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 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 founding 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 cardio (after eating), 12 healthy males in their early 20s were observed by researchers. The purpose of the study was to determine whether running before or after eating affected energy intake, appetite, and metabolism. Three separate trials were conducted over 10 hours for fasted exercisers, postprandial exercisers, and a control group (sedentary).
  
  In one trial, the men performed a fasted 60-minute treadmill run upon waking. On another day, the same men consumed breakfast 90-minutes before running. The control group did not exercise. All three groups were allowed to eat from a buffet 5.5 hours and 9.5 hours following the morning test phase. Their food intake was then recorded.
  
  Results showed greater appetite suppression later in the day for the participants who had eaten breakfast before running. By the end of the day, there was no difference in total energy intake between the groups who had fasted before exercise or eaten before. However, both the fasted and postprandial cardio subjects had an overall negative energy balance when compared to the control groups who did not exercise.
• Another 2013 study examined the impact of breakfast on exercise. A sample of 12 physically active males participated in this randomized crossover study. It consisted of four different trials, including rest without breakfast (fasting rest), exercise without breakfast (fasting exercise), breakfast consumption followed by rest (breakfast rested), and breakfast consumption followed by exercise (breakfast exercise).
  
  A continuous, moderate-intensity run was completed during the workout segment. A rest period for the same length of time was used for the resting trials. All participants consumed a test drink 90 minutes after working out or resting. This was followed by a leisurely lunch with no limitations on food intake.
  
  The findings showed that factors, including the amount of energy burned during morning exercise and the amount of energy eaten at breakfast do not impact the amount of energy consumed shortly after. Participants did not adjust their immediate food intake or compensate for their morning exercise.
  
  Furthermore, energy balance was most positive when participants ate breakfast and rested. It was least positive in participants who fasted and exercised. The results stated, "When exercise is performed, it may be more pertinent to omit breakfast if a negative fat balance is desirable, although the findings of the present study are unable to predict the longer-term outcomes of energy and fat balance."
• 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 fasted cardio, other research has found that athletes who fast in observance of religious holidays 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, it appears that fasted cardio provides 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. Fasting participants experienced some negative effects, including dehydration and reduced renal function.

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

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.

• 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.