The ketogenic diet has been everywhere these days in the media, from pro athletes to health bloggers and beyond. If you take a look at Google Trends, you can see that it’s really exploded in popularity over the last year.
But when it comes to the science, does this diet hold up?
I’m hoping I can give you some insight into the evidence out there and explain why – as a sports RD – I do not personally promote the ketogenic diet for endurance athletes. Keep in mind I’m talking about chronically low carb intake, not occasional depleted runs (which I do think can have a role in training).
Let me start with a quick description of energy systems, because that shapes the theory behind the ketogenic diet.
When you are doing endurance exercise, your body is pretty much always using a mixture of fat and carbohydrate. Now if you’re doing a very short sprint of some sort, you’ll really only be using the carb-energy system. But for long distance running or cycling, you’re using a mixture of both.
Keep in mind that you cannot shift your body to only use fat. It is impossible. The body just doesn’t do that. I wish it did because we have way more fat than carbohydrate to use, but it just doesn’t work that way.
The theory behind going keto…
Proponents of the ketogenic diet claim that it should make you a more efficient exerciser and able to train longer. Their theory is that by eating high fat and low carbohydrate, you’re training your body to adapt to using fat rather than carbohydrate for fuel.
The substrate part of this statement is mostly true. When people eat an ongoing low carb diet and they test their RER – which lets scientists see a breakdown of what fuel is being burned – those people do burn more fat compared to those eating a normal carb diet. Of course, that doesn’t mean they use all fat. They still burn some carbohydrate as part of that.
What’s key here is the lack of evidence that supports this substrate change makes someone a better endurance athlete.
Just about all the (limited) research studies out there suggest that there is no change (or possibly a decrease in performance) when on a ketogenic style diet. So even though the body shifts the way it burns fuel, it does not translate to better performance.
Why? We don’t know the exact answer to that, but part of it may be related to changes in the muscle. It appears that eating a low carbohydrate diet affects an enzyme called PDH. PDH is an enzyme in the muscle that is involved in making energy from carbohydrates. This is down-regulated in fat-adapted athletes, which makes the body less efficient at carbohydrate metabolism.
In other words, even though the body is burning a bit more fat, it’s now become a bit more impaired at making energy from the carbohydrates it does have. In research, this has reduced sprint power performance. This means fat-adapted athletes will have trouble with maintaining intensity and the end-of-the-race sprint. These athletes are also known to experience an increased in perceived effort in some studies.
Even in low-carb athletes that are re-fed high carb diets for 24 hours prior to competition, it does not erase the effects of the previous low-carb diet. That reduced PDH activity still occurs.
The most recent research supports the conclusions that keto does not improve performance.
At the session I went to at FNCE presented by John Hawley, he shared the results of his new study (currently under review for peer-reviewed publication) which I think will be one of the best pieces of evidence that we have on this topic.
In this study, he recruited elite race walk athletes (think Olympic-level and other similarly talented). The athletes underwent intense training and were randomized to either a high fat, periodized, or high carb diet for several weeks.
The athletes on the high fat/low carb diet experienced an increase in the submaximal oxygen cost of walking at different speeds. That means that the effort it takes to walk at a certain speed increased after the high fat/low carb diet – but not after the other two diets. In addition, performance in a 10km race walk event improved for the two other groups, but did not improve for the high fat/low carb group.
This, along with previous research on the topic, suggests that there is no benefit to performance with the keto diet compared to a higher carb diet (all else equal).
Of course, one of the nuances in this topic is weight loss.
I’ve had several athletes tell me that they are passionate keto followers because they lost weight on it and now they perform better than ever. Well, yes – cycling and running at lower (but still healthy-for-the-body) weights will typically lead to better performance. You’ll commonly hear that a 10 lb weight loss can result in knocking off about 20 seconds per mile. That’s a simplification and the change in speed depends on several factors, but it’s a good generalization of the effect.
However, this doesn’t mean that the ketogenic diet itself created that effect. If someone had lost the same amount of weight while eating a standard balanced diet, they’d probably experience the same performance benefit because it’s most likely due to the weight loss, rather than the keto diet.
Heck, if I could design my own study, I would love to take people who have lost a significant amount of weight on keto and compare their performance on that to switching back to a calorie-controlled carb-rich diet. Something that would maintain their new weight but boost their carb intake. But that’s a story for another day when someone wants to put me in charge of a research lab 😉
Long term side effects outside of athletics?
Last but not least, I worry about the potential long term effects of a ketogenic diet – namely the lack of plant-based foods in the diet which we know have a large role in overall health promotion. Think vitamins, minerals, phytochemicals, and fiber. For example, lack of fiber in the diet could have significant impacts on digestive and gut health. Emerging research regarding the breakdown of fiber into short-chain fatty acids by gut bacteria may play a role in a whole host of beneficial health effects in the body, from reducing anxiety to cancer prevention.
OK, really the last but not least point – I’m a firm believer in doing what you feel works best for your body. If that’s keto, then more power to ya. My concern is when media and sensationalism take over and try to denounce the research we do have on a standard sports nutrition diet, so I’m hoping this post will clear things up for you.
Here are a few great review articles you can take a look at if you want more info:
Phinney SD. Ketogenic diets and physical performance. Nutrition & Metabolism. 2004;1:2.
*Interestingly this is actually a pretty pro-keto review article, but it still says the same thing – performance does not get better but it’s possible it might be able to be maintained. It also suggests some interesting concerns to keep in mind if you do decide to do keto, including adequate protein intake and mineral intake.
Share with me: What do you think of the ketogenic diet? Are you a carb lover or hater? Do you find food affects how you perform?
- Pesto Burrata Toast - April 17, 2024
- Healthy Strawberry Shortcake - April 15, 2024
- How to Break in New Running Shoes (And Do You Even Need To?) - March 7, 2024
Fiona @ Get Fit Fiona
Can I just say that it’s refreshing to read something related to nutrition online that’s backed up by science and research? I took an exercise physiology class last year and we were taught the same thing.
Chrissy Carroll
Love love love this comment! That’s my biggest pet peeve in the online world these days – articles not based in science. I am all about differing interpretations and opinions of the research, but not about anecdotal stories based on one person’s experience when it comes to recommending extreme diet choices.
Kim Kremer
I agree! I’m a firm believer that we’re all the best judges of what works for us, but I was empirical evidence, not “This worked for me so you should try it, too.”
David @ Spiced
Wow, this is super interesting, Chrissy. Thank you! I’ve been hearing about the keto diet, and I had to go Google it not too long ago just to understand what it is. I love that you come at this from an academic perspective. It totally resonates with me. I’m not a long distance (or even a short distance…hah) runner, but I do appreciate this post. 🙂 So thank you!
Chrissy Carroll
Thank you so much for popping over to read this! I’ve been hearing about it so much these days; I was really happy there was a continuing ed session on it at my nutrition conference last week to confirm my own research/opinions.
Mikki
I need to send this to my son. He lifts a lot and eats a lot of animal protein and low carbs. We’ve talked about it not being healthy for him but this would explain it a lot better than I do.
Great post.
Chrissy Carroll
Glad you found it helpful Mikki!! Strength is a little different than endurance athletes, but adequate carbs are definitely important for heavy lifting too.
Janelle @ Run With No Regrets
I’d heard a little bit about the keto diet, seemed like the next trendy thing, but I had no idea that it doesn’t really work as advertised. Thanks for the info! I’ve never been a fan of restrictive diets, especially the long-term effects.
Chrissy Carroll
I feel like any diet that’s super restrictive is either going to a) be realllly hard to follow long term, or b) set you up for nutritional problems long term, even if it’s easy to follow. There are a few cases where keto can be useful, but I don’t think endurance athletes (for the most part) is one of them.
Kim Kremer
Good read! Thanks for sharing!
Chrissy Carroll
You’re so welcome Kim – glad you enjoyed it!
abbey
Very insightful post, thanks for sharing this!
Chrissy Carroll
Sure thing, glad you found it helpful Abbey!
Karin Hosenfeld
Chrissy, Thanks for the great post! I was wondering what your resource was for the increase in Keto diets over the last year? I’d like to use it for a nutrition talk I’m doing on hot topics. Thanks!!
-Karin Hosenfeld RDN, CSSD ?
Chrissy Carroll
Hi Karin – Sorry for the delayed reply! That was anecdotal on my part – I’ve just seen a big increase in the number of clients asking about it and the portrayal of keto in sports nutrition media. I wish I had a better answer for you! 🙂
Rachel
Thank you for this info. My daughter is on a modified keto diet for medical reasons and I have been dipping my toes in to see if it would help my running performance. I did find it curious that the first 2 articles that were not supporters of MKD were funded by Gatorade and PepsiCo.
Chrissy Carroll
Hi Rachel!
Yes, there is definitely a place for keto diets in the medical realm. As far as running, I don’t think the overall body of research supports it (at least right now). I agree the the industry funding for the studies may complicate things, but my view is that sports nutrition research is not as likely to be paid for by national research grants or anything like that (unlike perhaps cancer prevention, diabetes, etc) – so industry funding *may* allow some studies that we might not otherwise see for financial reasons. That said, I understand the perceived conflict of interest there.
Here’s the link to the most recent study I mentioned seeing at FNCE which I think is the strongest of all the literature – https://www.ncbi.nlm.nih.gov/pubmed/28012184. This looks to be funded by Australian Catholic University and Australian Institute of Sport, so not industry partners in this case.
Of course, I just like sharing the info – it’s all about figuring out what works best for YOUR body! 🙂
Jennifer Stilger
Thank you for this article. I was actually contemplating trying this plan. My pal started high fat low carb January 1st and she seems to be doing really well with it. While I feel like I am constantly eating but still sluggish she doesn’t eat much on long runs and she seems to run on rocket fuel! I know I need to work on my nutrition big time and that will help. I feel better to sticking with improving my nutrition and not switching to keto after reading your SCIENCE and fact based research for endurance athletes. I’m finding all kinds of wonderful info on your blog…thank you so much.
Chrissy Carroll
Sure thing Jennifer! Glad I could help break down the science. I will say that I am all about individualized nutrition, so if someone is on keto and feels amazing – then more power to them. I just don’t think the science supports it (at least right now) from a performance standpoint. 🙂
Kaylyn
I know keto is not for everyone, but it has helped me a lot. In fact, I was not a runner at all until about a month into keto (last May). I started with the C25K app and have now run two 5k races and a 10k (and will hopefully be doing my third 5k this Saturday!) with no carb loading at all. I have tried a few carb ups in the past year, with healthy carbs like sweet potatoes, corn, and higher-sugar fruits, but they don’t do much for me, so I keep those to only about once every two weeks at most.
Again, everyone is different, but it seems that you are saying that those athletes for whom keto works are an exception, not the rule. I’d highly recommend The Art and Science of Low Carbohydrate Performance by Dr. Stephen D. Phinney and Dr. Jeff S. Volek. I just wanted to share another perspective, because some people really benefit from keto, not just physically, but also with the food freedom aspect of eating lots of healthy fats. 🙂
Chrissy Carroll
Hi Kaylyn! Always open to other view points here. I know several athletes on keto who really enjoy it! And if something works for someone, that’s awesome.
I’m about the research though – when it comes to endurance performance, there’s no compelling peer reviewed research (right now) to support *improved* performance. Equal – or possible worsened performance – there are studies that show that. Phinney and Volek provide interesting info but there are plenty of scientists and researchers that would argue counterpoints to them (as I’m doing right now, haha – but I’m a big fan of Asker Jeukendrup, who I mentioned in this post). And given that a true keto diet tends to be very low in plant based foods, and I do worry about long term implications of that on gut health and other issues.
I’ll argue the counterpoint of your food freedom aspect – I think keto can set up some strict restrictions that can backfire for *some* people.
So all that said – while I may not agree with the diet choice for athletes from a research perspective, I DO agree with every athlete finding what they feel is best for themselves. And if keto is best for you or anyone else, then more power to them! 🙂 Nutrition is definitely very individualized.
ivann de la torre
hello! Thank you very much for this article so complete and clarifying ideas! I see that it was written in 2016, now 6 years later, what would your opinion be? Is it still the same or are there new studies that are clearer? I am thinking together with my nutritionist to do my first ironman based on keto and fasting, I already did the 70.3 and it worked well. thank you!
Chrissy Carroll
Thanks for commenting! You’re right, this article is due for a refresher. *Disclaimer – below is for informational purposes only; this is not individual advice 🙂
I would say much of the information here is still applicable. There is not adequate evidence to support an improvement in training on keto; there are a few studies that did show benefits but also a few that noted reduced performance. There was an interesting study that came out in 2020 which compared a nutritionally balanced reduced diet with a cyclical keto diet; it found better impacts on exercise with the nutritionally balanced diet (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7551961/). The ISSN position statement from 2019 has a good section on keto including theorized benefits, what happens in practice, and potential risks (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6839090/).
My personal/professional opinion is that I do not see a nutritional or performance reason for most athletes to embrace a ketogenic diet when good performance and eating habits can be achieved on a far less restrictive diet. That said, if someone was eating a keto diet and feeling good, not having side effects, achieving performance goals, and working with a professional to ensure they’re meeting all nutritional needs – I also think that is completely fine.
IVANN
THANKS FOR YOUR ANWER! SALUDOS!!!!
Chrissy Carroll
No problem! Good luck! 🙂