Working out fasted: does it boost fat loss?


Do you usually work out fasted or have you ever tried it? This is a popular theme that raises many questions regarding its efficacy.

The main goal of individuals who do this type of workout is to activate fat burn faster during the night to improve the availability of body fat storages and use them as an energy source. It is a popular method among competition athletes during their preparation period when they need to reduce the amount of body fat dramatically.

Theoretically, waking up in the morning and hitting the gym on an empty stomach will force your body to use fat instead if glucose to maintain its energy levels. When you’re fasting, fat is available to be burned and to provide energy. However, this process is limited. There’s a moment (depending on the workout, intensity and activity time) when your body is going to start using muscle mass to get energy. If that’s not the goal, there’s a problem here.

Before trying working out fasted:

1. Run a test on your physical condition.

It is not advisable to perform an aerobic workout while fasted for those who started working out recently. It is recommended to have a good nutritional support and be followed by a professional to eliminate the chances of muscle injuries, fatigue and an unexpected fall.

2. Think about your goal.

This type of exercise should be strategically included in your workout program and supervised by a certified professional. Don’t use this method just because you’ve heard that it gets you immediate results. Performing an aerobic workout while fasted may help, but it also may not be the most indicated for you. As with any other workout program, working out fasted should be performed under supervision and take into account other factors like: diet, supplementation, and the evaluation of your physical condition.

A study published by the Strength and Conditioning Journal refers that people who work out on an empty stomach burn more lean muscle mass than fat. The analysis was performed by two groups of cyclists: one group ate before working out and the other didn’t.

According to the author, Brad Schoenfeld, the analyzed result showed that those who worked out on an empty stomach burned about 10% of their calories from protein, that is, the body ended up using muscle mass for energy.

Schoenfeld also raises one question: the difference between lipolysis (chemical decomposition of fats) and fat oxidation. The author explains that, during moderate and high intensity exercises, there’s a bigger break in fat molecules, but not all of them are oxidized. What happens with these broken and non-oxidized fat molecules is that they are deposited back in your body. Besides that, half of those molecules come from the muscle itself.

To Schoenfeld, eating before working out brings more benefits than not eating. In fact, there are studies that show that the body burns much more calories after exercising if you eat before working out, rather than doing in on an empty stomach.

On the other hand, a study conducted by investigators of the Leuven’s Catholic University, in Belgium, stated that working out fasted does not only burn more fat, but it also promotes metabolic adaptations that improve performance in resistance sports.

There are many theories in favor and against this method, but no absolute conclusions on its efficacy; each body reacts differently to different stimuli and, in the end, it’s you who should know what works better for you. Some people adapt just fine to this situation; others need to eat before working out to guarantee they’ll feel fine while exercising. If not eating limits the intensity and duration of the exercise, this workout’s caloric expenditure will be smaller and, for that reason, fasting won’t have been helpful.

However, you can always try working out on an empty stomach twice a week and see how your body reacts. If you don’t adapt to this method, feel nauseous or decrease your performance, stop immediately and eat/drink something sweet, like a sugar sachet. When performed wrong, this practice can end up provoking severe hypoglycemia (decrease of the blood’s glucose level), so it is advisable to do it with caution and start with 15 minutes on the first week and gradually increase to 20, 30, until you reach your workout time goal.

To get good results during your workouts, and even if you want to lose body fat, we still recommend that you’re well-nourished before exercising. The ideal is to eat 1h before working out and add to your meal protein and low glycemic index carbs that will supply energy gradually and provide satiety for longer. Example: a whey protein and fruit smoothie.

whey isolate


Brad Schoenfeld, The Myth of cardio before breakfast, April 2011, translation and adaptation of the original version

Renata Fernandes, Treinar em jejum emagrece: mito ou verdade?, translation and adaptation of the original version

N/d, Aeróbico em Jejum, August 2013, translation and adaptation of the original version


Please enter your comment!
Please enter your name here