Sometimes, however, you may want to approach exercise with caution. Abstaining from food and drink for a set time — a practice known as fasting — is one of those instances.
Is It Safe to Work Out if You’re Fasting?
For the most part, it’s okay to work out if you’re fasting, “especially if you’re already generally healthy,” says William Dixon, MD, an emergency medicine physician and a clinical assistant professor at Stanford University in California. Certain populations should be careful, though. People with coronary artery disease (a condition in which the arteries struggle to deliver blood, oxygen, and nutrients to the heart), as well as those taking medication for hypertension (high blood pressure) and diabetes are the primary groups who need to be cautious. “Someone who has coronary artery disease should be careful with exercising in general, especially exercising too hard,” says Dr. Dixon, who is the cofounder of Signos, an app for continuous glucose monitoring. These folks are more susceptible to cardiac events when their heart rate goes up, and they should keep a close eye on exercise intensity. Fasting is a behavior that can make physical activity feel more strenuous, and therefore put people with these types of conditions at higher risk of running into complications than if they weren’t fasting. People with type 2 diabetes who take medications to increase insulin sensitivity or lower blood sugar levels, or those with type 1 diabetes who take insulin, run the risk of having their blood sugar drop too low if they go long periods without eating. This is known as hypoglycemia, and symptoms include shakiness, sweating, confusion, fast heartbeat, nausea, and headaches, according to the American Diabetes Association (ADA). Exercising in a fasting state may cause or worsen hypoglycemia in people with diabetes, Dixon says. Meanwhile, those who take blood pressure medications like beta-blockers may struggle to get their heart rate up, no matter how intensely they exercise. “They may be more prone to feeling light-headed, especially if they’re not drinking enough water,” Dixon notes. If you have one of these conditions, it’s important to talk with your doctor about how to safely exercise if you’re fasting (for either dietary or religious reasons), and if it’s safe to do so. Additionally, for anyone fasting and planning to exercise, it’s important to keep in mind that low-intensity and short-duration workouts are better options than longer or high-intensity workouts. You can use the rating of perceived exertion (RPE) scale to help you gauge your intensity, says Matthew Accetta, CSCS, an exercise physiologist with the Hospital for Special Surgery in New York City. The RPE scale runs from 0 to 10, where 0 corresponds to how much effort it takes to sit in a chair (none), and 10 is the effort it takes to complete an exercise stress test or other difficult activity (very heavy), per the Cleveland Clinic. Accetta recommends aiming for an effort level of 4 (somewhat heavy) or lower while fasting. “Activities such as walking, cycling, swimming, yoga, and Pilates are all great options.” It’s okay to work out in both heat and cold while fasting, “but precautions should be taken to ensure safety,” Accetta says. Be sure to fuel your body properly before starting the fast and limit your intensity (again, go no higher than a 4 on the RPE scale). You’ll also want to be sure to stay adequately hydrated and get plenty of sleep, Dixon says. “Both of those are important for safe workouts and recovery" — especially in more intense heat and cold weather. It’s important to eat a well-balanced diet once the fast is over, Accetta says. That means enough calories to meet your needs, and a full spectrum of vitamins and minerals, he says. “Your daily food intake should include complete proteins such as meat, eggs, and milk, or quinoa, chia seeds, and soy if you follow a plant-based diet, along with fruits, vegetables, and carbohydrates. Carbs should make up about 40 to 60 percent of your overall calorie intake, protein should account for 20 to 30 percent, and healthy fats (such as avocado, fatty fish, nuts) the remaining 10 to 15 percent,” Accetta says. A registered dietitian can guide you if you’re unsure where to start.
Does the Type of Fasting Affect Whether It’s Safe to Exercise?
Fasts vary; some last only a few hours and others span weeks. Spiritual fasting practices such as Ramadan, a holy month in the Islamic religion, specify that eating and drinking can take place only during set times (dusk to dawn) throughout the month, according to the Academy of Nutrition and Dietetics. Other eating practices, such as intermittent fasting, are done for health or weight loss reasons, per Johns Hopkins Medicine. Some of these fasting types may not mix well with exercise. In particular, fasts that stretch 24 or more hours, fasts in which you’re restricting calories and nutrients (such as a juice cleanse), and fasts where you’re not drinking any water are types of fasting during which exercise is unsafe. “The body hasn’t had a chance to ingest new nutrients and replenish its stores,” Accetta explains. At this point, your body is running on fumes. Not only will exercise be difficult, but it can also increase your risk of dizziness, fainting, nausea, and headache. Exercising while intermittent fasting is generally safe, Accetta says. Some people choose to restrict their eating to a single six- to eight-hour window each day, and fast for the remaining (plus or minus) 16 hours of the day. Meanwhile, other people prefer to eat normally five days per week and limit themselves to one 500- to 600-calorie meal for the other two days, notes Johns Hopkins. It’s generally fine to exercise when fasting before a medical procedure, provided you clear it with the doctor performing the procedure, Accetta says.
Tips for Working Out if You’re Fasting
Even if you take precautions, exercising while fasting can be risky. If you’re taking medications, have one of the health conditions mentioned above, or have another health issue that may interfere with your ability to safely exercise (or safely fast), talk with your healthcare provider before working out and fasting simultaneously, Dixon says. If you want to exercise during a fast, follow these tips for a safer, not to mention more comfortable, experience.
Watch your intensity. To conserve energy for the rest of your day, stick to lower-intensity workouts while fasting, especially if you’re exercising at the beginning or midway through your fast, Accetta says. Aim for an exertion level no higher than a 4 on a scale of 1 to 10, where 1 is resting and 10 is maximum intensity. That said, you can likely increase your intensity if the workout is short. “If you only have 20 minutes to exercise, you can probably do a higher-intensity workout and get similar benefits as a longer, lower-intensity workout,” Dixon says.Keep it short. Doing a low-intensity activity for long enough can make for a higher intensity workout. That means going for a three-hour walk while fasting may not be a good idea, even if the walk is low-intensity. If you normally work out for 30 minutes or an hour at a time, stick to the low end of that time frame if you’re exercising while fasting.Stay hydrated. Drink plenty of water while fasting and check your urine to gauge how hydrated you are (or aren’t): “The darker the color of your urine, the more dehydrated you are,” Accetta says. He adds that a minimum of eight glasses of water is recommended.Exercise at the beginning. If your schedule allows, try to work out toward the beginning of your fast. As you’ve recently eaten, your body still has plenty of stored nutrients to fuel the workout, says Mary Wirtz, RD, CSSD, a board-certified sports dietitian based in Colorado Springs, Colorado. This may make exercise more manageable than if you try to work out toward the end of your fast when your body is depleted of nutrients.Know when your body is telling you you’re overdoing it. Pay attention to your body’s signals, and stop exercising if you feel nauseated, light-headed, or have a headache, Wirtz says. You may also want to replenish with a small snack.
