• Weight Management
• Exercise

Working Out While Fasting: What Should I Know About Fasting and Workout Performance?

Medically reviewed and edited by Jessica Davis, FNP-BC

What is Fasting?

Fasting refers to voluntarily abstaining from food — and in some cases, drinks — for defined time periods.

People may fast for religious reasons, to reap various health benefits, to improve their focus and mental clarity, or to manage their weight.

We say that someone is in a fasted state if they no longer rely on digested food for energy but start burning body fat as fuel.

Most people enter a fasted state after going without food for around 10-12 hours, but some may experience it sooner.

Insulin levels drop significantly during fasting, which lowers the risk of certain diseases — especially diabetes and prediabetes.

Intermittent fasting — which is fasting according to a regular schedule — has become a popular choice for those wanting to lose weight, but there are various other types of fasting.

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What are the Benefits of a Fasted Workout?

Engaging in physical activity while fasting can be difficult — not everyone has the strength or determination to exercise on an empty stomach.

If you feel this strategy may suit your workout schedule and it’s something you’d like to try, speak to a personal trainer or health care provider.

Here are some benefits of working out in a fasted state:

• Improves fat oxidation: Fasting can promote body fat loss and weight loss.

• Increases mental strength: People who exercise in a fasted state typically require more motivation to get started than those who enjoy a pre- or post-workout protein shake when they train, so they develop mental stamina.

• More convenience: It can be difficult to remain consistent with your exercise efforts if you’re on a tight schedule, and working out in a fasted state can make your exercise routine easier. It means you can get your workout in without first needing to eat a meal or prepare a shake before or after your session.

• Enhances growth hormone production: Fasting during exercise increases growth hormone production, which can help the body build muscle and burn fat.

• Induces autophagy: This can be thought of as your body’s natural cleaning process where it breaks down old or damaged cells and allows the body to reuse them. Research suggests there is a link between fasting and regular exercise that can trigger this process.

Should You Be Exercising While Fasting?

Training while fasting increases your caloric deficit — which is an essential aspect of weight loss — but there are also downsides to exercising while fasting:

• Decreased exercise performance: Exercising on an empty stomach can negatively affect the quality of your workout. Some people report decreased performance when they exercise in a fasted state.

• Dizziness and fatigue: Low blood sugar levels during a fasting period can cause you to feel lightheaded or fatigued during exercise.

• Post-workout binging: Increased hunger and post-workout appetite could lead to overindulgence during your eating window.

• Potential muscle loss: Many people exercise to build muscle, but extended fasting and exercise without proper nutrition could lead to muscle breakdown.

• Compromised recovery: Your recovery after exercise can be negatively affected if you wait too long to have a meal or a post-workout protein shake.

• Carries high risk for certain individuals: Most of the research on fasting and exercise focuses on healthy subjects, but older people and certain individuals — like those with specific health conditions and eating disorders — may not be able to exercise without eating first.

Fasting and exercise — especially fasted cardio, which we’ll discuss shortly — have become very popular with those wanting to lose body fat and reap other health benefits.

If you’re unsure if exercising while fasting will work for you, but you’re intrigued by the potential benefits, schedule a consultation with a medical professional for more information.

What are the Limitations of a Fasted Workout?

Most of the studies conducted on fasted workouts explore the effects of fasted cardio — also known as aerobic exercise — and the results seem positive overall.

One study found that fat loss from fasted resistance training was equal to fat loss from fasted cardio, but more research is required on the long-term effects of fasted strength training.

Some limitations of a fasted workout include:

• Slower recovery due to lack of post-workout nutrient intake.

• Decreased mental clarity. Not everyone performs best when they’re hungry and your focus can be affected further if you’re exhausted, which also increases the potential for injury.

• High dehydration risk for those who are dry fasting (not consuming any liquids while fasting).

Exercise and Fasting Types: What Works Best if You Want to Work Out?

There are different fasting types, and the one you choose should complement your workout plan as well as your health goals.

For example, if you want to build muscle mass — which means your workout plan probably consists of resistance training — you’ll need to schedule your workout routine really close to the time you break your fast. This ensures that you get enough nutrients during your post-workout recovery.

If you’re doing high-intensity workouts, you’ll want to avoid 24-hour fasts as these would likely delay your post-workout meal and could result in muscle loss.

When you exercise in a fasted state, the body taps into glycogen — a type of glucose that’s stored chiefly in the muscles and liver — and eating after you’ve exercised helps replenish these stores and reduces your recovery time.

If you plan on exercising while fasting, you may want to consider water fasting — fasting that permits the consumption of liquid — as dry fasting can be challenging and dangerous to do if you’re working up a sweat in the gym.

Whether you’re trying to lose weight, maintain muscle mass, or simply lower your calorie intake and get healthier, there’s probably a fasting pattern that would suit your needs.

To ensure you exercise safely with intermittent fasting, get in touch with one of our healthcare providers for expert guidance and tips.

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How Does Fasting Affect Energy Levels?

People who are new to fasting may experience an initial energy dip, but as the body gets used to tapping into its glycogen stores and using fat as fuel, this will begin to normalize.

When you do intermittent fasting, it’s important to prioritize proper nutrition.

If someone fasts regularly for long periods and doesn’t consume balanced meals between fasts, they may well see a reduction in their body weight, but nutrient deficiencies can occur — and these may lead to other health conditions.

Exercise performance and recovery

There are several ways in which fasting can affect performance and recovery when it comes to working out.

Without a consistent intake of nutrients, especially after exercise, you may experience challenges with muscle repair and replenishing glycogen stores.

Protein intake is vital for muscle maintenance and growth, while carbohydrates are essential for restoring energy reserves.

Fasting can also affect your hormonal balance by reducing hormones like insulin and testosterone that play a role in muscle growth and recovery.

Research has found that fasting from dawn to sunset has the potential to optimize the immune system’s ability to fight viral infections like COVID-19, while results from a mouse study show that fasting for 24 hours could weaken the immune system.

Always remember that individual responses to fasting may vary. Some people may have an easier time exercising on an empty stomach, while others may feel sluggish and weak during an intense workout.

Strategies for Working Out While You’re Fasting

If you want to work out while fasting, there’s a lot to consider.

No matter which fasting method you choose, you’re ultimately skipping meals, so you’ll be engaging in physical activity with far less readily available fuel.

Think carefully about your fitness goals and what you want to achieve with your fasted exercise.

Choose the right fasting window

Some things to keep in mind when you choose the hours of the day when you’ll be eating include:

• Your daily schedule

• Your fasting days

• Your workout schedule

• When you’ll need to eat — either before or after you exercise — to maximize your results

Remember to ease into it and be flexible. It’s okay to adjust your eating windows until you learn what works best for you.

Focus on proper nutrition

As we said earlier, proper nutrition is crucial when you’re doing intermittent fasting. Whether you’re aiming to increase muscle mass, lose weight, or maintain your existing physique, you’ll need enough fuel to get the job done.

You’ll also want to opt for nutrient-dense foods and avoid consuming too many empty calories. Working with professionals like dieticians and personal trainers can help you stay on track.

Listen to your body

Your body will take time to adjust to your new eating window and you won’t be able to jump straight into intense workouts once you begin.

Your adjusted meal timing can also affect your sleeping patterns and mood, so pay attention to your body. Fasted workouts should not make you feel miserable or severely affect other aspects of your life.

For example, if you’re invited to dinner but the suggested meal time does not fall within your eating window, it’s okay to go out. Don’t be too hard on yourself and take time to enjoy the little pleasures in life even as you make changes to prioritize your health.

Drink enough water

Hydration is very important. If you’re doing dry fasting, be sure to drink enough water during your eating windows. Depending on your level of physical activity, this could be between eight to twelve glasses per day.

If you’re intermittently fasting and you drink throughout the day, opt for calorie-free beverages, like water, tea, and coffee. Keep in mind that drinking flavored sparkling water — even unsweetened options — can trigger cravings because of its natural flavors.

Ease into it

The best thing you can do when you start working out during intermittent fasting is to take it easy. Don’t swing for the fences and give up before you’ve even seen and felt the results.

A good way to ensure success on your health journey is to start with an overnight fast and then delay your breakfast the following day. This means no more midnight snacking, but it might be easier than going without food for the entire day.

Where Can I Learn More About How to Safely Exercise While Fasting?

Whether you’re just starting your weight loss journey, looking to increase muscle mass, or you just can’t seem to break through that weight loss plateau, LifeMD can help.

We provide online access to doctors and valuable resources about fasting, weight management, and exercise. Visit our portal and book your appointment to get started.

Jessica Davis, FNP-BC

Jessica earned her Bachelor of Science in Nursing from UMASS and her Master of Science in Nursing from the University of San Diego. Jessica is a happily married mom to a 4-year-old girl — and is passionate about animal rescue, travel, cooking, and outdoor life. She’s also a lover of exercise including half marathons, Lagree pilates, and Barry’s Bootcamp.

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