What The Science Actually Says About Fasted Cardio & Who Should Try It

Fasted cardio involves hitting the gym while your body is in a fasted state and fueling up after your workout. And though there are plenty of fitness pros who swear by fasted cardio, it's also a subject of heated debate among others in the industry.

Fasted cardio involves increasing your heart rate through aerobic exercise while in a fasted state (having gone around eight to 12 hours without food).

In most cases, this means simply hitting the treadmill or heading to the gym before your morning meal. However, it can also involve shifting your schedule to fit your workout between meals, especially if you're intermittent fasting or following a midday or late window for time-restricted eating (TRE).

While most foods are off-limits right before you exercise with fasted cardio, calorie-free drinks are still on the table. This means that you can still sip on a cup of black coffee or unsweetened tea if you need a quick fix of caffeine to help get you going.

Some research suggests that fasted cardio can be a good option for managing your weight. For example, a 2016 review of 16 studies found that fasted cardio could be an effective option to boost fat burning3. In another study, fasting before evening exercise reduced net energy intake6 and increased fat burning more than exercising two hours after a meal, which could theoretically translate to weight loss.

Still, the science isn't totally clear-cut when it comes to fasting and weight loss. In fact, a 2020 review even concluded that there is little evidence to back up the idea that fasting increases fat burning, with researchers noting that endurance athletes should avoid high-intensity training1 altogether during a fast. Plus, another small meta-analysis reported that fasted cardio had very little effect on body mass and body fat7.

Though there has been a slew of studies on the effects of fasted cardio on exercise performance, the jury is still out on whether skipping breakfast can help or hurt your workout1.

For instance, one study found that exercise performance6 was 3.8% lower in participants who were fasting. Not only that, but energy levels, readiness, pre-exercise motivation, and post-exercise enjoyment also took a dip while fasting. On the other hand, a 2016 study on highly trained endurance athletes found that performance9 was at its peak in the afternoon during Ramadan, while athletes were fasting.

"The research into this question is very young, and we don't have a lot of high-quality studies to inform us," explains Steve Hendricks, a journalist, fasting expert, and author of The Oldest Cure in the World: Adventures in the Art and Science of Fasting.

Hendricks also points out that the effects of fasted cardio on performance can vary depending on a handful of factors1, including the length of your fast, the type of exercise that you're doing, and what your specific goals are, among others.

As an example, one meta-analysis found that eating before exercise had minimal impact on shorter aerobic sessions but could actually enhance performance10 during prolonged workouts. Therefore, adjusting your meal schedule based on your workout may be the best option.

Fasted cardio is definitely not a great fit for everyone and could be associated with some serious side effects.

According to Kelsey Gabel, R.D., Ph.D., a researcher at the University of Illinois at Chicago who specializes in intermittent fasting, it may be best to break your fast if you experience any symptoms of low blood sugar, such as dizziness, shaking, or a fast heartbeat.

Gabel also points out that fasting in general is also not recommended for certain groups. More specifically, she notes that there is limited research on the safety of fasting for people who are pregnant or lactating and individuals over age 70.

"If you are taking medications with meals or have any other health complications, speaking with a doctor and or dietitian prior to starting fasting is recommended," she adds.

Research on the possible pros and cons of fasted cardio has been pretty inconsistent, making it hard to draw any definitive conclusions about its impacts on health.

While there are some studies suggesting that it could increase fat burning and rev up athletic performance, there are many others indicating that it might actually have the opposite effect.

If you're interested in fasted cardio for weight loss, there's likely no harm in giving it a try, as long as you stay hydrated, listen to your body, and fill up on nourishing foods afterward.

It's also a good idea to start slow and opt for low-intensity activities over-vigorous or prolonged cardio workouts. Opt for zone 2 training over HIIT-style workouts if you are exercising on an empty stomach.

If you're already trying intermittent fasting or TRE, it might be easier for you to take advantage of the potential benefits of fasted cardio.

"There isn't a ton [of research] out there about TRE with exercise, but placing the eating window around when you are physically active may be the easiest [approach]," explains Gabel. She also notes that in studies that combined alternate-day fasting and exercise, fasting didn't interfere with the participants' ability to exercise.