Have you noticed some changes to your typical exercise routine since starting the keto diet? Perhaps you're running an extra couple of miles without even realizing it, or maybe you hit a wall halfway through your favorite workout. The way your body responds to the keto diet is individual, but some distinctive shifts tend to happen as your body switches up the way it uses fuel.  

In this article, we'll explore how the ketogenic diet can impact your energy utilization, either boosting or inhibiting exercise performance. We'll also take a look at some of the unique metabolic benefits that happen when you combine keto dieting with exercise. 

Understanding What a Ketogenic Diet Is

The ketogenic diet is a low-carb style of eating that focuses primarily on fat and protein, allowing your body to reach a state known as ketosis.

What is Ketosis?

Ketosis is a metabolic state that your body shifts into when you are no longer providing it with enough carbohydrates to maintain its energy needs on glucose alone. To reach a state of ketosis, you must deprive your body of carbohydrates efficiently enough that both your blood glucose and muscle and liver glycogen are depleted. 

How does one accomplish this? When you dramatically reduce the number of carbohydrates you consume, your body will start to use the storage form of carbohydrates, known as glycogen, to pick up the slack. Eventually, your glycogen stores will also become depleted, leaving your body with only one option – start burning fat. As your metabolism shifts into fat-burning mode, it will produce ketones, which are the product of broken-down fat. 

Once you are adapted to ketones, and they are your primary source of fuel, you have entered ketosis.

What Is Keto-Adaptation?

While the process of moving from primarily burning carbohydrates to primarily fat for energy may seem pretty straightforward, your body actually requires a little time to make this adjustment. This adjustment period may last anywhere from a week to a couple of months, depending on your specific body and metabolism.

With that being said, most people report feeling keto-adapted within a couple of weeks of starting the diet. 

During the adaptation period, you may feel weak, tired, brain-fogged, fatigued, irritable, or achy. These are all symptoms of what is known as "keto flu," and they occur because your body is going through a significant shift. One of the biggest changes that happen during keto-adaptation is a transition in how your body uses fuel, which means that for a little while, your cells may not pick up energy as easily. 

Keto Diet Exercise: What Happens to the Body

As you shift into a state of ketosis, you may notice that your exercise regimen feels a little different. At first, exercise may be more challenging as your cells are still adapting to using ketones for fuel. However, once you're keto-adapted, you'll likely notice that certain types of exercise are much easier. Why? Because your body is now relying on a much more abundant fuel source. 

Keto Diets Burn Fat Faster

Generally speaking, your body can only store about 500 grams of carbohydrates or 2000 calories worth of energy. This means that if you're exercising and relying on glucose for fuel, there is a limit to what your body can provide. On the other hand, just one pound of fat contains 3500 calories, and most people have plenty of fat to spare. 

Picture a marathon runner, for example. If they're relying on glucose to fuel their run, they have to replete their stores every hour or so with glucose tabs, goo, or some other form of fast-acting sugar. Without this constant flow of glucose, they'll hit the all too familiar "wall" and start to bonk. 

On the other hand, research shows that once athletes are keto-adapted, their workouts are fueled primarily from fat, relying very little on their body's carbohydrate stores. This allows endurance exercise to become a lot easier as there is no longer the threat of bonking or depleting their fuel source.[1]

This is demonstrated in keto-adapted endurance athletes who are not only better able to meet their fuel needs but also experience enhanced endurance during their training.[2][3]

Keto Is Good for Maintaining Muscle Mass During Weight Loss

Another benefit of exercising while on the keto diet is that it tends to have a muscle-sparing effect, especially during weight loss. 

Typically on any weight loss diet, your body will burn more fat, but your muscle mass will also take a hit. On the ketogenic diet, however, research shows that athletes tend to lose more body fat while preserving their muscle mass.[4][5] 

This is an important factor as muscle is one of the most metabolically active tissues – meaning it helps you burn more calories. Therefore, a loss in muscle mass will result in a dip in metabolism, which is one of the reasons many people find themselves in a weight loss plateau. 

High-Intensity Exercises Might Be Tougher

While moderate-intensity endurance exercise tends to become much easier on keto, high-intensity exercise may take a hit. This is likely due to the time lag it takes your body to create ketones. 

When you're engaging in high-intensity workouts, your body requires a large amount of fuel in a short amount of time. Carbohydrates are very readily available, which makes them an excellent fuel source during high-intensity training. 

Unfortunately, ketones aren't as rapidly available as glucose, which means that you may find yourself fuel-deficient during a high-intensity workout – even if you're keto-adapted.[6] 

Your Body Might Feel Less Energy When Working Out

While some people will move into keto-adaptation and feel great using fat for fuel during their workouts, others may notice that although they can complete the workout, their energy isn't where it used to be. 

Some research suggests that using ketones as fuel requires the use of more oxygen, which isn't ideal.[7]

Furthermore, since ketones aren't as readily available as glucose, you may find that the intensity of your workout is beyond what your body can handle, even if it's not classically considered "high-intensity."

The good news is that even if you do feel less energy during your workout, most people bounce back with more energy once their body has a chance to catch up with more ketone production.[6]


The only way to be sure how a keto diet would impact your exercise regimen is to try it out for yourself. While most people do find that they have more success with endurance exercise, some high-intensity workouts may not pack the punch that they used to. 

Understanding how the keto diet will impact your body is a very individual process, but there are experts out there that can help you navigate the diet and set up the right exercise regimen for you. 

If you're interested in learning more about the ketogenic diet and how this way of eating could impact your exercise regimen check out BioCoach, where you will get access to one on one support for all your low-carb dieting needs. 


  1. Phinney, Stephen D., et al. "Capacity for moderate exercise in obese subjects after adaptation to a hypocaloric, ketogenic diet." The Journal of clinical investigation 66.5 (1980): 1152-1161.
  2. Volek, Jeff S., et al. "Metabolic characteristics of keto-adapted ultra-endurance runners." Metabolism 65.3 (2016): 100-110.
  3. Lambert, Estelle V., et al. "Enhanced endurance in trained cyclists during moderate intensity exercise following 2 weeks adaptation to a high fat diet." European journal of applied physiology and occupational physiology 69.4 (1994): 287-293.
  4. Paoli, Antonio, et al. "Ketogenic diet does not affect strength performance in elite artistic gymnasts." Journal of the International Society of Sports Nutrition 9.1 (2012): 1-9.
  5. Gregory, Rachel M., et al. "A low-carbohydrate ketogenic diet combined with 6-weeks of crossfit training improves body composition and performance." Int. J. Sports Exerc. Med 3.2 (2017): 1-10.
  6. Zinn, Caryn, et al. "Ketogenic diet benefits body composition and well-being but not performance in a pilot case study of New Zealand endurance athletes." Journal of the International Society of Sports Nutrition 14.1 (2017): 1-9.
  7. Burke, Louise M., et al. "Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers." The Journal of physiology 595.9 (2017): 2785-2807.

You may also like View all