Ketogenic diets are designed to induce the body to enter ketosis, in other words the state where fat is preferentially burned for fuel. Ever since this eating style first became popular sports scientists have been intrigued with the possibility that this could give endurance athletes a boost.
Seductive argument
The argument in favor of this notion is seductive on the surface. Endurance events, whether it’s a running marathon, a long race walk or a distance cycling event contested at a high tempo, will take athletes to a place where their stores of ready carbohydrate and stored muscle glycogen are exhausted and the body must resort to fat reserves to make up the difference. If athletes had been predisposed to this from the get go as a result of pursuing a ketogenic diet, wouldn’t that give them an edge?
To see what the literature has to say on the subject two researchers from Tufts University, Caitlin P. Bailey and Erin Hennessy, published a review paper recently in the Journal of the International Society of Sports Nutrition. The pair did a search on the subject on both the PubMed and Web of Science databases and found 38 potential papers for review.
Small data set
After eliminating duplicates and applying their inclusion criteria, the researchers were left with a fairly small data set. Only seven studies made the grade. One was a case study and six were prospective trials, one of which was set up as a randomized crossover trial.
The subjects were a mix of endurance athletes, including race walkers, runners and cyclists as well as a smattering of athletes from other disciplines. Most of the subjects were men, but two of the studies looked at women as well.
The primary outcome the authors were looking for was an improvement in VO2 max, a measure of peak oxygen uptake. This measure is strongly correlated with maximum performance in aerobic exercise. The authors were also looking at secondary measures such as improvements in race times, which are to some degree more subjective and unpredictable.
Two studies did record VO2 max improvements associated with the ketogenic diets. But athletes using other diets improved as well, which confounded the results. The other five studies showed no significant difference in VO2 max between the groups.
The one study that showed a decrease in race times associated that with carbohydrate-rich or periodized carbohydrate diets, not with the ketogenic diet.
Data doesn’t support notion that keto diets can help
Overall, the authors seemed unwilling to close the door on the potential for an athletic performance boost from ketogenic diets. But they concluded that the state of the science at the moment does not support the use of ketogenic diets for an improvement in performance in endurance events.
“Despite popular interest in the ketogenic diet as an ergogenic aid in endurance sport, there are few published studies examining the effect of EAKD [endurance athletes ketogenic diet] consumption on VO2 max and other outcomes. When compared to a high carbohydrate diet, there are mixed findings for the effect of EAKD consumption on endurance performance. This may be partially due to the heterogeneity across studies and/or variability in athletes’ individual genetic factors, especially those that directly influence metabolism,” the authors concluded.
Practicing nutritionist says data supports carb-rich diets
Susan Kleiner, PhD, a clinical nutritionist who is one of the founding members of the International Society of Sports Nutrition, has consulted with many athletes and teams in the Seattle area. She said her own experience leaves her more sure than the authors of the review seem to be about whether ketogenic diets make sense as a training aid.
“While those with pure research as their ultimate end goal may find the continued pursuit of this question purposeful, in my world of sports nutrition practice there is nothing in this review that inspires me to experiment with the keto diet on my clients. Placing the variable quality of the small number of included studies aside, the data on the side of keto and endurance race performance are pathetic compared to carb-rich diets. Of course, research may discover something new, especially the path investigating a genetic link to self selection into a specific diet type, as noted by the authors. But we are a long way from a practical understanding,” she said.
“Finally, we have more recent data from Dr Louise Burke and coworkers that reproduces and further corroborates her studies included in the review, demonstrating a clear performance deficit from subjects on the keto diet, and in a new addition those going from keto to carb after fat-adaptation as an energy sparing strategy. Performance enhancement is only shown with the carb-rich diet on its own,” Kleiner added.
Source: Journal of the International Society of Sports Nutrition
17, Article number: 33 (2020)
A review of the ketogenic diet for endurance athletes: performance enhancer or placebo effect?
Authors: Bailey CP, Hennessey E