Keto and Running: Everything You Need to Know

This post will review the keto running trend, and discuss nutritional implications for the keto diet and running, keto running fuel and how the keto diet may affect your running.

Keto and running diet graphic

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What is the Keto Diet?

The ketogenic diet (often shortened to the keto diet) is a high fat, moderate protein, very low carb diet that people often carry out in order to reduce their body fat percentage and total body weight.

Historically, the keto diet was originally designed to reduce seizures in children suffering from epilepsy, it was not intended to be a weight-loss diet for the general public.

The idea is that with a very low carb intake (generally 20 to 50 grams per day), the body will switch from using carbohydrates as the primary fuel source to using fat (ketones) for energy.

Ketosis is a metabolic state in which your body uses fat for fuel instead of carbs. Many people wonder if running and ketosis can exist. Achieving ketosis can be difficult for some people because it requires strict adherence to diet for a period of time, and then to stay in ketosis, there is not much wiggle room.

While “keto running” and not relying so much on carbohydrates may be helpful for longer endurance events, low carb and running may also have its side effects as well.

What Foods Are Allowed on the Keto Diet?

The majority of foods consumed on a keto diet should be high fat, moderate protein, and they may not be what you initially think of as keto running fuel.

This includes:

  • meats
  • fatty fish
  • eggs (more on eggs for runners!)
  • butter
  • cheese
  • nuts
  • seeds
  • oils
  • avocados
  • non-starchy vegetables (they typically have a lower carbohydrate content)
  • herbs and spices
typical foods on a keto diet

What Foods Are Not Allowed on the Keto Diet?

Any food that contains carbohydrates is essentially off limits.

This is why many people have major issues with keto and running, since carbohydrates play such a large role in energy production and activity.

This includes:

  • grains
  • beans
  • fruits and starchy vegetables
  • dairy products
  • sweets and baked goods
  • candy
  • juice
  • gels
  • alcohol

Many runners rely on simple sugars, gels and sports drinks during longer exercise, so adopting a keto diet and running will take some practice and should require oversight from a professional.

Side Effects of Keto Diet for Running

A low carb diet for runners and keto are often synonymous but there are some differences between them in terms of the amount of carbs. 

Both may lead to adverse performances and side effects, too. 

Often referred to as the “keto flu,” there are some side effects you may experience, especially in the beginning as you begin to switch your diet over to keto.

A consumer reports study published in Frontiers in Nutrition studied online forums and symptoms associated with keto flu and found 256 symptom descriptions involving 54 discrete symptoms that people wrote about.

runner lying down on track

These may include poor energy and mental function, increased hunger, sleep issues, smelly breath, nausea, GI issues, and decreased exercise performance.

All of these can impact your running in one way or another, which we will review shortly.

The long-term effects of the keto diet on health are unknown at this time as more research is needed.

Does Keto Running Reduce Body Fat?

There is some evidence to suggest that the keto diet does result in reduced body fat, though the mechanism for this is unclear.

It is thought that it may be due to the restrictive nature of the diet. Since individuals do not have as many food choices or may not like the foods available, they may eat less overall than if they were not on the keto diet. Running and intermittent fasting has gained popularity also, for similar reasons. 

There is also evidence that the high-fat content of the keto diet results in increased satiety and decreased appetite, which may result in weight loss.

It is also thought that there may be an increase in energy expenditure (calories burned) when following the keto diet, though the reasons for this are unclear.

One thing that often occurs when body weight is reduced is a subsequent loss in fat-free mass (muscle mass). Research published in the Journal of the International Society of Sports Nutrition indicates that the keto diet can help prevent this loss of muscle mass, if a moderate to high protein intake is consumed.

However, it’s important to distinguish that using fat for energy does not always equivalent to burning fat.

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How Keto Running Can Impact Performance

While many athletes may be interested in following a keto diet for reducing weight, a keto diet for runners may impact or impair performance as well.

Sources of Fuel

We know that consuming a very low carb, high fat diet has an impact on the source of fuel your body has available and utilizes when exercising.

Typically, carbohydrates (glucose) are the primary fuel source for the brain and the muscles. Without consuming carbohydrates, the body has to switch to using fat (ketones) to fuel the muscles for exercise instead.

This is a result of hormonal shifts that result in higher fat oxidation (fat being used for energy) when following the keto diet for running. These hormonal responses result in higher fat oxidation and may also be a reason why individuals following this diet often show a decrease in fat mass.

Findings are mixed regarding improvements to running performance following a keto diet. The idea behind endurance runners trying out the keto diet is that the body would rely less on carbohydrate intake for fuel. 

Since the body can only store a certain amount of carbohydrates, athletes must consume carbs every 1-3 hours during exercise in order to replenish their carb stores and practicing carb loading.

In contrast, the body can store a large amount of fat that can be used as fuel in the absence of carbohydrates. The idea behind using fat stores over glycogen stores is that we have pretty much unlimited fat stores to use, whereas there is a limit to how much glycogen the body can store.

If we can train the body to use this fat more efficiently and rely on low carb running fuel, it may lead to longer bouts of energy without as much reliance on carbohydrates and glycogen stores.

Vo2 Max and Keto Running

One measure of performance that studies have looked at regarding the keto diet and running is maximal oxygen uptake (VO2 max). VO2 max is considered the gold standard for measuring aerobic fitness.

Higher levels of VO2 max indicate greater endurance capacity. So far, research studies measuring VO2 max outcomes have seen mixed results. Additional performance-related outcomes were also mixed.

man with mask testing vo2 max on treadmill

Recovery from Running

Keto diets have been shown to reduce lactate accumulation after exercise, contributing to enhanced recovery and decreased inflammation.

One study showed that the exercise-related rise in blood ammonia concentration was decreased in the group of participants on a ketogenic diet. Further research is warranted to determine the exact mechanism of action.

Current studies suggest that exogenous (supplemental) ketones may play a more prominent role in post-exercise recovery.

Adaptations For Running and Keto

Keto-adaptation tends to impair high-intensity endurance performance.

One study found that a 31-day keto-adaptation reduced running speed at VO2 max in trained runners and another found that 3 weeks of keto-adaptation during intensified training in elite race walkers negated improvements in a 10-km time trial performance compared with a high-carb diet.

Keto-adaptation may be ergogenic for prolonged, moderate-intensity endurance events limited by endogenous carb availability; however, not many studies have been done to determine the strength of these claims.

In terms of improving speed and PR’s, taking in carbohydrates is the best way to do that since carbohydrates are the quickest fuel source to be used and oxidized by the body.

Overall, the performance benefits of the keto diet and using fat as fuel are unlikely to outweigh high-carb fueling strategies, which can prevent bonking and hitting the wall in a marathon.

However, future research is needed.

ultra runner with backpack and mountains in distance

The keto diet can also lead to an imbalance of electrolytes, like sodium (very necessary if you’re a salty sweater), potassium and magnesium. Magnesium for runners, for example, is a very important micronutrient, involved in over 300 processes.

Vitamin C is also prevalent in fruits and veggies, which may be limited. Vitamin C for runners has many antioxidant properties.

What About Training Low?

Another similar thought to sparing muscle glycogen use is the idea of training low.

Training low means training on a low carb intake or low carb stores for energy. Research indicates that training with low muscle glycogen availability promotes molecular changes that enhance training-derived endurance adaptations.

Here are some food ideas for a low carb diets for running.

Typically, training low involves training on low carb intake but replenishing carb intake and stores prior to competition or race day. To date, there has been no detectable performance benefit from a low-carb intervention as a strategy before a competition or race performed with high carb availability.

In conclusion, if you are considering the keto diet for running, it is important to know all about all of the potential side effects and challenges, as well as have alternates for meeting your nutrient needs. 

Keto and running performance do not necessarily go hand in hand, but it’s not necessary for everyone to follow a vegetarian diet for running, either. Each person will find what works for him/her. 

Someone who prefers to eat real food for ultra running may do and feel better with that option. 

While running a keto marathon or following keto during marathon training is unlikely to yield any PR’s or improved race times, it may help someone feel better, have less GI upset during long runs, and feel more stable.

This may be a case-by-case basis and should be evaluated with a sports dietitian.

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  • Bailey and Hennessy (2020). A review of the ketogenic diet for endurance athletes: performance enhancer or placebo effect? Journal of the International Society of Sports Nutrition, 18:41.
  • Burke LM, et al. (2020) Crisis of confidence averted: Impairment of exercise economy and performance in elite race walkers by ketogenic low carbohydrate, high fat (LCHF) diet is reproducible. PLoS ONE 15(6): e0234027.
  • Coleman et al. (2021). Body composition changes in physically active individuals consuming ketogenic diets: a systematic review. Journal of the International Society of Sports Nutrition, 17:33.
  • Mansor, L. S., & Woo, G. H. (2021). Ketones for Post-exercise Recovery: Potential Applications and Mechanisms. Frontiers in physiology11, 613648.
  • Shaw DM, et al. (2019). Exogenous Ketone Supplementation and Keto‑Adaptation for Endurance Performance: Disentangling the Efects of Two Distinct Metabolic States. Sports Medicine.
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