Very Low Carb (Keto) Diet and the Endurance Athlete, Part 3

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Written by: Stephanie Boville MSc, RD, Registered Dietitian and Sports Nutritionist

Welcome to the third part of the discussion focusing on ketogenic diets and whether they are beneficial for endurance athletes.

The first blog discussed the fuels we use during exercise, and especially intense endurance exercise. To summarize we learned that exercise at or around 75-85% VO2max largely relies on carbohydrates as the main fuel source. Carbohydrate breakdown is more efficient and can be more readily broken down compared to fat. We also learned that bursts of intense exercise (ie running up a hill) will increase that reliance on carbohydrates, existing ATP and phosphocreatine over fat oxidation.

In the second blog we learned that ketogenic diets do in fact increase fat oxidation during exercise but that it also comes with a cost! The costs include; suppressed ability to metabolize carbohydrates, decreased training response, decreased ability to work at maximal effort, and decreased running economy or efficiency. In general, most studies find that performance is reduced with ketogenic diets in endurance athletes.  However, some athletes do respond well, and there may be some sports (week long treks) that may benefit from keto adaptation.

In this last blog we will dive into whether or not ketone diester supplementation along with traditionally high carbohydrate diets can be helpful for performance in endurance athletes. Does it help to tap into the best of both worlds? Let’s look at the research!

Indoor cyclist

Study 1: Can ketone supplement help acutely during a cycling time trial?

The theory revolves around this question: Can an athlete ingest a ketone supplement that will force that individual into a “ketogenic state” temporarily without having a high fat, low carb diet?  With the ketone supplement absorbed in the blood stream, the body, in theory, will burn more fat without having to do a high fat, low carb diet.

Take for instance this study by Leckey et al. (2017).  In the study, 11 elite cyclists complete a 31.17km time trial that simulated the 2017 Bergen World Championship time trial course. They had optimal race nutrition strategies (high carbohydrate) prior to the trial and completed a placebo trial and a ketone diester trial. The ketone diester trial resulted in side effects for most athletes ranging from such severe dizziness, nausea and vomiting that one participant had to drop out, to moderate-mild nausea, reflux or minor discomfort.  Not good!

All participants completed the time trial faster and achieved higher power output in the placebo trial compared to the ketone diester trial. Further investigation is needed as these findings could have been a result of gastrointestinal discomfort. Fat and carbohydrate oxidation rates were not measured and therefore we cannot explain what happened metabolically but, due to the large amounts of ketones found in the urine, the authors speculated that the ketones were not used for energy production.

Study 2: Carbs vs. Carbs + Ketones

To play the devil’s advocate, let’s take a look at another paper.  Cox et al. (2016) conducted a study to show the effects of a carbohydrate + ketone beverage vs carbohydrate alone vs a control on endurance performance in a cross over designed study. They showed that consuming the carbohydrate + ketone beverage increased circulating ketones (no surprise), increased free-fatty acids (no surprise), decreased blood lactate levels, had a glycogen “sparing” effect (maybe good, maybe bad) and showed that about 10-18% of energy production came from ketones during exercise.

This supports other theories and research findings that ketosis inhibits the use of carbohydrates as a fuel source, even if it is available for use, and decreases ability to reach maximal intensity of exercise shown by the reduced blood lactate (the anaerobic by-product of glycolysis). The body likely does this to protect and save the available glucose for the brain, as sugar is its preferred fuel source.  Not good if the goal is to go fast!

We have also discussed why shifting to fat as a major fuel source can decrease our economy and efficiency in the previous blogs. However, this study found that there was a 2% increase in performance with the carbohydrate + ketones. That being said, in the hour steady state ride and 30 minute time trial, there was no carbohydrate intake, which very likely could have increased performance vs the ketone drink prior to exercise as glycogen stores can be limiting after 1h of exercise. Therefore, more testing is necessary to compare the high carbohydrate + ketones vs high carbohydrate + carbohydrates during exercise to determine the efficacy of using a ketone diester supplement.

It would also be interesting to see the impact of long term use of a ketone supplement throughout a periodized training plan that utilizes optimal levels of carbohydrate consumption. Would this allow for enhanced fat oxidation while also reaping the benefits of ingesting sufficient carbs?

Practical Applications:

Based on studies like the ones above and in my previous blogs, in general, the best practice for endurance athletes is to:

  • Continue with a high or periodized carbohydrate fueling regiment compared to the ketogenic diets. This way, when athletes are fuelled with carbohydrates (the fuel used for high intensity endurance athletes) they are more equipped to train harder, longer and reap the benefits from all of their hard work on the road, track or treadmill.
  • Remember you are still burning some fat! Athletes don’t need to be too worried about maxing out their ability to metabolize fats because most athletes have a very active and healthy metabolic system and are very efficient at burning fat as a fuel source. Therefore, you don’t have to risk decreased training intensity or injury due to inadequate carbohydrate intake while training.
  • It is also worth noting that some athletes may benefit from pushing their metabolic systems by doing a fasted run on their easy run days, one where you are not sacrificing quality or volume.  However, make sure you’re getting those important sessions without issue before you start adding these sessions!

Stephanie Boville dietitian GRSM Cambridge
Stephanie Boville MSc, RD
Registered Dietitian, ACcepting virtual appointments 

Stephanie is our Registered Dietitian and sport nutritionist. She graduated with Honours from the University of Guelph with a Bachelors of Applied Science specializing in Applied Human Nutrition. She then pursued her passion for sport performance nutrition by completing her Masters of Science degree specializing in Exercise, Nutrition and Metabolism at the University of Guelph. Here she was involved in studies investigating the nutritional adequacy of young hockey players and hydration habits of amateur, varsity and elite athletes to name a few. She then completed her internship at London Health Sciences Centre and is currently working there on the Medicine unit. She also has experience working with mental health and eating disorders. She also working towards being a Certified Specialist in Sport Dietetics.

Stephanie spent most of her childhood in the rink as a competitive figure skater, and later was involved in volleyball, track and cross country. During her university years she was drawn to lifting and has continued with this ever since. She is currently enjoying learning the art of Olympic weightlifting. Stephanie believes that every food fits in moderation and truly believes that nutrition has a huge impact on our sport performance and health.

References:

Leckey JJ, Ross ML, Quod M, Hawley JA, Burke LM. (2017) Ketone diester ingestion impairs time-trial performance in professional cyclists. Front. Physiol. 8 (806) 1-10.

Cox PJ, Kirk T, Ashmore T, Willerton K, Evans R, Smith A, Murray AJ, Stubbs B, West J, McLure SW, King MT, Dodd MS, Holloway C, Neubauer S, Drawer S, Veech RL, Griffin JL & Clarke K (2016). Nutritional ketosis alters fuel preference and thereby endurance performance in athletes. Cell Metab 24, 256–268.

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