Does Fasted Cardio Lead to Greater Weight Loss?

Fasted cardio is the fitness theory that working out on an empty stomach or after a prolonged period of fasting can lead to greater fat loss. While there are fitness professionals who adhere to the theory, it is still a hotly debated topic in the fitness world.

The efficacy of the fasted cardio theory remains contested, and ultimately, the decision of whether or not to exercise on an empty stomach is up to you. Here's a look at some of the research that's been done to test the ideas behind fasted cardio.

Does Fasting Burn 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 indicates 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 our 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 fasted cardio theory argues that your body then relies on body fat for energy to fuel your workout.

Are Fasting Workouts Effective?

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

Take a look below at some of the notable research on fasted cardio:

• In a 2012 study comparing fasted cardio to postprandial (after eating) cardio, results showed greater appetite suppression later in the day for the participants who had eaten breakfast before running. By the end of the day, however, there was no difference in total energy intake between the groups who had fasted before exercise or eaten before.
• A 2017 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 very little—if any—effect on body mass.
• A 2018 review from the University of Limerick in Ireland looked into 46 previously published studies and found that when study participants ate before a cardio session, they were able to work out for longer periods of time. Working out in a fed state led to longer aerobic workouts, showing that while fasted cardio may have other benefits, it does not typically lead to longer workouts.
• More recently, another 2018 study looked specifically into the effects of a fasted state on resistance training. The researchers at the Department of Kinesiology in Samford University 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.

While the recently published research on fasted cardio is limited, 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 is 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.

Researchers have studied Muslim athletes during Ramadan to learn more about the metabolic impact of fasted cardio. One such study examined the effect of aerobic training in active men while fasting during Ramadan versus in a fed state (before and after Ramadan).

Body composition and metabolism were compared in 19 men, and the results indicated bodyweight reduction in both the fed and fasted groups. However, body fat percentage decreased only in the fasted group (by 6.2%). Although exercise promotes weight loss in general, this study suggested that fasted cardio may provide an advantage for burning body fat specifically.

Another study on trained bodybuilders evaluated body composition and metabolism changes during Ramadan. Sixteen male bodybuilders participated in the study, which lasted for 30 days. Body mass and body mass index (BMI) increased by 2.4% in the non-fasting participants. It was unchanged in those who were fasting.

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

In addition to potential body fat loss, fasted cardio has other lifestyle benefits. If you've ever experienced exercise-induced nausea, eating prior to your workout may be a factor. An older 2001 study found that nausea during a workout, especially a high-intensity session, is more likely to occur on a full stomach.

For those athletes who aren't early risers, overnight fasting and fasting prior to 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 a benefit of fasted cardio, it's important to note some of the potential drawbacks of this nutrition and fitness approach:

• The same 2012 study referenced above that followed fasting effectiveness in athletes during Ramadan found that fasting participants also experienced some negative effects, including dehydration and reduced renal function. (Because Ramadan fasting includes abstinence from both food and water, the negative effects of dehydration and reduced renal function may not apply to fasters who continue to drink water normally.)
• A 2011 review published in the Strength and Conditioning Journal found that working out in a fasted state could lead to protein loss. While performing a cycling workout, the study participants were found to have lost protein, and in turn, muscle. 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 the best option. The same 2018 review noted earlier found that 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.
• Another 2019 study measured the workout efficiency of 20 male cyclists who performed fasted workouts and fed workouts. After both sessions, the 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.

A Word From Verywell

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, which can include 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.