Very Low Carb (Keto) Diet and the Endurance Athlete, Part 2
Written by: Stephanie Boville MSc, RD, Registered Dietitian and Sports Nutritionist
Welcome to part 2 of my exploration of very low carb diets for endurance athletes. My last blog provided the basis for understanding this blog as it explored how and when our body choses to use carbs vs. fats.
In this blog we will learn:
- Do low carb diets actually enhance fat metabolism?
- Does that actually makes us faster or perform better?
Does a Very Low Carbohydrate Diet (VLCD) increase fat burning capacity?
Short answer = yes. The body has no choice but to increase fat use to support the energy needs during exercise. Research clearly shows that after adapting to a keto diet for as little as 3 weeks, there is significantly higher rates of fat oxidation (0.6g/min vs 1.5g/min) during exercise. Fat oxidation at moderate intensities (65% VO2max) in elite ultra-endurance athletes on a keto diet contributed 88% of the fuel for exercise verses 56% in athletes consuming high carbohydrate diets. Now, if we remember what we discussed in the last article, we learned that as we increase intensity we increase the amount of carbohydrates burned. This begs the question “will those high fat oxidation rates continue at intense exercise (80% VO2max)?”. Another study investigated the fuel usage of elite race walkers at 80% VO2max, and they too found that fat oxidation was elevated to the same levels as previous research (1.5g/min).
Research Outcomes of VLCD and Performance:
The more important question in my mind (and likely yours as well) is “well that is fine and dandy- my body will burn more fat, but what will happen to my performance?!”. We will review some key research studies that have looked at fat adaption diets (high fat diets for 3-7days), keto diets and their effect on performance. The majority of high fat diet adaptation and keto diets find that performance decreased and a handful found no difference and only two articles find a performance benefit.
Keto, Training and Performance
Louise Burke et al. (2017) conducted a large study investigating the effects of a keto diet, chronically high carbohydrate diet or periodized carbohydrate diet on race performance of elite race walkers after a 3 week intervention and training camp.
Athletes on the keto diet perceived the training to be significantly more difficult and experienced an inability to complete the exercise training sessions planned. This is a very important point because if an athlete cannot train as hard as they could they won’t see much improvement in their performance.
After the 3 weeks of intense training, the keto group had higher fat oxidation compared to the two high carbohydrate groups. All groups had significant increases in their maximal oxygen uptake (VO2max) as a result of the training. As we discussed in the previous article, burning fat is less efficient and this study clearly demonstrated that at race speeds there was significantly more oxygen being used in the keto group and there was no change in the percent of VO2max at various speeds. The high carbohydrate and periodized carbohydrate groups used less oxygen and were able to keep the same pace at lower percent of VO2max. In plain English, the two carbohydrate groups improved their running economy and efficiency with the training where the keto group did not reap the benefits of the training because the cost of burning fat is so high.
Lastly, this study compared pre and post training performance walk times in a real 10km race. They found that both carbohydrate groups had a reduction in their time by 5-6% (on average 190s and 124s faster for high carbohydrate and periodized carbohydrate group). There was no improvement in the keto group and on average their times were 23s slower. There was a wide variability in performance for the keto group, ranging from 162s faster to 208s slower, meaning that keto worked for some individuals but not others.
High Fat Diet With Carbohydrate Loading
What if we don’t go into ketosis and we use a fat adaptation strategy + carbohydrate load, best of both worlds right? Havemann et al. (2006) showed that when elite cyclists consumed a high fat diet (68%) for 6 days with 1 day of carbohydrate loading that there was no significant difference in time to complete a 100km simulated bike race compared to a traditional high carbohydrate diet. However, if we look at the time to completion, we find that the high carbohydrate trial was completed on average 3 minutes 44 seconds faster (likely significant in the real world), leading us to believe that, on average, high carbohydrate diets may be superior to high fat diets. Again, 3 out of 8 racers on the high fat diet did improve their time compared to high carbohydrate diet, demonstrating that there may be some athletes who may respond well to a high fat diet.
More importantly, this research included 1km sprints throughout the ride to simulate a race-like situation and found that the power output was significantly lower in the high fat diet group which led to slower sprint times. Despite having lower power output in the high fat trial, they perceived they were working as hard as they were in the high carbohydrate trial. There was no difference in muscle recruitment during the sprints, meaning the high fat trial worked just as hard as the carbohydrate trial but did not achieve the same results in the sprint performance. The researchers thought that the high fat diet + a carbohydrate loading period would result in glycogen sparing due to increased reliance on fat for fuel, thus improving sprint times as sprinting relies on glucose to provide fuel. This was not the case and it is possible that high fat/fat adaptation diets reduce the ability to burn carbohydrates effectively.
- VLCD do result in higher rates of fat oxidation during exercise
- VLCD may reduce response to training
- VLCD decreases economy in elite athletes
- VLCD decreases ability to work at maximal effort which is important when there is change in work intensity- ie running up a hill, breaking away from the pack
- Most studies show that, on average, VLCD negatively affect performance in endurance athletes, however there are some that may respond well
- Remember that VLCD are not the same as training fasted or temporarily low carb diets to train your body to use fat more effectively, as this is an effective training method
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.
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