You often hear about the Ketogenic (keto) diet when people are trying to lose weight; often times a significant amount of it. People will often boast about how they lost 10-15 lbs within the first month on the keto diet. For those who are accustomed to eating high-carbohydrate diets, this is normal and perhaps even expected. This is mostly the result of the release of excess glycogen and water that carbs often hold onto. Without the carbs, less water and glycogen is stored in the skeletal muscle tissue. 

But how does this affect one’s endurance performance? Is this an effective dieting methodology to utilize to prepare for competition? Is it an effective nutritional intervention whatsoever?


It’s Not So Simple

There are several studies out there that support the notion that it doesn’t make a difference where it counts. You’ll figure out what I mean by that in a minute.

In a study that tested ultra-marathoners and triathletes, they found out that after performing both a maximally graded incline treadmill test along with a 180 minute submaximal run at 64% of VO2 max, there were no differences between the high-carbohydrate and low-carbohydrate diet groups in the level of muscle glycogen in the muscle after exercise and even 2 hours post-workout [1]. 

Now, this occurred while the energy utilization completely differed between the two groups. In the low-carb group, fat oxidation (the process of fat being released into the bloodstream to be burned for use) was over 2x higher than in the high-carb group.

Okay, well they may have performed similarly, but the low-carb group lost more fat, right? Well again…

It’s not so simple!

Keto Diet

Effects on Body Composition

You’d probably be surprised to find out that following a low-carb keto diet won’t make you lose fat any faster either! To this date, all studies comparing keto diets to standard moderate carb diets, with protein and calories equal between the groups, have shown no fat loss advantage [2]. Researchers postulate that favorable fat loss advantages are often seen in those who follow the keto diet because they also inadvertently increase their protein intake [3].

However, there does seem to be an advantage that the keto diet has that standard diets don’t; it effects appetite regulation. Study after study has shown that individuals often unconsciously reduce their calorie intake when following a strict keto diet, as dietary fats have a greater effect on hunger than carbohydrates do, independent of protein consumption [4]. Because of this, it may be a smart move to utilize this type of diet if you constantly battle with hunger while dieting.  

Should You Even Bother?

Well. that’s a tough question. Because there’s actually some compelling research out there to prove that utilizing it to an extent may enhance performance. 

Without getting too deep into the physiological mechanisms of keto, basically, keto uses an energy source called ketones (once you’re adapted to the diet) instead of glucose for energy. Glucose, when converted to glycogen, is what is normally used for energy from carbohydrate. However, in its absence, it utilizes the next best thing; ketones, also known as ketone bodies. 

These ketones have become available in many sports supplements in the form of salts; often referred to as exogenous ketones. Researchers believe that while utilizing a moderate carb diet, supplementing with these salts may help to increase glycogen replenishment after exercise and help to promote skeletal muscle recovery [5]. In this scenario, we get the “best of both worlds” and we theoretically have energy coming from multiple sources, increasing our efficiency and output. 

But don’t get too excited just yet. These keto supplements are still relatively in their infancy stages on the supplement market. Therefore, more evidence is needed in order to make this a more definitive claim. 

Keto Diet

It Just Doesn’t Seem Meant To Be…For Now

With the current research we have out right now (which is surprisingly limited in athletes), the general trend seems to fall towards a negative impact in several training variables. These include heart rate, level of perceived effort, and overall training quality [6]. This seems especially evident when performing at higher levels of intensity, as one reaches closer to the muscle glycogen utilization threshold, where carbohydrate loading techniques would often be employed.

On the other side of the coin, there’s no one-size-fits-all answer here either. Some researchers have trouble understanding why some people are “high-responders” and “low-responders” to the keto diet. For example, a few of the individuals that were a part of a group of ultramarathoners appeared to perform better while on the keto diet. Researchers believe this to be because ultramarathons are more slowly paced than standard marathons, they rely more on fat stores, therefore, making it the superior fuel source [1]. But again, like with all things in research, this necessitates further examination. 

The Bottom Line

I hate to leave you all like this, but that is what I must do. I must leave you with an inconclusive ending.

There’s still much research to be done in this field. There simply haven’t been enough tests conducted and hypothesis’ tested in order to get an in-depth grasp of keto’s mechanisms on performance. But for now, it seems to be leading in the direction that carbs are still king. 

You may hear even more conflicting evidence in places such as social media, with advocates touting how great it is for performance. But remember, what works for them may not work for you. Hey, some people have actually performed better with it. Sometimes, you must treat yourself as your own test “subject” in order to see how these things affect you personally. Therefore, at least you can give yourself some data to work with!

Subscribe to the GC Coaching Blog

References

  1. Volek, J. S., Freidenreich, D. J., Saenz, C., Kunces, L. J., Creighton, B. C., Bartley, J. M., . . . Phinney, S. D. (2016). Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism, 65(3), 100-110. doi:10.1016/j.metabol.2015.10.028
  2. Aragon, A. A., Schoenfeld, B. J., Wildman, R., Kleiner, S., Vandusseldorp, T., Taylor, L., . . . Antonio, J. (2017). International society of sports nutrition position stand: Diets and body composition. Journal of the International Society of Sports Nutrition, 14(1). doi:10.1186/s12970-017-0174-y
  3. Soenen, S., Bonomi, A. G., Lemmens, S. G., Scholte, J., Thijssen, M. A., Berkum, F. V., & Westerterp-Plantenga, M. S. (2012). Relatively high-protein or ‘low-carb’ energy-restricted diets for body weight loss and body weight maintenance? Physiology & Behavior, 107(3), 374-380. doi:10.1016/j.physbeh.2012.08.004
  4. Sumithran, P., Prendergast, L. A., Delbridge, E., Purcell, K., Shulkes, A., Kriketos, A., & Proietto, J. (2013). Ketosis and appetite-mediating nutrients and hormones after weight loss. European Journal of Clinical Nutrition, 67(7), 759-764. doi:10.1038/ejcn.2013.90
  5. Evans, M., Cogan, K. E., & Egan, B. (2016). Metabolism of ketone bodies during exercise and training: Physiological basis for exogenous supplementation. The Journal of Physiology, 595(9), 2857-2871. doi:10.1113/jp273185
  6. Burke, L. M., & Hawley, J. A. (2002). Effects of short-term fat adaptation on metabolism and performance of prolonged exercise. Medicine & Science in Sports & Exercise,34(9), 1492-1498. doi:10.1097/00005768-200209000-00015

Avatar

About the Author Shayne Gaffney

Shayne holds a bachelors degree in Health Science in Professional Development and Advanced Patient Care, is a USA Cycling Level 1 (expert level) Certified Coach, a level 2 certified Training Peaks coach, a USA Cycling certified power based training coach, USA Olympic Committee Safe Sport Certified, and a licensed physical therapist assistant. He is also the creator of Zwift's "Build Me Up" Flexible Training Plan. He can be contacted directly via info@gaffneycyclingcoaching.com for any cycling or training related questions.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.