Working Out While Fasting: Yes or No?

There is substantial debate about working out while fasting. The intersection of fasting, a practice often tied to religious observance, longevity, and weight management, with exercise raises many questions. 

Does it enhance fat burning? Could it deplete muscle stores or lead to better focus during workouts? As with most fitness strategies, the answer isn’t a simple “yes” or “no.” The mileage of working out while fasting can vary greatly depending on individual goals, body composition, and how you approach the practice.

In this post, we’ll break down the pros and cons of working out in a fasted state and take a look into the biochemistry behind what happens during fasted exercise. Let’s get to it!

What Happens to Your Body When You Work Out While Fasting?

Before choosing a side of this debate, let’s first wrap our heads around the science. Your body fuels physical activity through different energy systems, depending on the intensity, duration, and availability of fuel (food).

When you eat, your body stores glucose in the liver and muscles as glycogen. This is your primary fuel source for higher-intensity activities like weightlifting, sprinting, or intense cardio. However, when you fast, glycogen stores gradually deplete, forcing the body to switch its energy source to fat. This metabolic shift, called lipolysis, is where fatty acids are broken down for energy through a process known as beta-oxidation.

But the shift isn’t instantaneous. During a fast, there’s a delicate interplay between hormones such as insulin, glucagon, adrenaline, and cortisol. When insulin is low (which happens during fasting), the body becomes more efficient at accessing fat stores. 

Meanwhile, increased glucagon and adrenaline levels further drive fat breakdown. This can lead to ketogenesis, especially during prolonged fasts, where the liver produces ketone bodies from fatty acids to fuel the brain and muscles in place of glucose.

The biochemistry suggests that fasting could lead to more fat oxidation, but whether that translates to improved workout performance is another question altogether.

These are some of the processes occurring behind the scene: 

1. Fatty Acid Oxidation

In a fasted state, the body shifts from using glucose (from glycogen stores) to breaking down stored fat into fatty acids. These fatty acids then undergo beta-oxidation in the mitochondria to generate ATP (the energy currency of the cell). This process is slower than using glycogen but provides a more sustained energy source, especially for low- to moderate-intensity activities.

2. Ketone Production

When fasting is extended beyond 12-16 hours, the liver begins producing ketones from fatty acids. These ketones can serve as an alternative fuel source for the brain and muscles. Ketones provide a clean, efficient energy source that may help preserve muscle mass while continuing to burn fat during exercise.

3. Gluconeogenesis

During prolonged fasting or intense exercise, the body may run low on glycogen. To compensate, the liver begins to convert non-carbohydrate sources, such as amino acids from muscle protein, into glucose through a process called gluconeogenesis. This ensures that the brain and red blood cells have a steady supply of glucose, but it can also lead to muscle breakdown if fasting and exercise are not managed carefully.

The Pros of Working Out While Fasting

1. Increased Fat Burning

One of the most commonly cited benefits of fasted exercise is the potential for increased fat burning. With glycogen stores depleted, the body turns to fat as its primary energy source. Research has shown that fasting can increase lipolysis and the use of stored fat for fuel, particularly during low-to-moderate intensity exercise, such as jogging or cycling.

Who is This For?

This is great news for anyone whose primary goal is fat loss. Fasted cardio, in particular, is popular among bodybuilders and athletes looking to reduce body fat percentages before competitions. For those with a good base of fitness who are in a fat-loss phase, working out in a fasted state can help tap into fat stores more efficiently.

2. Improved Insulin Sensitivity

Exercising on an empty stomach has been shown to improve insulin sensitivity, meaning the body becomes more efficient at using glucose for energy when food is consumed later. Enhanced insulin sensitivity is beneficial for overall health and metabolic function, helping to prevent/manage diseases like type 2 diabetes. It can also support muscle recovery post-workout as nutrients are shuttled more efficiently into muscle cells.

Who is This For?

Individuals with insulin resistance, diabetics,  or those at risk of developing metabolic syndrome may see improved metabolic markers from combining fasting with exercise. For endurance athletes or those with specific health conditions, improved insulin sensitivity can also aid in long-term fitness.

3. Hormonal Benefits

During fasting, levels of human growth hormone (HGH) increase significantly. This hormone plays a vital role in muscle recovery, fat metabolism, and maintaining lean muscle mass. While the idea of working out in a fasted state can seem counterintuitive for muscle gain, the surge in HGH may help mitigate muscle loss during short-term fasting periods. HGH also assist with the process of lipolysis.

Who is This For?

Fasted workouts might be a good option for people who want to maximize fat loss while maintaining muscle mass, especially when paired with resistance training. Athletes aiming for lean gains can take advantage of this hormonal boost, particularly during fat-loss or cutting phases.

4. Mental Clarity and Focus

Many individuals report increased mental clarity and focus during fasted exercise, often attributed to the rise in norepinephrine and adrenaline during fasting. These hormones, alongside ketone production, help maintain energy levels and focus, even in the absence of food. For some, fasted workouts feel lighter and more focused without the heaviness or bloating that can come with pre-workout meals.

Who Benefits from This?

Busy professionals, those practicing intermittent fasting, or individuals who value the mental discipline gained from fasted exercise may find this benefit particularly appealing. It can also help people looking to work out early in the morning without the hassle of eating beforehand.

The Cons of Working Out While Fasting

Of course, fasted exercise isn’t all sunshine and rainbows. If this were the case there would hardly be debate around the subject.

1. Decreased Performance in High-Intensity Workouts

Fasted workouts can compromise performance, especially during high-intensity activities like sprinting, HIIT, or heavy weightlifting. Glycogen is the preferred fuel source for these exercises, and when glycogen stores are low, performance tends to suffer. Studies have shown reduced output in fasted conditions, particularly in sports that require explosive power or sustained high effort.

Who Does This Affect?

Athletes or individuals whose focus is strength, muscle gain, or performance in high-intensity sports might find fasted exercise less effective. Without readily available glycogen, strength and endurance can decrease, leading to suboptimal workouts and longer recovery times.

2. Risk of Muscle Loss

One of the biggest concerns with fasted exercise is the potential for muscle breakdown. When glycogen stores are low, the body can begin to break down muscle tissue to produce glucose through the process of gluconeogenesis. While this is more likely to occur in longer fasts (24+ hours) or during very intense exercise, it’s still a risk for those combining strength training with fasting over extended periods.

Who Does This Affect?

Bodybuilders, powerlifters, or anyone aiming to maximize muscle mass should be cautious about training in a fasted state. While short-term fasting may not cause significant muscle loss, consistent fasted workouts without proper nutrient timing can lead to accelerated catabolism over time.

3. Decreased Energy Levels

Many people feel lethargic or lack energy when they train without eating. While fat is a slower-burning fuel source, it may not provide the immediate energy needed for more vigorous exercise. This can make workouts feel more exhausting or lead to subpar performance, especially in the early stages of adopting to fasted training.

Who Does This Affect?

Individuals new to fasting, those who rely on intense cardio or endurance sports, or those with already low energy stores (e.g., due to a calorie deficit, for example) may find fasted exercise challenging. Transitioning into fasted workouts takes time, and energy levels may dip before the body adapts to the new routine.

4. Increased Cortisol Levels

Fasting and exercise both increase cortisol, the primary stress hormone. While some cortisol is beneficial for energy production, too much can lead to negative side effects such as poor recovery, impaired muscle growth, and fat retention (particularly around the abdomen). This is especially concerning if fasting is combined with high-stress exercise like endurance training or HIIT.

Who Does This Affect?

Individuals under high levels of stress or with existing hormonal imbalances might want to avoid fasted training, as elevated cortisol levels could exacerbate these issues. It’s essential to balance fasting, exercise intensity, and recovery to avoid burnout.

Who Is Fasted Exercise Best For?

Fasted exercise isn’t a one-size-fits-all solution, and its benefits largely depend on personal goals, metabolic health, and the type of exercise performed. Here’s a breakdown of who might benefit most from working out while fasting:

1. Endurance athletes who are looking to improve fat adaptation for long races or events may benefit from incorporating fasted low-intensity cardio into their training regimen. Fasted runs or rides at a moderate pace can help the body become more efficient at using fat as fuel.
2. Individuals aiming for fat loss, especially those already familiar with fasting or intermittent fasting, might find fasted exercise particularly effective for shedding stubborn fat, especially around the abdomen.
3. People with insulin resistance or metabolic issues may see improvements in blood sugar regulation and insulin sensitivity by combining fasting and exercise.
4. Busy individuals who prefer early-morning workouts and find eating beforehand a hassle might enjoy the simplicity of fasted training.

On the flip side, those focused on muscle gain, performance in high-intensity sports, or strength training may want to avoid fasted workouts, as the risk of muscle breakdown and compromised performance outweigh the benefits.

Maximizing Results with Fasted Workouts

If you choose to experiment with fasted exercise, here are some tips to maximize your results and minimize potential downsides:

1. Keep it low-to-moderate intensity: Fasted workouts are most effective for steady-state cardio or moderate-intensity resistance training. Save the high-intensity interval training (HIIT) or heavy lifting for after a meal when glycogen stores are replenished.
2. Time your workouts strategically: Exercising toward the end of your fasting window, right before you plan to eat, can help you refuel and recover immediately after your workout. This can help mitigate muscle loss and aid in recovery.
3. Use supplements wisely: Branched-chain amino acids (BCAAs) or essential amino acids (EAAs) can help prevent muscle breakdown during fasted workouts. Consuming these before or during exercise provides the body with the necessary building blocks to protect muscle tissue while still keeping the fast intact. Some people also opt for black coffee before fasted exercise, as caffeine can help boost energy levels and fat oxidation without breaking the fast.
4. Listen to your body: If you feel weak, dizzy, or excessively fatigued during fasted workouts, it might be a sign that your body isn't adapting well to training without food. Don’t push yourself too hard in a fasted state—start slow and gradually increase intensity as your body gets used to the practice.
5. Prioritize recovery: Fasting can stress the body, especially when combined with exercise. Make sure you're eating nutrient-dense meals post-workout to optimize recovery. Focus on a mix of protein, healthy fats, and complex carbohydrates to replenish glycogen stores and support muscle repair.
6. Experiment with different fasting windows: Some individuals thrive on shorter fasts (e.g., 12-14 hours) while others prefer longer fasts (16-18 hours or more). Depending on your workout goals, you may want to test different fasting periods to find the sweet spot for your performance and fat loss.

Final Words: Should You Work Out While Fasting?

Ultimately, the decision to work out while fasting should be made based on your specific goals, how you feel during fasted training, and whether the benefits outweigh any potential downsides. For some, fasted workouts can be a game-changer, while for others, they may be less effective or even counterproductive.

If you decide to give fasted workouts a try, approach it with patience and curiosity. Start with low- to moderate-intensity activities, monitor your body’s responses, and adjust as needed. As with any fitness regimen, consistency, recovery, and balance are key to long-term success.