Is Fasted Cardio Really Better for Fat Loss?

Bill Phillips, bodybuilder and author of Body for Life created the infamous fasted cardio theory in 1999. The cardio segment of his book indicates the human body maximizes fat loss in a fasted state during aerobic exercise first thing in the morning. The book was a success and his fasted cardio concept has remained popular but also controversial.

According to Phillips, fasting reduces our blood sugar and glycogen levels. Without energy from carbohydrate (glycogen) sources, the body would turn to stored fat as fuel for our workouts. His book also indicates fasting for exercise lowers our insulin levels which increase fatty acid breakdown. This news drives early bird athletes and bodybuilders to the gym without a morsel in hopes of achieving the best results for their cardio efforts.

Is time lost sleeping and feeling famished during exercise worth the sacrifice? Is the “fasted cardio for greater fat loss” theory fact or fiction? To answer these questions, we will look at some research. It will also be important for you to understand how fat loss occurs. 

Fat Loss

To lose fat, you must burn calories. Calories are units of energy stored in our body and obtained from the food we eat. Consuming more calories than we’re able to burn equals weight gain. Exercise helps with caloric burn and fat loss. Our metabolism also plays a role in how the body uses energy for fat loss.

A study was published in the National Institutes of Health comparing fasted cardio to postprandial (breakfast) cardio. The research examined how fasted, and postprandial exercise affected energy intake, appetite, and metabolism. Study participants included twelve healthy males in their early twenties. Three separate trials were conducted over 10 hours for fasted, postprandial and control feedback. The men performed a fasted 60-minute treadmill run upon waking for one trial. On another day, the same men consumed breakfast 1.5 hours prior to running.

Results showed greater appetite suppression when the participants ate breakfast. There was no difference in energy intake between fasted and postprandial. However, both fasted and eating breakfast cardio created a negative energy balance compared to a sedentary day. Conclusions indicated running for 60 minutes fasted or after breakfast showed the same caloric deficit. It appears exercise is what creates fat loss not when we eat our food. 

A study published in the British Journal of Nutrition examined the impact of breakfast on exercise. Accurate reports on postprandial (breakfast) metabolism, appetite, and energy balance were recorded. Twelve physically active males participated in the randomized study. Four trials were completed all including an overnight fast followed by rest without breakfast, fasted workout, breakfast followed by rest, and breakfast followed by exercise. A continual and moderate intensity run was used for the workout segment. The length of running time was also used on those participants resting instead of working out. All participants consumed a test drink 90 minutes after workout or rest followed by a leisurely lunch.

Results showed glucose uptake (sugar in the blood) slightly faster in those consuming breakfast with rest versus those fasting with rest. However, there was no comparable difference in those resting or exercising without breakfast. Better appetite suppression throughout the day was realized in those eating breakfast in all categories. It will come as no surprise the most significant energy stores were recorded after lunch in the breakfast followed by rest group.

The study did reveal less energy (calories) remaining in those performing fasted exercise. Research also indicated regardless of eating breakfast, exercise appears to be the primary reason for lowered energy (calorie) balance. Those not eating breakfast had slightly better results according to the findings. Trial participants eating breakfast showed improved overall food cues throughout the day. The study also discovered eating breakfast interferes with appetite suppression provided by fasted exercise.

This research is a great example of going either way with fasted cardio. Better appetite control throughout the day was realized eating breakfast. This means less daily calorie intake overall. Negative energy balance and fat reduction occurred with both fasted cardio and eating breakfast. The fasted group showed slightly more positive results.

It really becomes a personal preference of how you feel prior to exercise. Waking up hungry and forcing an aerobic session may not be your best option. On the flipside, if you feel ready to go, performing fasted cardio may work just fine for you. 

The International Journal of Sport Nutrition and Exercise Metabolism published a study on physically active men. The research examined aerobic training in a fasted or fed state during Ramadan. According to Wikipedia, “the predominant practice during Ramadan is fasting from dawn to sunset.”

Body composition and metabolism were compared in nineteen men separated into two groups. Ten men completed aerobic training in a fasted state while the remaining nine trained postprandial (after a meal). The research duration was for one month. The participants were lab tested on 4 separate occasions during the trial period.

Research results indicated body weight decrease in both fed and fasted groups. Body fat percentage decreased only in the fasted group and by 6.2 %. Blood samples did show dehydration and reduced renal function in the fasted participants.

Conclusions of this study indicate “aerobic training in a fasted state lowers body weight and body fat percentage.” In addition, findings show eating before aerobic exercise only reduced body weight.

Realistically, this wouldn't be maintainable fasted cardio and isn't the usual circumstance. The research is covering a thirty day fast where food or water is not consumed from dawn to sunset. This study concludes fasted cardio reduces both body fat and weight which is awesome. However, for the typical exerciser, Ramadan fasting is not ideal for everyday life.  

The International Journal of Sport Nutrition and Exercise Metabolism published a study on trained bodybuilders. The research evaluated body composition and metabolism during Ramadan (no food or water during daylight hours).

Sixteen male bodybuilders participated in the study lasting 30 days or typical time for Ramadan. There were nine Ramadan fasters and seven non-fasters. The men continued resistance training during the trial period. Body mass and BMI (Body Mass Index) increased by 2.4% in non-fasting participants.

Those fasting experienced no change in body mass or BMI. Weight resistance training through Ramadan proved to have “no effect on body mass and body composition of bodybuilders.” Fasting participants did experience dehydration and reduced renal function during the test period.

Although not cardio related, this study provides further insight into fasting and workouts. It appears the only adverse effect of Ramadan fasting and resistance training is dehydration and reduced renal function.

Body mass percentage and index remained unchanged. However, one could consider no change in body composition a negative, especially with fasting. The goal of bodybuilding is to decrease fat while increasing in muscle. Given the research, it seems no fat reduction was realized with Ramadan fasters. 

Fasted cardio is a personal choice. Performing exercise regardless of when you eat will provide weight and fat loss. Research has shown nothing superior about doing aerobic exercise on an empty stomach. Consider how you feel in the morning with energy and hunger cues. Do what works best for your body and lifestyle.

Exercise intensity is directly related to fat loss. Burning fat equals working hard in the gym. This requires energy (food).

Longer workouts require energy. Fasted cardio or intense workouts over 60 minutes may not be sustainable without a meal on board.

You may risk burning hard earned muscle in a fasted state. Exercising on an empty stomach can increase protein breakdown directly affecting muscle growth. The point is to burn fat while maintaining our lean mass.

Exercise post-exercise oxygen consumption (EPOC) is increased when you eat before exercise. EPOC is the “after burn” or accelerated metabolism we experience for several hours post exercise.

Every person burns fat and loses weight differently. Fasted cardio may work for your friend but not the best option for you. Don’t get caught up in your friend’s fat loss results and stick to what science says on the subject.  

Eat smart and exercise smarter. Healthy food intake plus exercise is what will get those fat loss results.