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Is It Okay to Workout While Fasting?

Updated: Dec 22, 2023

A man is lifting heavy dumbell while intermittent fasting

Fasting and exercise are two terms that are often mentioned together in the health and fitness world. In this article, we'll delve into the intricacies of exercising both cardio and weight training in a fasting state, scrutinizing the benefits, potential drawbacks, and the science that supports this practice.

Fasting and Cardio: Can We Burn More Fat?

Yes, exercise, especially cardio, like running, swimming, or cycling, combined with fasting, has been noted for its fat-burning effects.

When we engage in cardio workouts in a fasted state, the body's lipolysis activity gets a boost. Lipolysis refers to the breakdown of fats (lipids) into free fatty acids (FFAs) and glycerol. The body uses these molecules for fuel during exercise.

As insulin levels are low during fasting, the body tends to use fats as the primary source of energy. This way, fasting workouts can aid in reducing body fat, with the potential for increased fat oxidation even during rest periods.

That said, consuming food before a workout session can spike insulin levels, potentially dampening the metabolic responses that come with fasting workouts.

How Does Our Body Fuel the Muscles During Cardio?

During cardio exercises, the body's muscles need fuel to generate energy (ATP). The primary sources of this energy are fat and carbohydrates. For endurance-based activities, such as long-distance running or cycling, the body uses both fat and stored carbohydrates (glycogen) for energy.

Endurance and Fatigue in a Fasting State

Endurance during exercise depends on several factors. These include the heart's ability to pump oxygen (maximal cardiac output) and the muscles' efficiency in using fats and carbohydrates for energy. A depletion of glycogen stores can cause fatigue, making it difficult to continue the exercise.

Performance of Exercising in a Fasting State

Short fasting periods (11-24 hours) may not significantly affect exercise performance. Some studies even suggest that habitual fasting training could lead to physiological changes that boost endurance. However, longer fasting periods (>24 to 55 hours) combined with high-intensity exercise could lead to decreased performance.

Biochemical changes during fasting, such as increased sodium and protein levels, along with decreased sympathetic tone, can influence physical work capacity and athletic performance.

A research study conducted by Dr. Carson found no difference in exercise performance between those who did cardio exercises in a fasted state versus a fed state for workouts less than 60 minutes.

However, the impact of fasting on exercise performance can depend on various factors. These include the fasting duration, dehydration, exercise intensity, and individual fitness levels.

Fasting and Weight Training: Does Muscle Catabolism Happen?

No, weight training in a fasting state doesn't cause muscle loss. In fact, it's possible to build muscles just as effectively as in a fed state. The body also tends to burn more fat during a fasting state workout, even during resistance training.

After a workout, the body's insulin sensitivity is heightened. This principle applies to fasting, too, as it increases insulin sensitivity. This means that after a workout, the protein and carbohydrates consumed are more likely to contribute to muscle growth rather than being stored as fat.

As per post-workout nutrition, it's advisable to consume lean protein and a small amount of carbohydrates right after a fasting workout.

Studies on Weight Training in a Fasting State

A study published in the Journal of Translational Medicine examined the effects of weight training in a fasting state. The study involved 34 male participants who had been doing resistance training for at least five years. They were divided into two groups: the 16/8 intermittent fasting (IF) group and the Normal Diet (ND) group for a duration of eight weeks.

Positive Outcomes

The IF group saw a significant reduction in fat mass (about 16.4%), while the ND group had a smaller reduction (about 2.8%). Both groups showed the same fat-free mass and increased muscle size, arm, and thigh muscle cross-sectional areas, and significantly increased strength.

The IF group also showed significant reductions in blood glucose and insulin levels, improved insulin resistance (HOMA-IR), an increase in adiponectin (a hormone associated with improved metabolic health), and lower levels of inflammatory markers TNF-α and IL-1β.

Negative Outcomes

The IF group experienced a drop in testosterone (about 20.7%) and IGF-1 (about 12.92%) while the ND group didn't show significant changes in these hormones. IGF-1, or insulin-like growth factor 1, is a type of growth hormone that stimulates growth, muscle growth, cell growth and division, regulates metabolism, and aids in anti-aging and tissue repair.

Fasting and Growth Hormone Release

Some experts suggest that weight training on an empty stomach might trigger the release of growth hormone, potentially aiding in muscle building. However, a study by the University of Virginia found that while fasting exercise might lead to the release of growth hormone, it's likely the body might release growth hormone as a way to protect itself during fasting exercise.

Exercising During Ramadan

A study conducted by the University of Sfax in Tunisia investigated the effects of resistance training in a fasted versus a fed state during Ramadan on 16 male bodybuilders. The study found that Ramadan fasting did lead to changes in urinary and some biochemical parameters, but these changes did not significantly differ between the two training groups.

The Long-Term Benefits of Fasting Workouts

A study published in the Journal of Physiology found that exercising in a fast state can lead to better endurance in the long run. The study found that subjects who trained in a fasted state showed an increase in anabolic response, leading to better muscle adaptation, and improved metabolic health.

The Neuroprotective Effects of Fasting Workouts

Fasting workouts can also have neuroprotective effects, thanks to a protein in the brain called BDNF (Brain-Derived Neurotrophic Factor). BDNF plays a crucial role in neuroplasticity, learning, and memory. A study found that light cycling exercise, regardless of whether participants were fed or fasted, increased serum BDNF levels. High-intensity exercise led to a significant increase in both plasma and serum BDNF levels.


Working out in a fasting state can offer various benefits, from increased fat burning to improved endurance and potential neuroprotective effects. However, the potential impact on exercise performance can vary based on several factors. As always, it's advisable to consult with a healthcare or fitness professional before starting any new fitness regimen.


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