Intermittent Fasting And Working Out: How To Schedule Exercise While Fasting

If you decide to take on an intermittent fasting diet, one of the first concerns you may have is how to exercise while fasting. Intermittent fasting and working out aren’t absolute incompatibilities, but working out while fasting certainly has its challenges.

So, what are the recommendations for working out while intermittent fasting? When should you exercise when intermittent fasting?

For example, is a 16/8 intermittent fasting morning workout better, or should you exercise in the middle or end of the day during your eating window?

In this article, we will discuss how to structure your workout routine when you are following an intermittent fasting diet and the important factors to consider with intermittent fasting and working out to help you minimize the challenges of exercise during fasting.

We will discuss the following: 

  • Can You Work Out While Fasting?
  • Tips for Intermittent Fasting and Working Out

Let’s get started!

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Can You Work Out While Fasting?

Before we go into specific recommendations for intermittent fasting and working out schedules and structures, it is helpful to first look at whether you can exercise while fasting.

The most important thing to establish is that everyone has unique metabolic and biochemical needs. What may work in terms of working out while intermittent fasting for one person may not necessarily be optimal, let alone safe, for another.

Overall, studies have found that exercising in the fasted state does not cause any significant differences in body composition changes compared to exercising in the fed state.

Furthermore, the terms “working out“ and “intermittent fasting“ are both extremely broad.

There are upwards of a dozen or more common intermittent fasting schedules, and you may have your own personal structure for intermittent fasting that deviates from even this large list of diet options.

Certain intermittent fasting diet schedules will pose much less of an obstacle to working out while fasting than others simply due to the fact that some intermittent fasting schedules are less restrictive than others.

A green clock and a plate of food.

For example, if you are trying to fit in exercise while fasting with a 14/10 or 16/8 intermittent fasting diet schedule, it will be easier to accommodate your workout routine given the relatively long eating window compared to a more strict intermittent fasting diet such as the 20/4 or the one meal a day (OMAD) diet.

Note that with intermittent fasting, the first number before the slash refers to the number of hours per day that you are fasting, and the second number refers to the number of hours that you have for your permissible eating window

The two numbers together sum to 24 hours because there are 24 hours in a day.

For instance, with the 16/8 intermittent fasting diet, you will have a 16-hour fast every day, and you will be allowed to eat over the course of an eight-hour window. The specific hours that you are fasting and allowed to eat are up to you.

For example, you might finish your last meal of the day at 7 PM and then fast for the next 16 hours, which would bring you to 11 AM the next day.

This means that you would be eating all of your intended meals and snacks each day between the hours of 11 AM to 7 PM and then fasting for the next 16 hours.

Similarly, “exercise“ and “working out“ can range from something as low intensity and gentle on the body, such as a walk or Hatha yoga workout, to vigorous or long endurance workouts like marathon training long runs, HIIT workouts, and resistance training with heavy weights.

Two people doing lunges.

Here again, it is theoretically much less of a challenge to do a low-intensity workout while fasting than a long-duration or high-intensity workout.

This is because the primary difficulty with intermittent fasting and working out is having enough energy to perform your workouts and being able to fuel properly beforehand and refuel afterward.

Of course, a longer or more intense workout will require higher fueling needs and refueling requirements.

Therefore, fasting will have much less bearing on an easy cardio workout, a short mini workout, or a slow walk than a long run, vigorous indoor cycling workout, or high-intensity weightlifting session at the gym.

There are several factors to consider when deciding how to work out with an intermittent fasting diet.

In terms of the exercise side of the equation, the type, intensity, and duration of your intended workout are the most important factors to consider.

People working out outside.

As mentioned, long and vigorous exercise will be more significantly impacted by fasting and either a lack of pre-workout fueling or post-workout fueling or exercising during the fasting window rather than the eating window.

The other factors to consider are the intermittent fasting diet schedule you are following, the logistics of your schedule with work/life relative to the number of hours you are fasting and eating, as well as your own personal preferences.

For example, it’s easy enough to make a blanket recommendation that if you are doing 16/8 fasting, you should do a 16/8 intermittent fasting morning workout.

However, if you have to get to work first thing in the morning, but you really aren’t a morning person, or you can’t exercise on an empty stomach without feeling like you have zero energy, this recommendation simply won’t be practical or effective for your needs.

In most cases, people who are trying to figure out when to work out while fasting are best served by experimenting with how they feel exercising on an empty stomach versus exercising at the end of the day when you can’t refuel afterward. 

Or, if you have a relatively lax intermittent fasting schedule with a long eating window and some flexibility in terms of your job schedule such that you can step away to exercise in the middle of the day, it may be best to exercise during your eating window rather than your fasting window.

Alarm clock, a plate and utensils.

Tips for Intermittent Fasting and Working Out

Here are some tips for intermittent fasting and working out:

#1: Try Different Intermittent Fasting and Workout Schedules

Experiment with working out at different parts of your fasting and/or eating window to see when you feel best

#2: Listen to Your Body

Even if you normally feel okay exercising while fasting, there may be days when your body has no energy in the tank, and you should either change when you are going to work out or alter the intensity, type, or duration of your planned workout.

#3: Use Electrolytes

During the fasting window, consider having electrolyte tablets or Himalayan sea salt with your water, particularly if you exercise during your fasting window and will not be able to eat right after your workout. 

This will help replenish electrolytes to minimize the consequences of your post-workout recovery due to delayed refueling.

A person holding battleropes.

#4: Document As You Go

Keep a workout journal to document how you feel exercising while fasting at different portions of your fasting/eating window and with different types of exercise.

You can be an experiment of one to help dial in the best time of day to exercise while fasting.

#5: Start Low and Slow

Whether you are new to intermittent fasting and are trying to layer this new type of diet onto your existing exercise plan, or you have been fasting for a while and want to start working out consistently, once you add the two variables together, you should build up slowly. 

If you’ve been exercising consistently, reduce the intensity and duration of your workouts at first to minimize the challenges of exercising while fasting. 

On the other hand, if you are just starting an intermittent fasting diet, rather than jumping right in with an aggressive intermittent fasting schedule like a 20/4, start with a 12/12 or 14/10 and gradually reduce your eating window so that your body can get used to daily fasting while working out.

Two people smiling and walking.

#6: Eat After Working Out to Build Muscle

If you are lifting weights to build muscle, make sure you time your workouts so that you can refuel afterward.

To support muscle protein synthesis, start refueling and rehydrating with at least 20 to 25 grams of protein in your post-workout snack and 60-80 grams of carbohydrates.

Lastly, exercising in the fasted state may not be safe for certain individuals, such as those who are pregnant, have an eating disorder, or have a metabolic condition.

Consult your physician before combining intermittent fasting and working out if you have concerns about the safety and best practices.

To learn more about specific intermittent fasting diets, check out our complete guide to 18/6 intermittent fasting here.

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Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a NASM-Certified Nutrition Coach and UESCA-certified running, endurance nutrition, and triathlon coach. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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