At a glance
Fasting and working out can be beneficial for metabolic health. While fasted exercise may accelerate fat oxidation and improve insulin sensitivity, maintaining proper hydration and a healthy lifestyle are crucial factors for supporting optimal physical performance, energy levels, and bodily comfort.
Many people worry that fasting and working out will slow results. However, when performed properly, being physically active while in a fasted state may support improved fat burning and metabolic health, especially when combined with efficient hydration and other healthy habits.
Let’s explore the benefits and possible drawbacks of fasting and exercise, and discover practical strategies to combine them to support your health goals.
What is fasting?
Generally, fasting is a practice of avoiding all calorie-containing foods or beverages for a period of time. It’s one of the oldest health practices in human history, used across cultures for both religious and therapeutic purposes.
While there are many ways to fast, most approaches fall into two main categories: intermittent fasting and prolonged fasting.
Intermittent fasting
Intermittent fasting (IF) is one of the most popular and accessible fasting methods. It involves shorter, flexible fasts that vary in schedule and intensity, making it easier to adapt to different routines, including time-restricted eating, alternate-day fasting, or 24-hour fasts.
One of the most common IF methods is time-restricted eating (TRE), which sets specific daily eating and fasting windows. The 16:8 approach, which includes 16 fasting hours followed by an 8-hour eating window, is among the most widely used TRE protocols.
Prolonged fasting
Prolonged fasting usually refers to fasts lasting 24 hours or longer. It’s often pursued by those seeking the deeper health benefits of fasting, such as supporting enhanced cellular renewal through autophagy.
“Fasting is the most powerful trigger for autophagy—your body’s built-in system for recycling damaged proteins, cleaning up old cells, and stimulating genes for long-term survival,” explains Dr. Berg.
Because prolonged fasting involves severe calorie restriction, it produces more significant metabolic shifts and requires careful planning—particularly around electrolyte intake, hydration, and how you reintroduce food.
For most people, starting with intermittent fasting and gradually extending the fasting window can offer a safe way to build up to longer fasts.
Watch the video below to learn the best approach to working out while fasting.
Potential benefits of fasted workouts
Any food consumed, especially carbohydrates, raises glucose levels and triggers the release of the hormone insulin, which helps shuttle glucose into cells for energy and stores the excess as glycogen and body fat.
When you stop consuming calories, insulin levels drop, signaling the body to shift from primarily burning glucose to utilizing more stored fat for fuel. This helps explain why many individuals choose fasting to support weight loss, as this metabolic shift has been associated with beneficial changes in body composition.
Research published in the British Journal of Nutrition shows that fat oxidation was greater during fasted exercise than fed-state exercise, particularly during aerobic activities, which include cycling, running, or walking.
A study published in Physiology & Behavior also found that intermittent fasting combined with resistance training reduced fat mass and body fat percentage without significant loss of lean mass.
Furthermore, exercise stimulates a protein transporter called glucose transporter type 4 (GLUT4), which moves glucose into cells for fuel even in the absence of insulin.
Because exercise also improves insulin sensitivity for hours afterward, combining fasting with physical activity may support weight loss by enhancing the body’s metabolic response to subsequent meals.
Possible challenges of exercising while fasting
While fasted workouts can offer metabolic advantages, they often require an adaptation period and can affect individuals differently based on lifestyle, dietary habits, and overall health.
Personal routines and professional obligations can make it difficult to maintain a fast until a workout. Many people fast from evening into the next morning, and those with busy schedules may struggle to fit in a fasted workout.
Fasting can be especially challenging for individuals accustomed to eating a higher-carbohydrate diet, which can intensify food cravings and energy fluctuations.
External factors such as poor sleep or high daily stress can make it harder to adhere to a fast. These stressors increase the release of hormones like adrenaline and cortisol, which can trigger blood sugar fluctuations and intense cravings.
For some, this heightened stress response may even hinder muscle building and prolong recovery times, making it essential to monitor overall stress levels when training fasted.
Finally, people with certain health conditions, including diabetes, a history of disordered eating, or blood pressure concerns, should discuss fasting and working out with a healthcare provider before making any changes to their routine.
How to approach fasting and working out safely
With the right approach, fasting and working out can fit well into a healthy routine, so long as it prioritizes efficient hydration and aligns workouts with the length of your fast.
Here are two primary considerations for fasting and working out safely.
Maintain proper hydration
During calorie restriction, the body transitions to using stored glycogen for energy. Because glycogen is stored with three to four parts water, its depletion triggers a significant release of fluids and the excretion of minerals, including sodium, potassium, and magnesium.
These electrolytes support cellular hydration, energy metabolism, and facilitate nerve signaling required for muscle contractions. This may help explain why fasting can sometimes be accompanied by increased thirst or lower energy levels.
A study published in the Journal of Exercise Science & Fitness found that fasted exercise was associated with improvements in body composition and reductions in fasting insulin levels. While thirst and fatigue were reported slightly more frequently in the fasted group, these differences were not statistically significant compared with the fed group.
Drinking plenty of water and incorporating electrolytes are often recommended during both exercise and fasting to support muscle comfort, physical performance, and proper hydration.
Align your workouts with fast intensity
Generally, the longer the fast, the more conservative your workout should be. Intense exercise during a prolonged fast may not be the best option to support optimal body composition or long-term health.
When managed correctly, shorter intermittent fasting windows are typically compatible with a wide range of exercise routines, as periodic food intake provides the nutrients necessary to support muscle building and metabolic functions.
Research published in Nutrients confirms that, “in terms of capacities, intermittent fasting does not have a negative impact on sports performance and could be considered an adequate type of diet for sports practice.”
During a prolonged fast, however, high-intensity exercise may be counterproductive. Without adequate fuel, intense physical stress can elevate cortisol and contribute to muscle breakdown.
For extended fasts, shifting to low-intensity, restorative movements, such as light walking or stretching, is generally recommended. The primary benefits of prolonged fasting center on cellular renewal processes, which are best supported by avoiding overexertion.
Key takeaways
- Fasting and working out can effectively support significant fat loss and improve metabolic health when implemented correctly.
- Time-restricted eating protocols such as 16:8 or 18:6 fasts are sustainable fasting routines that can help enhance insulin sensitivity and fat loss when paired with moderate exercise.
- Inadequate hydration, as well as poor dietary and lifestyle habits, can lead to low energy, reduced physical performance, and elevated stress markers during fasting and exercise.
- Those with health conditions, such as diabetes, disordered eating, or cardiovascular issues, should consult a healthcare provider before combining fasting and exercise.
