Who has never heard about working out while practicing intermittent fasting? The list of so-called advantages goes a long way: from improving weight loss to generating antioxidants in the bloodstream, you’ve probably heard it all. But in the end, is it or is it not advantageous to combine intermittent fasting with exercise? Should you skip breakfast when exercising in the morning? It’s difficult to blatantly reply to these questions, mostly because research is still active in this area. In the meantime, we can review the best knowledge to date.

Does intermittent fasting plus exercise help you burn more fat?

 Multiple studies have shown that combining intermittent fasting and exercise, aka working out before your first meal of the day, makes you burn more fat. Regular intermittent fasting already makes your body use more lipids than other sources of energy, such as sugars. So, naturally, when you combine intermittent fasting and exercise, your body will do similar processes to use fat as fuel. A 2018 review analysed 46 scientific studies about the effects of fasted vs fed-state exercise. The researchers found more free fatty acids circulating in the bloodstream of people who exercised while practicing intermittent fasting compared to those who exercised after a meal. This suggests that intermittent fasting mobilised more triglycerides from fat tissue to use as fuel during work-outs, thus burning more fat. Another study also showed that exercising while practicing intermittent fasting increased lipolysis – the breaking down of fat molecules. This study also found higher concentrations of stress hormones and enzymes that triggered fat burn and increased fatty acids in the bloodstream.   

Does intermittent fasting plus exercise help with weight-loss?

It’s complicated. Some studies have pointed out that during fasted workouts, your body could use more fat and thus make you lose more weight. However, other studies suggest otherwise. In 2014, researchers studied 20 healthy young women who worked out regularly but who were not athletes. At the beginning of the month-long study, women reported to the lab to get their measurements taken: body mass, height, body composition, and waist circumference. Afterwards, 10 women would work out while practicing intermittent fasting, while the other 10 had a meal prior to exercise. All females performed low-to-moderate-intensity work-out routines for 1 hour, 3 days per week. By the end of the study, their measurements were taken once again. All women lost a significant amount of weight and fat mass, and researchers found no differences in body composition between women who worked out while practicing intermittent fasting and women who worked out after a meal. This means intermittent fasting plus exercise didn’t appear to impact body composition parameters such as waist circumference, BMI or percent body fat. All women lost weight similarly.

A DSLR photo of burning white paper with a word FAT written on it. It's a real fire with real flames. Can be used as a concept of dieting, weight loss, etc.
Intermittent fasting plus exercise increases fat burn, but this may not translate into increased weight loss.

Other studies involving different people and more intense workouts led to the same conclusions: people lose weight equally, regardless of fasted or non-fasted workouts. Thus, even though you can burn more fat by combining intermittent fasting and exercise, it seems that this fat burn doesn’t translate into weight loss.

Does intermittent fasting plus exercise improve your performance?

While some people put in the effort of exercising for the sake of burning calories, others focus on achieving fitness goals. However, is combining intermittent fasting with exercise really helping you improve your performance? Could fasted work-outs jeopardise your fitness outcome? Scientists have been trying to answer these and other questions, and so far results seem to be inconclusive. 

On one hand, several studies found no changes in performance between fasted and fed conditions. Particularly, in shorter exercise routines (under 60 minutes), athletes who were practicing intermittent fasting were not performing better than fed athletes: not running faster nor putting in more reps per workout session. The same happened with aerobic exercise: Intermittent fasting did not lower performance, but it didn’t improve it either. On the other hand, some studies have showed that working out after long fasts could worsen performance. According to this  review, a study showed that fasting for 4 days decreased capacity to perform endurance exercise – fasted participants felt fatigued faster than non-fasted ones. Another study showed that after 24h fasts, cyclists would be fatigued. This could happen because a withdrawal of carbs might reduce exercise performance, but scientists are still looking for less equivocal answers. 

As frustrating as it may be, there are no definite answers to working out while practicing intermittent fasting just yet. However, intermittent fasting and exercise have a multitude of advantages worth going for. While we wait for science to answer these questions, combining intermittent fasting with exercise can still be a healthy goal for 2021.

Barbara Pinho

Award-winning science communicator with a track of experience from top science institutions worldwide. I tell stories about biochemistry, medicine and all things health.

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