Is Fasted Cardio Safe? Risks Vs Benefits, According to the Experts

Many gym-goers and fitness enthusiasts are often on the lookout for new ways to exercise and improve their workouts, and cardio on an empty stomach has become one of the latest fads.

But what exactly is fasted cardio and is it safe to practice? Newsweek spoke to fitness experts to find out.

What Is Fasted Cardio?

Fasted cardio is performing cardio exercise before eating, when your body is in a fasted state and isn't digesting anything.

Usually, you would need to stop eating between 10 to 14 hours before the workout for the body to be truly fasted. However, depending on the speed of individual digestive systems and the size of your last meal some people may only need to wait between three to six hours.

Traditionally, most people practice it in the early morning, shortly after waking up. However, if you practice intermittent fasting it can also be done later in the day.

The Benefits of Fasted Cardio

The practice has been branded as a method of accelerating weight loss, though its effectiveness is still unclear. Nutritionist and personal trainer Andy Griffiths told Newsweek about how it works.

"The main benefit to fasted cardio is increased fat burning. Research shows that aerobic exercise in a fasted state induces higher fat oxidation than a fed state. Essentially, as you haven't eaten through the night, you'll wake up with pretty low glycogen levels, which is your body's preferred energy source. This means when you're out on your morning run, your body will need to burn more fat as an energy source," Griffiths said.

According to one small study, people who practiced fasted cardio by running on a treadmill burned 20% more fat compared to those who exercised after having breakfast.

Woman on treadmill
A woman runs on a treadmill as part of a cardio exercise Getty Images

However, the effectiveness of this tactic as a weight loss technique is contested.

One study found that fasted exercise resulted in a higher metabolic performance once the workout was concluded, though noted that for extended aerobic exercise eating prior to the workout improved performance.

The British Journal of Nutrition assessed 27 studies on aerobic exercise in 2016 and found that fasted cardio "induces higher fat oxidation than exercise performed in the fed state."

However, despite that finding, the Journal also noted that the difference in calories burnt between fasted and non-fasted cardio exercise over the course of a full day was negligible.

As Griffiths told Newsweek: "Just because we burn more fat during our fasted morning run, that doesn't mean we'll burn more fat throughout the day. A systematic review found that fasted compared to fed exercise does not increase the amount of weight and fat loss. Ultimately, it comes down to calories in vs calories out, and cardio, whether that's fasted or fed, is a great tool to burn more calories."

Another benefit is more logistical as fasted cardio allows you to save time if you're on a tight schedule, as you can forgo pre-workout meals.

Two people run up stairs
Two people exercise together by running up some stairs. Getty Images

The Risks of Fasted Cardio

Despite its benefits, fasted cardio can also be counterproductive as an exercise method.

Exercising on an empty stomach may hinder muscle-building. While practicing fasted cardio, your body may start a process known as gluconeogenesis which converts protein into fuel, meaning it is harder to use that protein to build muscle instead.

One study found that an hour-long fasted cardio workout led to double the amount of protein breakdown in muscles in comparison to a non-fasted workout.

Additionally, high-intensity workouts such as HIIT, CrossFit, or boot camps will likely become considerably more challenging on an empty stomach. Such workouts require carbohydrates for fuel and energy, without them you could nauseated, weak, tired, dizzy and sore.

Antonia Osborne, MSc, registered nutritionist and personal trainer added that fasted cardio may even contribute to higher levels of stress.

"Fasting increases the stress hormone cortisol and exercise increases the stress hormone cortisol. By doing these actions together, this may contribute to chronically elevated cortisol levels. As well as chronically elevated levels of cortisol being damaging for numerous health reasons, it is also associated with increased fat storage around the middle and increased muscle breakdown," she told Newsweek.

Is Fasted Cardio Safe?

Overall, it is considered a safe form of exercising if done so in moderation and if no pre-existing medical conditions will hinder the process, Osborne explained.

"If you are generally healthy and do not have any contraindications, then fasted cardio may not pose any risks. If you are pregnant, fasted cardio may not be safe and for individuals who are very lean or have trouble managing their blood sugar, fasted cardio may also not be safe.

"Factors such as overall daily energy intake, training volume and lifestyle may impact an individual negatively in the longer term, if the balance is not appropriate for that individual."

Experts recommend short or moderate-length sessions of fasted cardio over long, high-intensity ones.

Griffiths told Newsweek: "Like most things, when done sensibly, fasted cardio can absolutely be safe — just ensure you're being sensible. If you're planning a long hard run, then it's wise to eat beforehand. Otherwise, you run the risk of your low blood sugar causing dizziness, lightheadedness, and consequently passing out."

Griffiths added that it is best to work progressively towards fasted cardio exercises.

"I wouldn't recommend diving straight into fasted cardio if you're not used to training in a fasted state. Instead, I'd start off with eating something small before your session, perhaps a banana and protein bar, and progressively reduce that until you feel comfortable enough to train fasted."

It's also important to fuel your workouts and avoid this practice if you have low blood-sugar or blood pressure levels, or if you're pregnant.