By Holley Samuel MEd, RD, LD, CPT
The ketogenic diet has been around for decades as a therapeutic diet for pediatric patients with epilepsy as a potential intervention to decrease seizures and other neurological symptoms. More recently, the ketogenic diet (also called “keto”) has been touted across the weight loss, diabetes, and athletic communities as a potential option to improve health, lose weight, improve blood sugar regulation, and enhance athletic performance through “fat adaptation.”
What is the Keto Diet?
The ketogenic diet is traditionally high in fat (70-80% of calories), low in protein (10-20% of calories), and extremely low carbohydrate (5-10% of calories). Being in ketosis requires the body to have little access to carbohydrates through the diet or glycogen stores and instead use ketones for energy.
The brain and muscle’s primary fuel source is from glucose derived from carbohydrates, and most healthy human brains require at least 130g of carbohydrates per day to function properly. Keto dieters are recommended to reduce carbohydrate intake to just 50g or less to be in ketosis, where the brain is required to use ketones for energy instead of glucose. Ketones are made by the liver as the body’s backup source of fuel for the brain to use for energy when carbohydrates are absent.
The idea behind using the ketogenic diet to fuel endurance performance is to become a more “fat adapted” athlete, as endurance sports require the athlete to use both fat and carbohydrate for energy during long events. Fat has more energy per unit than carbohydrates, making it a potentially more energy-dense fuel source to teach the body to tap into during long endurance events. However, burning fat for fuel is less efficient biologically than burning carbohydrate for fuel.
There are also potential risks to restricting carbohydrates such as underfueling related issues like RED-S, which may entail hormone imbalances, eating disorders, and potential increased risk for injury. The keto diet can also lead to GI distress from increased fat intake, hepatic steatosis, kidney and gallstones, and potential nutrient deficiencies from cutting out major food groups to restrict carbohydrates (for example fruits, starchy vegetables, whole grains, legumes).
There is limited short term evidence (studies that range from 3 to 36 months long in sedentary populations and 3 to 12 weeks long in endurance athletes populations) to support that keto diet can lead to weight loss, increase in insulin sensitivity, and improvement in metabolic syndrome symptoms. There is currently not enough definitive research to support endurance athletes using the keto diet to improve performance, nor that these positive impacts are sustainable in the long term (over 5 years or more). In fact, there is a body of research that shows low carbohydrate and ketogenic diets lead to poorer outcomes in cholesterol over longer periods of time. Studies on low carbohydrate and ketogenic diets often are limited in quality due to higher dropout and noncompliance rates among participants, suggesting these dietary interventions are unsustainable long term.
There is research that suggests the keto diet can decrease performance in endurance athletes and that any potential benefit does not outweigh potential risks when compared with an evidence based moderate to high carbohydrate diet. Limited research has been completed on female endurance athletes.
The takeaways? There is evidence that shows going keto as a runner won’t make you a better runner when compared with following a less restrictive traditional moderate to high carbohydrate diet, and there is evidence that shows that it may actually make you a worse runner (measurements used in research to assess performance: VO2max, peak power, RPE, TTE, race times).
***If you have certain medical conditions, such as diabetes or polycystic ovarian syndrome, there is some short-term research that shows that going low carb or keto may help improve biomarkers observed in these medical conditions. There is lacking long term evidence and no evidence in runners with these conditions to show continued benefit, which may suggest it won't help you long term. Going keto may be too restrictive to be sustainable long-term when considering athletic performance.
Elite Athletes and Exogenous Ketones
Many elite athletes are starting to mention taking exogenous ketones in their training and on race day to improve their performance in long endurance events like the half and full marathon. It is important to note that while we may not be aware of exactly what these athletes consume, most of these athletes appear to not follow ketogenic diets, but just choose to include exogenous ketone supplements into their fueling strategy. While research is very limited on this new potential ergogenic aid, there are some interesting findings.
One study using a product that costs ~$28.33 USD per serving to support someone who is 150lbs or less showed that when carbohydrate intake during and following long efforts was adequate, adding the exogenous ketone product improved glycogen synthesis (IE: the ability to restock glycogen stores) by 50% due to increased insulin levels. Limitations of this study, like its findings, are significant. The study conditions were heavily controlled in a lab setting. Male participants were given glucose intravenous solutions post exercise with ketones rather than actual food, which may be challenging to create outside the lab for the everyday athlete. Furthermore, ketones may be expensive to be used properly, which is taking them before, during, and after exercise (3 servings costs about $85 USD for this particular brand, and more servings are needed for those who weigh more than 150lbs).
Another study looked into if ketone ester supplements can help male athletes prevent overtraining and recover well from their training. Their overall findings were that ketone ester supplements coupled with adequate carbohydrate intake could improve biomarkers associated with overtraining; however, there are many limitations to this study, such as that overall nutrition outside the study was not controlled for between the control and experimental groups. They found that the group who received ketone esters also tended to eat more carbohydrates than the control group, which may contribute to improved recovery on its own. While they acknowledge this and reference an unpublished study they conducted on females to support their argument, there is not enough substantial evidence to support their discussion nor negate the positive effect eating more carbohydrates has on performance and recovery. They found that ketone esters instead of ketone salts decrease instances of ketone-related gastro-intestinal issues in athletes. This study also implemented supplementing with ketones three times daily, which may be unrealistic financially for everyday athletes.
The takeaways? When coupled with adequate carbohydrate consumption, exogenous ketone supplements may improve recovery and glycogen storage efficiency in male athletes. However, many endurance athletes are not eating the recommended amount of carbohydrates. Addressing the optimization of overall caloric needs to support performance should prelude experimenting with ketone supplements, which are costly to an athlete's ergogenic aid toolkit.
Should runners go keto or use ketones?
No, there is no research that supports making widespread recommendations that runners will benefit from going keto. Runners should focus on fueling their bodies adequately on a day-to-day basis and implement evidence based “long run” nutrition recommendations.
"As a former "keto" runner, I can say my running took off as soon as I quit keto. It was not aligning with my goals. I was stagnant by restricting calories and felt terrible. Now, I no longer restrict carbs. I've seen insane gains in performance and have shaved 25 minutes off my previous half marathon PB."
~ Kendall (Pleasant Ridge, MI)
Bailey, C.P., Hennessy, E. A review of the ketogenic diet for endurance athletes: performance enhancer or placebo effect?. J Int Soc Sports Nutr 17, 33 (2020). https://doi.org/10.1186/s12970-020-00362-9
Batch JT, Lamsal SP, Adkins M, Sultan S, Ramirez MN. Advantages and Disadvantages of the Ketogenic Diet: A Review Article. Cureus. 2020;12(8):e9639. Published 2020 Aug 10. doi:10.7759/cureus.9639
Bob Murray, Christine Rosenbloom, Fundamentals of glycogen metabolism for coaches and athletes, Nutrition Reviews, Volume 76, Issue 4, April 2018, Pages 243–259, https://doi.org/10.1093/nutrit/nuy001
HOLDSWORTH, DAVID A.1; COX, PETER J.1; KIRK, TOM1; STRADLING, HUW1; IMPEY, SAMUEL G.2; CLARKE, KIERAN1 A Ketone Ester Drink Increases Postexercise Muscle Glycogen Synthesis in Humans, Medicine & Science in Sports & Exercise: September 2017 - Volume 49 - Issue 9 - p 1789-1795. doi: 10.1249/MSS.0000000000001292
Poffé C, Ramaekers M, Van Thienen R, Hespel P. Ketone ester supplementation blunts overreaching symptoms during endurance training overload. J Physiol. 2019;597(12):3009-3027. doi:10.1113/JP277831
Vitale K, Getzin A. Nutrition and Supplement Update for the Endurance Athlete: Review and Recommendations. Nutrients. 2019;11(6):1289. Published 2019 Jun 7. doi:10.3390/nu11061289