Is Fasted Cardio Really Better for Fat Loss?

Whether or not to eat before exercise remains a hotly debated topic in the fitness world. The theory that fasted cardio optimizes fat burning may motivate athletes and exercisers to hold off on breakfast. While some experts swear by this theory, others don't agree on its efficacy. Ultimately, the decision to exercise on an empty stomach is up to you. Here's a look at some of research that's been done to test it.

The Fasted Cardio Theory

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. Without available energy from stored carbohydrates (like glycogen), our body turns to body fat for fuel.

Fasted Cardio Research

A 2012 study comparing fasted cardio to postprandial cardio (after eating) sheds some light on this topic. Twelve 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."

Exercise offers several benefits whether it is done in a fasted state or after eating. If you can exercise on an empty stomach, you may be able to leverage greater fat-burning effects.

Athletes and Religious Fasting Research

Due to religious reasons, many athletes around the world practice periods of fasting. 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, separated into two groups. Ten men completed aerobic training in a fasted state. The remaining nine participants trained after eating. The total research duration was one month, and participants were lab-tested on four separate occasions during that time.

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. There were nine Ramadan fasters and seven non-fasters. The men continued resistance training throughout the trial period. 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.

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 include 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.