Intermittent fasting plus exercise: Helpful or harmful?

Intermittent Fasting (IF) has become one of the most popular weight loss strategies. But does it really work? Can it boost your efforts to lose weight and build lean body mass? Let’s dive in to see what the research says about intermittent fasting and exercise.

How intermittent fasting works

There are various ways to practice intermittent fasting, such as:

• Five days of regular eating followed by two days of severe calorie restriction
• Alternate-day fasting
• 24-hour fasting once or twice a week
• Time-restricted eating, which involves eating within a certain window of time and fasting the remainder of the day

A popular time-restricted eating window is 16/8: fasting occurs 16 hours a day with 8 hours to consume meals and snacks. For example, food is allowed from 10 AM to 6 PM and only water and/or non-caloric beverages are allowed outside of this window.

The idea is that fewer calories are consumed if there is less time to consume them.

The popularity of IF stems from claims of “effortless” weight loss through periodic fasting and other benefits. Research has shown support of IF for maintaining weight and improving metabolic health (lowering blood pressure and improving blood sugar).

But there are also some concerns. Namely, the need to ignore the body’s hunger cues and the risk of overcompensating meals during the allowed eating window. Both factors might make intermittent fasting impractical and unsustainable for some. However, others may thrive on the structure and find success with intermittent fasting.

Intermittent fasting combined with exercise

In general, it is not recommended to start strenuous activity while actively feeling hunger. Doing so risks impaired performance due to reduced stamina; dizziness or fainting due to low blood sugar; and the potential use of the body’s lean muscle protein as an energy source. Research supports having a meal or snack before exercise so that the body has energy to perform optimally and safely.

Typically, the goal of exercise is to increase or maintain lean body mass while reducing body fat mass. A review of multiple studies shows that practice of IF with exercise did result in the general maintenance of lean body mass, but notes these were short to medium-length studies. So, it’s difficult to know what the long-term results are.

Of the eight studies reviewed, one study showed a slight decrease in lean body mass of its IF participants while the others showed general maintenance (none describe a significant increase in lean body mass).

Therefore, the question arises: is the body capable of increasing lean body mass while fasting? Perhaps not. Think about it—the goal of intermittent fasting is to reduce calorie intake to force the body into a calorie deficit, resulting in weight loss. It becomes difficult for the body to build lean mass at the same time it is breaking down energy reserves for fuel.

Regarding physical performance, research shows conflicting results. Some studies showed a negative effect, some showed a positive effect, most showed insignificant effects indicating neutral or no effect on performance.

The verdict on intermittent fasting

The bottom line is that more research is needed to determine the effects of IF combined with exercise on body composition, physical performance and as a lifestyle over a longer period of time.

However, if your goal is to reduce body fat mass and not necessarily increase lean body mass or improve physical performance, combining IF and exercise may be a safe practice for you. It’s simply a matter of personal preference—some people enjoy it and others decide it’s not a good fit.

For more help in deciding on an eating plan that’s right for you, talk to a registered dietitian.

This article was contributed by dietetic intern Mia Vargas.