Should You Try Fasted Cardio? Evidence-Based Benefits and Risks.

Fasted cardio is when you exercise on an empty stomach, known as a “fasted” state, usually in the morning before your first meal. The theory is that working out on an empty stomach or after a prolonged period of fasting can lead to greater fat loss. While some fitness professionals adhere to this theory, it is still a hotly debated topic in the fitness world and lacks scientific evidence.

The efficacy of the fasted cardio theory remains contested, and ultimately the decision to exercise on an empty stomach is up to you. If you embrace the fasted cardio theory, there are drawbacks to be aware of. Here’s a look at research that tests the ideas behind fasted cardio.

How Fasted Cardio Works

Most people who engage in fasted cardio do their workout first thing in the morning, before eating breakfast. A 30-45 minute moderate-intensity workout, such as brisk walking on a treadmill, is recommended if you can endure for that long. Shorter workouts also contribute to the recommended amount of exercise per week, which is currently 150-300 minutes of moderate-intensity exercise, according to the U.S. Department of Health and Human Services.

Does Fasted Cardio Burn More Fat?

Bill Phillips, bodybuilder and author of “Body-for-LIFE,” has been credited with introducing the fasted cardio theory in 1999. In his book, he suggests that the human body maximizes fat-burning while exercising in a fasted state. The book was a success, and his concept of fasted cardio has remained popular ever since.

According to Phillips, fasting overnight reduces blood sugar, insulin levels, and glycogen stores. Typically, our bodies rely on these glycogen stores, which come from carbohydrates in our diet, for energy. However, if you exercise in a fasted state (i.e., with reduced glycogen), the theory is that your body relies on body fat for energy to fuel your workout.

Are Fasted Cardio Workouts Effective?

While some fitness industry professionals follow the fasted cardio method and note it has some fat-burning benefits, scientific research has not fully supported the theory’s effectiveness.

One scientific review by researchers in Australia looked into five separate studies and a total of 96 participants to measure the effectiveness of exercising after an overnight fast. In their research, the review authors determined that working out post-fast had little—if any—effect on body mass.

Fasting may make it challenging to complete a workout, whereas eating could help you complete your exercise plan. One review from the University of Limerick in Ireland looked into 46 previously published studies and found when study participants ate before a cardio session, they could work out for longer periods. Working out after eating led to longer aerobic workouts, showing that while fasted cardio may have other benefits, it does not typically lead to longer workouts.

It’s important to remember that exercise offers benefits whether it is done in a fasted state or after eating. The fasted cardio theory argues that if you can exercise on an empty stomach, you may be able to leverage greater fat-burning effects. Overall, weight loss and reduction of body mass are likely a result of an overall calorie deficit, whether exercise is completed on an empty stomach or not.

Benefits of Fasted Cardio

While scientific studies have found limited support for the theory behind fasted cardio, other research has found that athletes who fast have experienced measurable fat loss.

One study looked specifically into the effects of a fasted state on resistance training. The researchers studied 12 female NCAA Division I athletes, each of whom completed two workouts. One resistance workout session happened after a 10-hour fast, while another happened after a fat- and carb-loaded meal. Following the two workouts, the researchers found that the fasted session allowed the body to use more fat as energy than carbohydrates.

Although this study looked at weight training instead of cardio, it provides further insight into the effect of fasting on exercise and body composition.

For athletes who aren’t early risers, overnight fasting and fasting before a morning workout may also save you time after waking up. With a no-meal-before-workout schedule, you don’t need to wake up early to build time for eating before a session. Instead, enjoy those extra few moments of sleep.

Risks of Fasted Cardio

While body-fat reduction may be one benefit of fasted cardio, it’s important to note some potential drawbacks of this nutrition and fitness approach.

When you know you are working out in a fasted state, it is possible to compensate by eating more at other times. One study showed that participants who knew they would work out without eating in the morning consumed more calories the evening before.

Other studies have shown that fasting generally can lead to protein and muscle loss. These findings show that if you’re trying to build muscle mass, fasted cardio may not be the optimal choice.

Similarly, fasted cardio may not lead to longer workouts. If endurance workouts—marathon training, for example—are your goal, fasted cardio may not be your best option. One study shows consuming food pre-workout can naturally lead to a longer endurance aerobic session. On the other hand, working out in a fasted state can result in a shorter workout.

Finally, another study measured the workout efficiency of 20 male cyclists who performed both fasted and fed workouts. After each session, study authors found that fasted workouts negatively impact workout intensity and volume. Such research suggests that working out on an empty stomach can adversely impact how intensely you perform a workout.

The choice to practice fasted cardio is a personal decision. Performing exercise consistently, regardless of whether or not you eat before, provides clear health benefits, including healthy, sustainable weight loss. If you have never done a workout before eating breakfast, it’s best to start slow. Give your body a chance to adapt to any changes in your exercise and eating plan, and find ways to be active that work best for you and your lifestyle.