Intermittent fasting and working out safely

The intermittent fasting fad hit bookshelves back in 2013 and has continued to grow in popularity since. 

An alternative to the tried and true “calories in, calories out” weight management method, intermittent fasting involves periods of eating followed by periods of fasting (Collier, 2013). 

Tons of information about intermittent fasting is out there. However, there’s no guarantee what you’re reading is rooted in science. Here’s what the science says about intermittent fasting and working out so you can keep up your routine while staying safe.  

In general, people can workout and fast at the same time. But you need to be in tune with your body as low blood sugar and dehydration can easily happen if exercising during a period of fasting. 

If you’ve never fasted before, keep in mind it takes anywhere from 3-6 weeks for your body to adjust to its new normal (Longo, 2014). It’s not uncommon to have a headache while your body adjusts to the fast, which may make it harder to exercise (Sanvictores, 2020). 

For some people, intermittent fasting can lead to undesirable side effects. You may feel irritable and frustrated, be bothered by hunger, and have less energy. All this combined can make workouts less pleasant (Patterson, 2017). 

If you experience side effects or are still adjusting to the fasting process, you can change when or how you workout.

Save workouts for windows where you’re eating, so you have more energy. Engage in lower impact exercises while fasting like walking, doing yoga, and stretching.

Regardless of when you choose to workout, you’ll still experience changes in how your body uses energy as this process happens more quickly if working out while fasting.  

Usually, your body uses the most accessible energy source. That typically comes in the form of glucose after you eat. 

When you fast, there’s less glucose available to use. If you haven’t eaten in 10-14 hours, your body undergoes a shift in its primary energy source—in this case, from sugar to fat.

After glucose in the bloodstream is used up, your body burns through any glucose stored in your liver. When you work out, your body needs glucose. If there isn’t any available, it switches to fat as an energy source (Mattson, 2018). 

This switch from sugar to fat happens around the 12-hour mark of a fast for those who don’t exercise. Conversely, people who exercise can decrease the time it takes to switch. 

Moreover, the switch that occurs during intermittent fasting and exercise has benefits like improving brain function and maintaining a healthy weight, which we’ll get into below (Mattson, 2019). 

There is some evidence that suggests working out while fasting can increase weight and fat loss. 

In one study, people who followed an alternate-day fasting schedule combined with endurance exercise for 12 weeks lost more weight and fat compared to those who just did one or the other (Bhutani, 2013).

As long as you’re feeling okay, you can work out during your fasting or eating period. 

If you’re experiencing signs of dehydration or low blood sugar—like headache, dizziness, weakness, and nausea—you may want to postpone a workout. 

When you fast, blood glucose drops quickly. This can lead to trouble concentrating, which may be exacerbated by exercise. Fatigue is also more likely to occur in high-intensity workouts. 

People who establish a workout routine before beginning a fasting program have seen improved tolerance to workouts as their bodies are more efficient at managing glucose levels (Zouhal, 2020). 

Limiting food and liquid intake inevitably leads to dehydration. Take breaks often and make sure to replenish fluids before exercising (Correia, 2020).  

If you have diabetes, make sure a healthcare provider knows you’re fasting as you may be at higher risk for side effects.

The easiest way to adjust to a new workout routine is to start with short, low-intensity workouts. Only increase intensity and duration if you can tolerate it. 

Since no two bodies are the same, there’s no ideal scientifically proven time to exercise. But there are ways to help you stay healthy, hydrated, and energized like the four Rs: rehydrate, refuel, recover, and rest.

Rehydrate

You lose lots of fluid when you sweat during a workout. It’s essential to replenish fluids both during and after physical activity. 

Water is excellent, but your body often needs sodium as well, which you can usually get from a sports drink. There’s no need to slam a bottle of Gatorade right after a workout—rehydration can take place between four and 24 hours after. Eating foods that contain salt can also help with this (Bonilla, 2020). 

If you’re hydrating during a fasting period, make sure you choose sugar-free options. You can even increase your fluid intake with liquid nutrition.

Refuel  

After you workout (especially when fasting), you’ll need to replenish low glucose stores with high-quality carbohydrates. The amount you need depends on the type of exercise. 

People who prefer endurance activities like running, biking, and HIIT classes need more carbs than those doing strength or resistance training. Here’s a rough breakdown (Bonilla, 2020):

• Low-intensity training, moderate duration: 5-6 g per kg of body weight

• Moderate to heavy endurance training: 8-10 g per kg of body weight

• High-intensity training: 10-12 g per kg of body weight

It may sound counterintuitive, but you can help your body by eating consistent levels of carbs before exercising. For example, runners might eat a carbohydrate-rich meal early in the day for a run in the evening. 

Pairing carbs with protein as a post-workout snack enhances recovery and helps with muscle repair. A bonus of eating high-quality carbs is that they can also help with sleep—more on that below.

Recover 

Most people need to eat 1.2-2 g of protein per kg of body weight every day. To put this in context, a 135-pound adult woman needs about 73 grams of protein daily, and one boneless skinless chicken breast is about 43 grams of protein. 

It’s best to spread your protein intake throughout your eating period, eating 0.25-0.40 grams per meal. After working out, your body can use protein in liquid form within an hour. In whole food form, it takes about two hours (van Vliet, 2018). 

Since liquid protein is used more quickly by the body, many people opt for protein shakes to recover after intense workouts. Whey, casein, pea, soy, and hemp are all popular protein powders you can mix with water or add to a smoothie. 

Good sources of protein include meat, seafood, and dairy products like yogurt and cottage cheese. Those on a vegan or vegetarian diet can fuel up on nuts, seeds, and legumes (Vliet, 2018).

Rest

Perhaps the best part of a workout is after when it’s time to rest. Rest not only helps you recover after intense workouts but can also prevent injuries during subsequent activities. 

What you eat before bed can enhance these effects. Opt for carbohydrate-rich meals, antioxidant fruits like berries, and protein sources high in tryptophan like milk, tuna, chicken, turkey, and nuts (Bonilla, 2020). 

Another strategy is to consume 30-40 grams of casein protein before bed to aid muscle recovery and metabolism (Kerksick, 2017). If you’re vegan, skip the casein protein as it comes from animal sources.   

Research shows it’s safe to work out while fasting, though it’s best to start slow and see how your body reacts. 

If you’re uncomfortable working out and fasting, shift workouts to your eating windows and focus on post-workout nutrition to optimize recovery. 

You may find that you meet your healthy weight goals faster if you add exercise to your intermittent fasting schedule. Plus, adding physical activity to your daily schedule is good for overall health and disease prevention. 

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Current version

May 06, 2021

Written by

Caitlin Knudsen, RN, BSN

Fact checked by

Yael Cooperman, MD

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About the medical reviewer

Yael Cooperman, MD, Ro

Yael Cooperman is a physician and works as a Senior Manager, Medical Content & Education at Ro.