Surprisingly, studies have shown that after the body fully fat-adapts on a ketogenic diet, the body can replenish carbohydrate storage (glucose) through a process called gluconeogenesis, and it can do it in a way that doesn't use up any significant body proteins. So some weight lifters have been able to perform well and build muscle while on a keto diet. A keto diet is also well-suited to endurance athletes who do long distances like ultramarathons.

However, most people, including competitive athletes will prefer a non-keto diet. And the most competitive bodybuilders will likely want to supplement with extra carbs for best performance. This is because high-intensity exercise is best done with fast-acting fuel (carbohydrates stored as glycogen) to keep the muscles working hard. When glycogen stores are low, you will run out of energy quickly and exercise will be miserable or impossible to do. To fix that issue, you can use a Carb Refeed strategy. Flex Meals may help as well. Or avoid high-intensity exercise and resistance exercise altogether and stick to lower-intensity exercise like walking or slow endurance running.

The lower your calories, the less exercise you can handle. Generally, rapid weight loss (or very low calorie diets) have the greatest issues (see below). However, it is possible to run into trouble on even more moderate weight loss diets if you go crazy with exercise (see Too Much Exercise).

In contrast, you can handle almost any kind of exercise if you are in maintenance mode or have chosen slow weight loss tactics. Exercise is especially important to avoid weight regain. Be sure to read the Exercise and Weight Control page for general exercise information and suggestions.

Making Adjustments

Ultimately, how you feel (energy levels, exercise performance) combined with the results you're getting will tell you if you are exercising too much or if you need to eat more to fuel the exercise you're doing. If your weight loss stalls and/or you start to feel terrible then you probably need to make some adjustments to your diet or amount of exercise. See Too Much Exercise and Diet Vacations. and Troubleshooting.

Exercise on a Rapid Weight Loss Diet

On a rapid weight loss plan, your calorie intake is very low. Most likely the diet is also low carb or a ketogenic diet.

You have three options on a rapid weight loss diet:

No exercise. If you choose to not exercise, to avoid losing lean muscle mass you must consume protein or carbs at certain levels (see Protein and Carb Grams Per Day for formulas and calculations to use).
Low-intensity resistance exercise.
Easy walking.

Avoid:

A lot of aerobic exercise, especially if it is long in duration or high intensity. You are not consuming enough calories to handle it.
Strenuous resistance exercise.

Advanced Option: For serious athletes, there is actually a fourth option: doing high-intensity exercise in conjunction with a Carb Refeed strategy. However, it requires more precise timing and monitoring of calories and amount of carbs eaten.