low carbohydrate, high fat diet (LCHF) also referred to as a ketogenic diet, is a diet in which an individual consumes less than 20% of energy from carbohydrates, more than 50% from fats and variable amounts of protein, though many studies use a more extreme version, containing less than 5% of energy from carbohydrates (Chang, Borer, & Lin, 2017). There is a lot of controversy discussing the impact the LCHF diet may have on endurance athletes especially because of the delay, from cutting out energy produced from carbohydrates, in production of ketones many studies discuss. This period is also controversial, as some researchers do not acknowledge the keto-adaptation period. Nevertheless, after a few weeks of starvation known as the keto-adaptation period, when glycogen levels have decreased significantly, hepatic ketone production ramps up dramatically to replace the brain’s previous glucose energy source. Fatty acids supply the majority of energy for skeletal muscle during this period (Volek, et al., 2015). Ketone bodies are lipid-derived compounds that are produced in the liver during levels of low carbohydrate intake, similar to what happens during starvation. Ketone bodies refer to three molecules, acetone, acetoacetate (AcAc) and β-hydroxybutyrate (β-OHB) (Sansone, et al., 2018). The physiological ketosis that results from this diet, raises the blood ketone concentration level to 7-8 mM and a blood pH around 7.4, which enhances cellular oxidation. (Chang, Borer, & Lin, 2017). Evidence also suggests that reliance on ketone body metabolism may offer an ergogenic aid for endurance performance. Additionally, the LCHF diet can promote fat loss in addition to stimulating fat oxidation. (Harvey, Holcomb, & Kolwicz, Jr., 2019).


Many of the studies reviewing high carb low fat (HCLF) diets were studied based off of research done on the short-term effects to both kinds of diets. Without a keto-adaptation period, of two to four weeks, athletes may show signs of poor performance, fatigue, weakness, and asthenia. Once through the adaptation period, it was observed that the LCHF was effective in reducing body mass and fat content and the increased reliance on fat oxidation increased mitochondrial function but seems to not be able to be sustained moderate-high intensity levels. It was found that there was not consistent content of muscle and hepatic glycogen stores. The reduction in glycolytic flux by decrease of pyruvate dehydrogenase may diminish high-intensity efforts that relay on the glycolytic pathways (Sansone, et al., 2018). Contrastingly, in another study conducted over 20 months observing 20 elite ultra-endurance athletes, 10 on a LCHF diet and 10 on a HCLF diet. This was the first study to review chronic keto-adaptation and its association with increased capacity for fat oxidation during exercise with the maintenance of normal skeletal muscle glycogen concentrations (Volek, et al., 2015). Another study noted that between their three test groups, high carbohydrate diet (HCHO), periodized carbohydrate cycle diet (PCHO), and LCHF diet, the LCHF experienced RER at maximal aerobic capacity to be lower than pre-treatment. Additionally, the heart rate of the HCHO and PCHO groups were lower at the same point in the post-treatment trial than the pre-treatment trial. This study also had a novel finding that the shift in substrate oxidation from carbohydrate-based to fat- based energy sources, resulted in an increased oxygen demand for a given walking speed across different velocities. An additional finding was the improvement in performance when training and competing with HCO and PCHO but not with the LCHF diet (Burke, et al., 2016). Building on that study, the same author conducted another study years later, in which further novel findings were found that include, “(1) adaptations that substantially increase fat oxidation during exercise in response to the LCHF occur in elite athletes in as little as 5–6 days, reaching the same rates as observed in endurance athletes who have adhered to this diet for medium (3–4 weeks) and long-term periods (>12 weeks);(2) muscle retooling to enhance fat oxidation with brief exposure to a ketogenic LCHF is maintained in the face of acute increases in muscle CHO availability, but is reversed by 5 days of chronic high CHO diet; (3) the economy of exercise at intensities relevant to real life endurance events is reduced by brief adaptation to a ketogenic LCHF when fat oxidation is maximized; and (4) strategies that attempt to integrate brief adaptation to a ketogenic LCHF diet with enhanced muscle CHO availability seem limited by the blunting of CHO oxidation and impaired performance of endurance events undertaken at high relative and absolute exercise intensities” (Burke, et al., 2020).


With these findings, a clear delineation between thoughts surrounding the LCHF diet in the scientific community can be observed. While it is agreed upon throughout the noted studies that the LCHF diet promotes fat oxidation, that is ostensibly where the similarities between studies for the most part ends. A notable difference found between the articles produced by (Sansone, et al., 2018) and (Burke, et al., 2020) were how long the keto-adaptation period truly took. A keto-adaptation period of two to four weeks in which athletes may experience negative side effects, is noted, though no clear definition of the adaption period was outlined in the study by (Sansone, et al., 2018). Contrarily, defined by the time the adaptations occur that increase fat oxidation during exercise (Burke, et al., 2020) found this to take place in as few as five to six days. Additionally, long-term keto-adaptations were observed over a 20-month period with (Volek, et al., 2015). Since athletes may show signs of poor performance, fatigue, weakness, and asthenia during these keto-adaptation periods, further research is clearly needed within this area (Sansone, et al., 2018). Additionally, it has been observed more consistently, that the intensity of exercise is extremely important. The LCHF diet is best at a steady state, lower (59-64% VO2max) intensity but longer distance. Relying on fat oxidation eliminates the need to carbohydrate load during events such as the ironman triathlon. There is clearly a wide variety of results that have been observed as researchers study subjects and their endurance performance when and after adaptation to the LCHF diet. While some studies have observed positive performance trends in individuals on the LCHF diet, most specifically related to their enhancement of fat oxidation, the draw backs and lack of consistency as well as negative effects of adapting to the utilization of fat-derived fuel sources from carbohydrate based, highlight the need for more research on the LCHF diet and endurance exercise performance. Currently, evidence does not consistently suggest that the LCHF diet does in fact benefit endurance exercise.


Diets consisting of less than 5-20% of carbohydrates, more than 50% from fats and varying levels of protein, are considered LCHF. An adaptation period is known to take place, though disagreement throughout studies of the amount of time and how to measure it seems evident. Some studies did not discuss the short-term adaptation period that takes place, is when glycogen levels have decreased significantly, hepatic ketone production ramps up dramatically to replace the brain’s previous glucose energy source. In one study, this was noted by the time in which enhanced fat oxidation was observed. The interest in utilizing fat oxidation and fat-derived fuel sources is an interesting topic, as carbohydrate energy is finite and not ideal for ultra-endurance events. There is disagreement among the science community related to the benefits of athletes relying on a LCHF diet for endurance events. While inconsistent benefits may be observed through current findings, more research on the topic is needed, including on the intensities at which benefits seem to outweigh drawbacks to this macronutrient breakdown, and on the adaptation period in which athletes may experience decreased athletic performance. Relying on fat oxidation would be helpful in ultra-endurance events in the sense that athletes would not need to refuel mid-race, but the drawbacks seem to significantly outweigh the benefits of it.



Works Cited

Burke, L. M., Ross, M. L., Garvican-Lewis, L. A., Welvaert, M., Heikura, I. A., Forbes, S. G., . . . Hawley, J. A. (2016). Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. The Journal of Physiology, 2785-2807.


Burke, L. M., Whitfield, J., Heikura, I. A., Ross, M. L., Tee, N., Forbes, S. F., . . . Sharma, A. P. (2020). Adaptation to a low carbohydrate high fat diet is rapid but impairs endurance exercise metabolism and performance despite enhanced glycogen availability. The Journal of Physiology, 771-790.


Chang, C.-K., Borer, K., & Lin, P.-J. (2017). Low-Carbohydrate-High-Fat Diet: Can it Help Exercise Performance? Journal of Human Kinetics, 81-92.


Harvey, K. L., Holcomb, L. E., & Kolwicz, Jr., S. C. (2019). Ketogenic Diets and Exercise Performance. Nutrients, 1-16.


Sansone, M. M., Sansone, M. A., Borrione, M. P., Romanelli, M. F., Di Luigi, M. L., & Sgro, M. P. (2018). Effects of Ketone Bodies on Endurance Exercise. International Federation of Sports Medicine, 444-453.


Volek, J. S., Freidenreich, D. J., Saenz, C., Kunces, L. J., Creighton, C. B., Bartley, J. M., . . . Phinney, S. D. (2015). Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism Journal, 100-109.


As always, please let me know if you would like to discuss this further! This paper was written in scientific terminology and I am happy to answer any questions you have!


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