Working Out While Fasting: Is It Safe?

Fasting involves freely abstaining from food and/or drinks for set periods. People may fast for health, religious, or cultural reasons. Fasting windows can last from several hours a day to a few weeks. No matter your reason for fasting, research shows that this type of eating can help you lose weight, prevent disease, slow aging, and enjoy other health benefits.

As fasting has become more widespread, there is more interest in the effects of merging fasting and exercise. Research shows that fasting workouts may enhance weight loss and other benefits of fasting. However, the adverse effects of exercising on an empty stomach may affect the results of your workouts.

Exercising During Intermittent Fasting

Intermittent fasting (IF) is a type of fasting that involves an eating plan in which you alternate between eating and not eating for a specific period during the day or the week. It focuses on when to eat food, rather than what you eat.

Intermittent fasting can be classified into three categories:

• Intermittent calorie restriction (ICR): This is also referred to as whole-day fasting. ICR involves fasting for more than 24 hours two or three times a week. There is unrestricted food intake on the other days. Each fasting day is separated from the next cycle by at least one week.
• Alternate-day fasting (ADF): ADF involves alternating non-fasting days during which you eat at liberty, followed by fasting days when you consume only 25% of your usual dietary intake or no calories.
• Time-restricted feeding (TRF): TRF involves a set number of hours of daily fasting while eating is allowed during the remaining hours. TRF can occur in one of three variations: 16/8 (16-hour fast, eight-hour feeding window), 18/6 (18-hour fast, six-hour feeding window), and 20/4 (20-hour fast, four-hour feeding window).

Because intermittent fasting allows you to choose the days and times that you go without food, you can adjust your eating/fasting schedule to align with your workout schedule. If you align your fasting schedule with your workouts, you can ensure that you work out when you have enough energy to get through a workout and achieve optimal results.

Working out while intermittent fasting ideally happens when you have the most hydration in your body. When you have limited energy, such as during times of fasting, it is best to preserve it to avoid dehydration. If you decide to work out during your fasting period, you may have better results if you keep your exercise intensity low to ensure that you have sufficient energy to complete your workout.

Can You Exercise During a Religious Fast?

Most healthy people can exercise during any type of fast, whether you are doing it as a religious fast or for other reasons. However, your options may be limited based on the rules of your faith and what they permit during the fast.

Like other types of fasting, it’s always best to conduct your workouts during periods when you are not fasting so you can properly hydrate and restore lost nutrients. However, this precaution can make it more difficult to exercise during a religious fast that does not allow you to drink water, especially during a fast that lasts for periods of 24 hours or more, such as the Jewish fasting holidays of Yom Kippur and Tisha B’Av.

Pros and Cons of Exercising While Fasting

Pros

• Increased fat burn: There is some evidence that fasted workouts can lead to fat loss. In a fasting state, your body doesn’t have glucose to fuel your energy during a workout. So, it pulls fuel from fat stores and you burn more calories from fat.
• Improved insulin function: Results from a study suggest that training on an empty stomach improves insulin sensitivity and can help you improve blood sugar control. Insulin is a hormone your pancreas produces to help metabolize food and use it for energy.
• Increased autophagy: Exercising while on a fast may increase autophagy, a process that helps destroy unwanted, damaged, or unnecessary cells to regenerate newer and healthier cells. Autophagy also destroys disease-causing pathogens, like bacteria and viruses, that can harm cells.

Cons

• Impaired athletic performance: Research indicates that fasting while working out may affect the quality of your workout and decrease your physical performance.
• Slower recovery: Research indicates that the lower level of nutrients available during your fasting period can challenge your ability to recover optimally between exercise sessions performed while fasting.
• Dizziness and fatigue: Both intermittent fasting and working out can lower blood pressure. Combining the two may lead to lightheadedness if a drop in blood pressure occurs.

How to Exercise Safely While Fasting

Choose the Right Time for Your Workout

Working out before breakfast allows you to exercise during your natural intermittent fasting period, which begins when you eat your last meal in the evening and continues until you eat breakfast. Working out upon waking, or shortly after, supports your body’s natural circadian rhythm. Working out too close to your bedtime can interfere with deep and REM sleep.

However, to reduce your risk of dehydration, the best and safest time to work out is the time of day when your body is hydrated the most. When fasting, this would require exercising after your last meal at the start of your fast or after your fast ends and you resume normal eating.

Eat Healthy Before the Fast Begins

Eating healthy before your fast begins can help you feel better and maintain strength and energy during your workout. Follow these strategies to eat healthy before your fast begins:

• Drink at least 2 liters (8 cups) of water before your fast begins or during your fast if it allows for drinking water.
• Eat plenty of protein. Protein slows your body’s digestion so your food remains in your stomach longer and prevents you from feeling hungry.
• Eat healthy fats. Healthy fats like eggs and avocados take longer for your body to digest, stabilize blood sugars, and help control hunger.
• Eat high-fiber foods, which can help control blood sugar levels during your fasting.

Hydrate Before and After (And Watch for Dehydration Signs)

Consuming plenty of fluids before, during, and after a workout is the best way to prevent dehydration. Working out while fasting can create extra stress on your body during a time when you can’t replenish water and nutrients. This can increase your risk of dehydration because the level of fluids in your body is less than ideal.

When you work out, you lose fluids through your skin as you sweat and through your lungs while you breathe. Without adequate fluids, you have a decreased volume of blood and a higher risk of the following problems:

• Decrease in the amount of blood pumped with each heartbeat
• Inadequate oxygen supply to exercising muscles
• Inability to flush out the by-products of exercise as needed
• Poor performance as a result of exhaustion caused by inadequate blood flow and oxygen

Your hydration levels have a direct impact on the function of your muscles, nerves, and organs. Early symptoms of dehydration during a workout can include:

If you don’t consume fluids in time, symptoms of dehydration can worsen and include;

Modify Your Workout Expectations

Despite the potential health benefits, you may have to modify your workout expectations if you choose to exercise during periods when you do not eat. Prepare for the following consequences:

• You may experience fatigue faster during your workout.
• You may not gain strength because you need a surplus of calories for that to occur.
• You may lose muscle because your body may resort to using the energy stored in muscle tissue when energy from carbohydrates isn’t available.

Cool Down After Your Workout

Cooling down after a workout is important because of the risk of dehydration. Follow these strategies to help your body temperature return to normal:

• Rest in a cool, shaded area.
• Raise your legs to a level above your head.
• Remove extra clothing and equipment.
• Soak in cool water or spray yourself with water.
• Sit in front of a fan.
• Place ice packs on your pulse points like your wrists, neck, and temples for a rapid cooling effect.
• Drink sips of cool water or a sports drink if you feel like you can drink, are not confused, and are not nauseated.
• If you have symptoms of sports-related heat exhaustion, continue to cool off until you start to shiver.
• Watch for signs of changes in your normal heart rate, blood pressure, breathing rate, and mental status.

If you do not return to your normal body temperature and health within an hour or two of treatment, seek emergency medical treatment.

Listen to Your Body

Despite your best intentions, working out while fasting may not be the right fitness decision for you. Listen to your body and the signals it may send telling you to stop and rest during your workout.

Elevated sweat rates and inadequate fluid intake can lead to a state of dehydration or exercise-related heat exhaustion. These conditions can cause the following symptoms:

Choose the Right Foods to Break Your Fast

Choosing the right foods to break your fast can contribute to the results you achieve from fasting. Though you may have to follow traditions if you are fasting for religious or cultural reasons, easing back into normal eating and drinking starts best with a small, light meal. Avoid excessive sugar, calories, fats, and carbohydrates.

Consuming a large meal can overwhelm your digestive system and increase your risk of abdominal bloating (puffiness or fullness, often accompanied by gas), distension (increased belly girth), and discomfort. Better options for breaking your fast involve foods that are gentle on your digestive system and provide essential nutrients.

Examples of some of the best foods to break your fast include:

• Smoothies: Blended drinks are a good way to consume nutrients with lower fiber content than whole, raw fruits and vegetables.
• Healthy fats: Healthy fats—like nuts, avocados, and olive oil—provide sustained energy without overeating.
• Soups: Broth-based soups that contain lentils, pasta, or tofu provide protein with easily digestible carbohydrates. Avoid soups with raw vegetables or heavy cream.
• Cooked vegetables: Cooked, starchy vegetables like potatoes are a filling option that is gentle on your stomach.
• Dried fruits: Dried raisins, apricots, and dates are nutrient-rich options.
• Fermented foods: Unsweetened yogurt or kefir are nutrient-rich options that are gentle on your digestive system.

As you return to your normal eating pattern, choose nutrient-rich healthy options such as:

• Nuts and seeds
• Whole grains
• Raw vegetables
• Beans
• Poultry
• Fish
• Lean meat

Make Sure Fasting Is Safe for You

Though fasting can provide many benefits for most healthy adults, following a schedule of time-restrictive eating isn’t always safe for everyone. Consult with your healthcare provider about the potential risks and benefits of fasting based on your age, health, and medical conditions. Fasting may not be appropriate if you are:

• Under the age of 25
• Pregnant or breastfeeding
• Taking any medication with food
• Taking insulin or other medications to control diabetes
• Diagnosed with a seizure disorder
• Operating heavy machinery at your job
• Working nights

Summary

Though there is little conclusive research on the value of working out while fasting, researchers believe it is safe for most people. Blending these two practices may help you lose weight. However, it is unclear whether doing so is any better than calorie-restricted eating.

No matter what type of fasting you practice, your body’s response to a lack of fuel triggers a switch to using body fat for energy. Working out while fasting may help you reach this point faster. Though this state can yield benefits such as better blood sugar control, it may also interfere with your athletic performance.

Before you start working out while fasting, contact your healthcare provider to ensure that it is safe for you to pursue. Though it may have benefits, it may also cause harm to people with certain health issues.