Sports nutrition guidelines have always recommended high-carbohydrate diets for endurance performance. However, some researchers claim that low-carbohydrate, high-fat diets are ‘the future of elite endurance sport’. Who is correct?
Introduction
As early as 1901, high-carbohydrate diets have been reported to improve exercise economy (or efficiency) (1). A couple of decades later, in 1920, researchers from Copenhagen demonstrated that when people were fed a high-fat diet of bacon, butter, eggs, and cabbage for three days, they struggled with fatigue during a two-hour exercise test (2). But, when they ate a high-carbohydrate diet of potatoes, flour, bread, cake, marmalade, and sugar, they found the exercise to be easy (2).
Fast-forward to the 1930s and it was demonstrated that as exercise intensity increases, so does the proportion of carbohydrates used by the body to create energy (3). Then, in the 1960s, researchers were able to perform muscle biopsies—or take a look inside the muscle—and saw that our muscles had the ability to store carbohydrates within them (3,4). In addition, researchers observed that the improvements in performance that were observed when people ate high-carbohydrate diets could be linked in large part to increases in these carbohydrate stores (known as glycogen) (3,4).
With all of this context presented, you may be wondering how low-carbohydrate, high-fat (LCHF) diets could be described as ‘the future of elite endurance sport’ (5). Those in favour of this approach argue that eating a high-fat diet allows our body to become more efficient at using fat for fuel and, because we store much more fat (as body fat) than carbohydrate (as liver and muscle glycogen), this could be beneficial in long-duration endurance sport (1,6). Indeed, as the theory goes, this would allow athletes to spare carbohydrates for when they matter most: the highest intensity moments of endurance events (e.g., sprint finishes).
While both schools of thought each have their own theoretical underpinning, the idea that higher carbohydrate diets are beneficial for endurance performance has a greater evidential base going back almost 125 years. However, key research in the last decade from Professor Louise Burke and colleagues has added a lot of evidence to this debate.
The Supernova Studies
In response to an increase in anecdotal claims of performance benefits to a LCHF dietary approach, Professor Louise Burke and colleagues set out to investigate whether high-carbohydrate or LCHF diets were optimal for endurance performance in athletes of the highest level—world-class race walkers.
Before you roll your eyes, the world record for males for the 50k race walk (set in 2014 by Yohann Diniz) stands at 3:32:33. This equates to a mind-boggling 4:15 /km; how many of you could run a 5k in 21:15, let alone 10 of them back-to-back!?
In the first of these studies, elite race walkers went through a battery of performance tests, followed by an intense three-week training programme where they either ate one of two kinds of high-carbohydrate diets, or a LCHF diet (1). Afterwards, they repeated these performance tests to see if their performance improved (and to compare improvements between groups). Researchers found that, on average, both groups eating a high-carbohydrate diet significantly improved their 25k race time (by ~2–3 minutes) whereas the LCHF group did not improve at all (1)!
In a more recent study from this group, elite race walkers performed a similar protocol and the researchers reported similar results, except this time over the 10k distance (7). However, this study went a step further. After the race walkers finished the three-week period of eating a LCHF diet, a proportion of them resumed a high-carbohydrate diet for two and a half weeks and then completed a 20k race. The race walkers who ate the LCHF diet showed no evidence of rebounding to better performance (7). In another publication from this group of researchers, feeding athletes a high-carbohydrate diet after 5–6 days on a LCHF diet did not help them to perform better than those eating a high-carbohydrate diet for the entire week-long period. In fact, six of seven athletes’ performance worsened (8).
The likely rationale behind the decline in performance appears to relate to exercise economy. The race walkers eating a LCHF diet used more oxygen at the same relative intensity as those eating the higher carbohydrate diets, meaning they were less efficient at using oxygen to create energy (1,7). Further, these athletes perceived the exercise to be more challenging compared to athletes consuming higher carbohydrate diets.
Cork man, Rob Heffernan, won the 50k walk at the 2013 World Championships. Image source: RTE
Other Research
While the Supernova studies are the most rigorous on this topic—and are the only studies performed in genuinely world-class athletes—there are other studies that warrant mention. Two separate (and recent) systematic reviews on this topic largely reported no significant differences between LCHF (ketogenic) diets and higher carbohydrate diets for a variety of endurance performance outcomes (9,10). However, there were some studies included in these reviews that showed higher carbohydrates diets performing better for certain outcomes, whereas no study showed LCHF diets performing better for endurance performance outcomes (9,10).
Summary
Taken altogether, the available evidence supports high-carbohydrate diets as optimal for fuelling endurance performance. Indeed, the highly controlled studies from Professor Louise Burke and colleagues were quite definitive in this regard. The take-home message for those of you reading who are endurance athletes, or team sport athletes that engage in both endurance and high-intensity activity (e.g., footballers), is to eat a high-carbohydrate diet. For exact recommendations, see our earlier articles on carbohydrates. With that said, if you’re not an elite athlete and prefer to eat a lower carbohydrate, higher fat diet—and feel good and perform well eating this way—I wouldn't necessarily suggest changing what you're doing.
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As always, thank you so much for reading!
Patrick Elliott, BSc, MPH
Health and Nutrition Science Communication Officer at Training121
Instagram: @just.health.info
Twitter/X: @PatrickElliott0
References
(1) Burke LM, Ross ML, Garvican-Lewis LA, Welvaert M, Heikura IA, Forbes SG, Mirtschin JG, Cato LE, Strobel N, Sharma AP, Hawley JA. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol. 2017;595(9):2785–807. Available at: https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP273230
(2) Krogh A, Lindhard J. The Relative Value of Fat and Carbohydrate as Sources of Muscular Energy: With Appendices on the Correlation between Standard Metabolism and the Respiratory Quotient during Rest and Work. Biochem J. 1920;14(3–4):290–363. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1263890/
(3) Jeukendrup A, Gleeson M. Sport Nutrition. 3rd ed. Champaign, IL: Human Kinetics; 2019. Available at: https://books.google.ie/books/about/Sport_Nutrition_3rd_Edition.html?id=SMVlDwAAQBAJ&redir_esc=y
(4) Hawley JA, Maughan RJ, Hargreaves M. Exercise Metabolism: Historical Perspective. Cell Metab. 2015;22(1):12–7. Available at: https://www.sciencedirect.com/science/article/pii/S1550413115002831
(5) Burke LM. Ketogenic low-CHO, high-fat diet: the future of elite endurance sport? J Physiol. 2021;599(3):819–43. Available at: https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP278928
(6) Volek JS, Noakes T, Phinney SD. Rethinking fat as a fuel for endurance exercise. Eur J Sport Sci. 2015;15(1):13–20. Available at: https://www.tandfonline.com/doi/full/10.1080/17461391.2014.959564
(7) Burke LM, Sharma AP, Heikura IA, Forbes SF, Holloway M, McKay AKA, Bone JL, Leckey JJ, Welvaert M, Ross ML. Crisis of confidence averted: Impairment of exercise economy and performance in elite race walkers by ketogenic low carbohydrate, high fat (LCHF) diet is reproducible. PLoS One. 2020;15(6):e0234027. Available at: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0234027
(8) Burke LM, Whitfield J, Heikura IA, Ross MLR, Tee N, Forbes SF, Hall R, McKay AKA, Wallett AM, Sharma AP. Adaptation to a low carbohydrate high fat diet is rapid but impairs endurance exercise metabolism and performance despite enhanced glycogen availability. J Physiol. 2021;599(3):771–90. Available at: https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP280221
(9) Cao J, Lei S, Wang X, Cheng S. The Effect of a Ketogenic Low-Carbohydrate, High-Fat Diet on Aerobic Capacity and Exercise Performance in Endurance Athletes: A Systematic Review and Meta-Analysis. Nutrients. 2021;13(8):2896. Available at: https://www.mdpi.com/2072-6643/13/8/2896
(10) Murphy NE, Carrigan CT, Margolis LM. High-Fat Ketogenic Diets and Physical Performance: A Systematic Review. Adv Nutr. 2021;12(1):223–33. Available at: https://www.sciencedirect.com/science/article/pii/S2161831322003763
Technical Terms
Exercise economy: This refers to how efficiently the body uses oxygen to create energy. It is also referred to as aerobic efficiency.
Glycogen: This is the storage form of carbohydrate (or glucose). Glycogen is stored in the liver and muscles and, when stored to capacity, can store about 2,000–2,500 calories of carbohydrate.
Systematic review: A systematic review is a comprehensive summary of all the relevant research on a specific topic. It includes multiple studies to provide a clear picture of what the science says about that topic. Studies are included such that they meet certain eligibility criteria, which are defined by the researchers. For example, the researchers may only want to look at a certain age group of people, or a certain study type. This helps to narrow the focus of the review to a particular research question.