Keto and Exercise Performance – A Look at the Research

As a way to lose weight and get healthier, keto is amazing. Many people regularly follow the diet and see amazing outcomes. But, one topic that often comes up is keto and exercise performance.

We’re taught that our bodies need carbs to build muscle and provide energy. Athletes often increase carb intake to maximize their performance.

This suggests that being on keto would be detrimental. Yet, research and theory don’t agree.

Instead, evidence suggests that you can maintain your exercise performance while in ketosis. You may even be able to improve it.

How’s that possible? To answer that question, this post takes a brief look at the underlying theory, along with the current research into the topic.

In a Hurry? Skip Straight to the Research!

Carbs do play a key role in muscle growth. For one thing, they promote the release of insulin. They also restore glycogen in the muscles. Glycogen is important – it’s what our bodies use for energy.

When you decrease carb intake, you lower glycogen too. That sounds like it should decrease performance.

But, glycogen is a secondary energy source. We don’t have to rely on it. Instead, our bodies can rely on fat instead, decreasing glycogen needs considerably. The little glycogen we do need can be obtained by breaking down protein. 

Decreasing carb intake may even be a good way to promote endurance, forcing the body to rely on fat for energy instead (1,2). This process also increases fat oxidation, which may provide benefits as well, while acting as evidence that fat is being used for fuel (3).

What about the insulin impact? As Bodybuilding.com points out, stimulating insulin like this doesn’t promote protein synthesis but it does decrease muscle breakdown. Likewise, carbs do have other impacts on promoting muscle growth and recovery.

Even so, none of those patterns make carbs essential. Instead, it’s entirely possible to build muscle while following keto. Our bodies have the ability to adapt and there are many alternative pathways that can be followed.

Such pathways can impact performance too. They mean that people may be able to perform as well, if not better, without relying on carbs (4). The prevalence of keto athletes highlights this point as well.

So then, what does the research say? How well do ketogenic athletes actually perform?

The popularity of keto has led to many different studies. This section examines the most significant of these, along with their implications.

Lambert, Speechly, Dennis & Noakes, 1994

This research looked at exercise performance of trained cyclists. Two groups were used, one on a high-fat diet (70% fat, 7% carbs) and the other on a high carb diet (12% fat, 74% carbs). Participants followed their respective diets for two weeks.

The authors looked at how the dietary differences impacted various performance outcomes, including muscle power and time to exhaustion.

For many outcomes, there was no statistical difference between the groups. This suggests that the high fat and high carb participants performed equally well. Both groups also had similar levels of muscle glycogen used, despite differences in carb intake.

But, during moderate intensity exercise, the high-fat group had longer exercise time to exhaustion than the high carb group. This is a desirable performance outcome.

Paoli et al., 2012

This study compared outcomes for eight male elite artistic gymnasts, who were around 20 years of age. In this case, participants followed a very low carbohydrate ketogenic diet (VLCKD) for 30 days, with outcomes measured before and after.

Three months after that protocol, the same participants followed a typical western diet for 30 days and the same details were recorded.

The authors found that there were no significant differences in physical performance between the two diets. However, participants did lose some weight and body fat during the ketogenic phase. There was also an increase in muscle mass but this change wasn’t statistically significant. 

Brinkworth, Noakes, Clifton & Buckley, 2009

This study examined the relationship between keto and physical performance, in the context of weight loss. In this case, there were 60 participants, with an average age of around 50 and an average BMI of 33.6 (which is considered moderately obese).

In the study, participants followed either a keto diet or a high carbohydrate and low-fat diet for eight weeks. The keto group lost more weight but other parameters were mostly the same between the two groups.

Even though the study didn’t consider athletes specifically, the conclusion is important. This suggests that people losing weight on ketosis can still engage in regular physical activity without any significant problems.

Volek et al., 2016

This study considered outcomes for 20 elite ultra-endurance athletes. One group of athletes was adapted for a low-carb keto diet, while the other was not. The participants underwent a three-hour run, with data being recorded before, during and after that event.

Participants adapted to keto had a much higher rate of fat oxidation, meaning that fat was the primary source of fuel. Yet, despite this, there were no significant performance differences between the groups.

This research is powerful, as it considers athletes who are adapted to keto, rather than monitoring people who are new to the diet. In fact, the participants on keto had been following the diet for anywhere from 9 to 36 months. This approach makes the outcomes a truer reflection on how keto can affect performance.

It also shows there may be a connection between the ketogenic diet and running. In particular, people can still run effectively while on a keto diet, without decreasing their performance. 

McSwiney et al., 2017

This research extended the work from the previous study, looking at the same two groups and participants. In this case, the study looked at outcomes across a 12-week period.

The longer period of time revealed some advantages to the keto diet, including improved body composition and performance.

The outcomes suggest that longer studies are needed to reveal the true implications of low-carb and ketosis for performance. Short studies simply may not have enough power.

Urbain et al., 2017

Here, the authors looked at outcomes for healthy adults. The study involved 42 participants, who went through a non energy restricted keto diet for six weeks.

The authors found a mild negative outcome on performance, including lower endurance capacity and power. Participants also reached exhaustion faster when on keto. Even so, the authors concluded that the effects were too small to be relevant in a practical manner.

It’s also worth mentioning that this study involved people new to the keto diet. These individuals wouldn’t have been fat-adapted and may have been experiencing some keto flu symptoms. This may make the negative performance outcomes irrelevant to anyone who has been on the keto diet for months (or longer).  

Wilson et al., 2017

In this study, authors compared a ketogenic diet to a traditional Western diet for 25 college-aged men. Participants were initially split into two groups and followed their designated diet from weeks 1 to 10. Carbs were then reintroduced from weeks 10 to 11 and participants then followed a resistance training program.

Key outcomes were as follows:

• Lean body mass increased in both groups from weeks 1 to 10
• Both groups experienced fat loss
• Only the keto group saw lean body mass increases in weeks 10 to 11
• Strength and power increased in both groups across the study, with no significant differences
• The keto group saw improved testosterone as a result of the study, while insulin levels remained the same

The study does reinforce the idea that performance on keto remains the same. Still, the conclusions are limited, as carb intake was increased when resistance exercise began.

Burke et al., 2017

The researchers here considered outcomes for elite race walkers, following three different types of diet. These diets are described below. Unless otherwise noted, the amount of each macronutrient was in grams per kilogram body weight per day (g kg^-1 day^-1).

• High carbohydrate availability consumed before, during and after training (Carbohydrate: 8.6 g, Protein: 2.1 g, Fat: 1.2 g).
• Same macronutrients as the previous diet but with daily amounts varying to create low-carb and high-carb days.
• Low-carbohydrate, high-fat every day (Carbohydrate: <50 g/day; Protein: 2.1g, Fat: 78% energy).

In this study, the researchers found that the low-carb high-fat diet did increase fat oxidation. However, the diet also increased the oxygen cost of the race walking, which had implications for performance.

In particular, the study involved two races, with intensified training between them. The high carb diet and the alternating diet both saw significant improvements in speed between Race 1 and Race 2. The low-carb high-fat diet didn’t provide any such improvements.

The authors suggested that the diet that alternated carbs offered the most advantages, without decreasing performance.

So then, how to these conclusions apply to the real world?

• It’s clear that people on ketosis can perform as well as those regular high carb diets
• In some situations, those on ketosis may have better outcomes
• The results are mixed. This suggests that some people may have better outcomes combining ketosis and exercise than others. This may be the result of the experimental designs or differences between people

Despite all of this, more research is needed. There are serious limitations with current studies and they don’t address all the parameters that we need.

• Many of the studies have been small, often involving 10 or fewer participants. They are frequently too short as well.
• Definitions of keto and/or low-carb vary considerably. This makes it tough to compare the outcomes to what you would experience personally.
• Many studies provide carb numbers in percentages only, so it's not possible to know whether participants were in ketosis or not. Some also never use the term ketosis, even though the macros suggest participants should be in ketosis. Likewise, ketone levels are often not tested.
• The groups used for comparison vary. For example, many studies compare a low-carb diet to a high-carb one. Yet, the high carb group sometimes involves an excessively high percentage of carbs.
• There is often no information about whether the diets compared had the same calorie intake or not.
• The strong differences between studies mean that there is little or no replication
• Many more types of exercise and groups of people need to be studied before we fully understand the implications of keto for exercise outcomes (5).
• Studies often don’t allow enough time for participants to become fat-adapted. This makes any outcomes questionable, as there is a transition period at the start of a low-carb or ketosis diet.
• Research outcomes are inconsistent. Some studies show a benefit to ketosis, others show no advantage.

Even so, the lack of research isn’t a reason to avoid ketosis if you’re an athlete. The only way you’re going to know whether it works for you is to try the idea out. We are all different after all. Even if the research was high-quality and amazing – it still wouldn’t answer every question about your own needs.

There are many authors who talk about this topic. Their posts give you the information you need to get started and show you the complexities that you need to consider.

One such author is Michael Joseph from Nutrition Advance, who highlights how being keto-adapted can offer performance benefits. His article discusses some of the recent research as well, along with additional considerations that you can make.

In a similar way, ruled.me talks about training on a ketogenic diet. That post aims to bust some myths surrounding keto and exercise, including the idea that you need carbs to build muscle. As that article points out, you can still build muscle while in ketosis and many people do.

And, of course, there are many ketoers who do actively exercise. Some of these individuals have a strong performance focus. The site Bodybuilding.com even goes into this in depth, including multiple articles on how to gain muscle and be effective while remaining in ketosis.

Overall, it’s clear that you can be an athlete while in ketosis. A keto diet may even increase your performance (6), especially if you plan well.

But, as many of the authors and studies point out, the process isn’t always easy. Exercising on keto does require additional considerations. You also need to pay close attention to your body, along with your energy levels and exercise performance.

You may also find the idea simply isn’t for you. There are many keto-adapted athletes out there, along with countless others who aren’t. There will never be a ‘one size fits all’ approach, nor should there be.

Still, it’s good to know that relying heavily on carbs isn’t the only way to go.

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