Working Out While Intermittent Fasting

Intermittent Fasting Clock

Intermittent fasting (IF) and exercise are components of cultivating longevity, but should you combine them? Let’s review all the questions regarding exercising while fasting. We will explore the benefits of exercising in a fasted state, including the difference between cardio, sprint training, and weight training, as well as some simple tips on how to implement it to reap the most benefits of your fasted workouts.

Can I Exercise While Fasting?

Yes, it is OK to work out while fasting because the key to weight loss and muscle gain is not just calories and exercise, but hormone optimization. Studies demonstrate amazing benefits to intermittent fasting alone, but combining fasting with sprint training takes the benefits of each to a whole new level. Combining the two raises growth hormone and makes you more insulin sensitive, which is the key to staying youthful and lean. 

Many people obsess over calories in versus out, and fear muscle loss that theoretically happens when you exercise without having refueled. But when you understand the beneficial impact that exercising during a fasted state has on the body’s hormones, you’ll see that fasting and exercise aren’t only OK, they are actually the optimal way to boost your health and body composition.

Can I Exercise on an Empty Stomach?

Not only is it OK to exercise on an empty stomach, but it actually increases the benefits of exercise and of fasting. This is considered a multi-therapeutic approach, where the synergy of two things that are individually health promoting actually boosts each other’s benefits to a level that surpasses the level of each combined.

One of the best ways to take advantage of these benefits is to work out in the morning before breakfast.

Intermittent Fasting and Working Out

Working out before breakfast is another way of saying that you exercise during your intermittent fasting period. An intermittent fast is the portion within the 24-hour clock that your body goes without food (including when you’re asleep). The IF window begins when you consume your last bite or food or drink (other than water) before bed, and ends the following day when you take your first bite of food. 

To reap the most benefits, the length of your intermittent fast should be between 16 to 18 hours. For example, eat between the hours of 10:30 a.m. and 6:30 p.m. 

When to Work Out While Intermittent Fasting

The best time to work out while intermittent fasting is usually upon waking, or shortly after, to support the body’s natural circadian rhythm. Studies show that working out (or eating) too close to bedtime can interrupt levels of deep and REM sleep, so better save exercise for the following day.

Ideally, you don’t want to eat directly after a workout for the same reasons you’re exercising in a fasted state: hormone optimization. Studies show that even waiting two to three hours after a workout before eating promotes a rise in growth hormone, which helps make you a fat burner and replaces the used energy (sugar). Adaptation to the stress created from a high-intensity workout is the reason a hormone shift occurs. If your schedule only permits a lunchtime workout, you can exercise in your available time, and then benefit from the hormonal boost by refraining from eating until two to three hours after exercising.

Cardio and Intermittent Fasting

The hormonal benefits you get from exercising in a fasted state are related to the depleted muscle and liver glycogen stores that occur when you fast. Doing cardio while intermittent fasting is fine, but your performance will hinge on how fat-adapted your body is (how good it is at burning fat for fuel, instead of glucose). If you’re new to fasting and exercise, you can expect your performance to drop a little; it can take up to six months for some athletes to fully adapt their endurance to this new fuel source. For example, if you’re a competing athlete, and your race performance is your primary goal, don’t switch to fasted training a couple of weeks before a competition.

If you are doing cardio in a fasted state, avoid extending the fast post-workout, and opt to refuel after you finish.

Sprint Training and Intermittent Fasting

Sprint training or high intensity interval training (aka HIIT) involves intervals of intense activity combined with rest for about 15-30 minutes. Not only is sprint training time very efficient, but studies show that it provides health benefits that you cannot get from aerobic exercise alone, such as the tremendous boost of human growth hormone (HGH). Some of the benefits of sprint training include increased strength and stamina in the muscles and brain, increased growth hormone, improved body composition, increased brain function, higher testosterone levels, and less depression. Sprint training with intermittent fasting increase all these benefits. Sprint training is the ideal exercise method to incorporate into your fasted period, and to increase the benefits even more, you can continue to fast two to three hours post-workout.

Lifting Weights and Fasting

Lifting weights while fasting if also OK, but you need to be mindful of the role that glucose plays in repairing your muscles after a major weight-lifting session, especially while in a fasted state. When you exercise in a fasted state your glycogen stores are already depleted. If your workout for the day involves heavy lifting, you can do so in a fasted state, but you should prioritize eating a meal directly following your workout. Unlike a burst exercise session, heavy lifting puts enough stress on the body to warrant an immediate refeed. Like doing cardio session, lifting weights while fasted might decrease your strength in the short run, as your body adapts to being a “fat burner.” For this reason, you may want to save your weight-lifting sessions for periods after you have eaten (in which case you can fast for two to three hours post-workout), and incorporate fasted exercise on the days when you perform burst-style training.

To sum it all up:

  • Exercise while fasting is not just OK, it’s extremely beneficial for hormone optimization (which is the key to many health benefits, including improved body composition);

  • You can maximize the benefits of burst training and intermittent fasting by combining the two, for a multi-therapeutic approach;

  • Cardio and weight training can also be performed in a fasted state, but your performance may suffer slightly in the short term;

  • The best time to incorporate exercise while fasting is early in the day, to match the body’s natural circadian rhythm;

  • Unless you’re participating in a heavy weight session or endurance cardio, you can benefit hormonally from fasting after your workout, too (for two to three  hours).