When I first started considering a low-carb diet for weight loss, my biggest concern was that I would be able to continue running half marathons and similar distances, lifting weights with my personal trainer, and continue taking Hot Vinyasa Yoga classes. The book The Art and Science of Low Carbohydrate Performance by Phinney and Volek answered most of the questions I had. Here is some of what I found out:

• Keto Adaptation
• Fuel Tanks – Carbs vs Fat
• Increased fat-burn rate during exercise
• Deepens ketosis
• Intensity
• Muscle Retention and Fat Loss
• Other Benefits

Keto Exercise: Keto Adaptation

The first few days of adapting to keto are what’s known as the keto-flu. You will be fatigued, get headaches, generally just feel like crap, and really should not exercise. I went through this myself and even tried to run on the fifth day I was on the keto diet, but had to stop halfway through a 3mi run because I just didn’t have the energy to do it. A couple days later, when my blood ketone levels went over .5 millimolars, I was able to start exercising again. However, the full keto adaption – where your body becomes more efficient at burning fat as fuel – takes at least two weeks. So, you may be able to exercise again after a week, but you won’t be performing your best for at least two weeks.

Keto Exercise: Fuel Tanks – Carbs vs Fat

Our bodies are designed to be run on either fat as fuel, or carbohydrates (carbs) as fuel. When we are eating enough carbs – let’s say more than 50g of carbs a day (the exact amount varies from person to person and how active they are that day), our bodies default to carbohydrates as fuel, and suppress fat burning.

When you are running on carbohydrates as fuel, your body will store, at most, an extra 2000 calories of carbs as glycogen that is available for energy during physical exertions. This is what you use when you are running, doing yoga, lifting weights, etc. This limited amount of carb fuel in the tank is why long distance runners run into what is commonly known as hitting the wall, or bonking. This is when you run out of energy and have to start refueling while you run.

Now, in contrast, when you are in ketosis and fat is your fuel, your own body becomes a near-limitless source of fuel. Typically, you have more than 40,000 calories of fat fuel available in in your body, even in a very lean athlete. If you are overweight, you will have more than that.

I have personally found that I do not hit a wall when running keto-adapted, though my longest distance has been 13.1miles (a half marathon), and that I feel more energized in general during my various exercises. I also found an article of an ultra-marathon won by a low-carb runner. Ultra marathoner runners (in the book) also reported having less fatigue later in an event on a low-carb diet.

Keto Exercise: Increased fat-burning rate during exercise

In a keto diet, you will increase fat-burning during exercise. A study of non-keto-adapted athletes in 2005 showed a fat burning rate of 11g-60g per hour. A similar study of keto-adapted athletes in 1983 showed a fat burning rate of 74g-112g per hour.

Keto Exercise: Deepens Ketosis

Exercise during a keto diet will help deepen ketosis. What this means in practical terms is that your fat burning will be more efficient, and it will also allow you more leniency with the amount of carbs you can eat on your diet on an exercise day and yet still stay on ketosis. I have found this to be very motivating to keep exercising while on the diet.

Keto Exercise: Intensity

How intensely you are exercising will affect whether your body uses fat or carbs for primary fuel. On average, the peak rate of fat burning occurs at 50% of maximal oxygen consumption (VO₂ max) if you do not regularly train, and 65% VO₂ max if you are trained. However, that number can vary quite a bit from person to person. In a study of 300 people, some people hit peak fat oxidation at 25% VO₂ max and others hit it at 77% VO₂ max.

What this means for you is that if you are doing high-intensity exercises and want to do them on a keto diet, you should try to focus on getting your carbs for the day right before and perhaps right after you do a high intensity exercise.

Also, according to a personal experiment of one by Dr. Peter Attia, it does seem like a low-carb diet may reduce your max all-out capacity.

Keto Exercise: Muscle Retention & Fat Loss

These diets have also been shown to help you hold onto your muscle better than higher-carb diets, and lose more fat tissue.

I know I experienced this for myself. During my low-fat, reduced calorie diet I lost 34lb, and of that weight, I lost 8.72lb of lean mass, of which 4.15lb was muscle, and 25.48lb of fat. During my low-carb, loosely reduced calorie diet, I’ve lost 27lb, 1.5lb lost of lean mass, .66lb is muscle mass of that lean mass, and 25.68lb is fat.

Keto Exercise: Other Benefits

• Breathe easier during exercise.
• Low carb diets are anti-inflammatory, producing less oxidative stress during exercise and more rapid recovery between exercise sessions. 

Sources/Suggested Reading:

The Art and Science of Low Carbohydrate Performance by Dr. Phinney and Dr. Volek
How a Low Carb Diet Affected My Athletic Performance – by Dr. Peter Attia, M.D.
The interplay of exercise and ketosis – Part I – by Dr. Peter Attia, M.D.
The interplay of exercise and ketosis – Part II – by Dr. Peter Attia, M.D.