What Happens To You When You Work Out While Fasting

If you're intermittent fasting, you might be wondering if you can work out before you've eaten. According to Prospect Medical, it's ideal to exercise while you're fasting for hormone optimization. Combining fasting with exercise increases your insulin sensitivity and growth hormone, which is key to staying lean and youthful.

A 2016 study published in the Journal of Nutrition and Metabolism found that fasting before exercise helps burn fat and reduces energy intake. Since your body uses up stored carbs while fasting, you'll be burning fat instead.

The best time to exercise is first thing in the morning — before breakfast, while you're still fasting. Working out in the morning helps support your natural wake-sleep or circadian rhythm. Exercising too close to bedtime can affect your deep sleep and REM sleep, which is essential for your health (via Prospect Medical). 

Prospect Medical recommends waiting two to three hours to eat after exercising, which can help you burn fat better. That might seem like a long time, but you'll be busy taking a shower, getting dressed, and generally getting ready for your day after you exercise. The time will fly by.

Your body may need up to six months to get used to working out while fasting, so don't start doing this before a competition. Some workouts are good to do while fasting, but some need caution. 

Jonathan Poyourow, RD, CSCS, told Women's Health that if you're exercising a lot and intermittent fasting, you need to drink a half-gallon to a gallon of water daily. He adds that if you sweat a lot, you might also take an electrolyte tablet to stay hydrated. 

Sprint training or HIIT are ideal during fasting. All of the benefits of sprint training and HIIT are amplified when fasting, including a boost of growth hormone, increased strength and stamina, improved brain function, fewer depression symptoms, increased testosterone, and enhanced body composition. You can further boost all of these benefits by delaying eating for two to three hours after working out.

You should be cautious of two types of workouts while fasting: weight lifting and cardio. You can do these workouts while fasting, but you should eat when your workout is done, to refill your depleted glycogen stores. Alternatively, you could eat before your workout, and then fast for two to three hours after (via Prospect Medical).