Is Fasted Cardio Safe?

If you’ve ever woken up at 5 a.m. to squeeze in a morning run and haven’t had time to scarf down a banana or an energy bar, then you’ve done fasted cardio. You may have heard about the fat-burning benefits of getting out of bed and doing a workout immediately, but is this new workout craze all it's made up to be? While fasted cardio may be beneficial for some people, read this before attempting it to see if it’s right for you. 

What is fasted cardio?

Fasted cardio refers to cardio performed when your body is in a fasted state, which means you’re not digesting any food at that moment in time. In other words, fasted cardio is simply when you do cardio on an empty stomach. This is normally done in the morning before having anything to eat, however, it can happen later in the day if you practice intermittent fasting. Some people may claim they’re doing fasted cardio by skipping lunch and heading straight to the gym, but the scientific literature specifies that the body needs a 10- to 14-hour period of not eating to be truly fasted. 

Why do some people prefer fasted cardio? 

If you’re the type of person that can’t stand the thought of eating breakfast in the morning, fasted cardio may be a good option for you. Other people may choose fasted cardio for the claimed benefits. While it’s nice to think that something can help us to lose weight easily, the research just isn’t there to say with confidence that this is the best way to burn fat and fast. In one study, 20 young females were split into two groups — one group did 1 hour of fasted steady-state cardio, and the other did 1 hour of non fasted steady-state cardio. Both groups exercised 3 days per week for 4 weeks and followed a diet with a calorie deficit[1]. Researchers found no difference in weight loss or body composition between groups[1]. However, a review of 27 studies published in the British Journal of Nutrition in 2016 illustrates that while fasted cardio may burn more calories than non fasted cardio during the session itself, the difference it makes to total daily calorie expenditure in a span of 24 hours is insignificant[2].

Who shouldn’t participate in fasted cardio? 

Avoid fasted cardio if you have a medical condition that’s affected by low blood sugar, low blood pressure or if you’re pregnant. If you’re someone who usually wakes up hungry, eat! You shouldn’t ignore your body's hunger signals just for the sake of doing fasted cardio because this can potentially be dangerous. It’s also best for beginners to shy away from fasted cardio — understanding what works for your body should be the first step on your exercise journey! 

Overall recommendation: 

In general, if you’re looking to lose weight, increasing your daily movement, whether fasted or not, is still the best plan. Moving more every day and finding a workout that you genuinely enjoy doing will have a larger impact on weight loss than a 30-minute session of fasted cardio.

Ultimately, do what works for you. While research on its metabolic effects is still inconclusive, fasted cardio may work better for your lifestyle or preferences, so if you’re generally healthy, feel free to give it a go! 

References: 

1. Body composition changes associated with fasted versus non-fasted aerobic exercise. Nih.gov. Accessed December 7, 2021.
2. Aird TP, Davies RW, Carson BP. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scand J Med Sci Sports. 2018;28(5):1476-1493.