Is it okay to workout on an empty stomach?

If you're anything like us, your IG feed has a high-volume of fitspirational belfies, smoothie bowls, and (recently) proud body hair pics. But there's another thing people love talking (nay, bragging) about on their social platforms: fasted cardio workouts.

What is fasted cardio, exactly?

At the most basic level, fasted cardio involves increasing your heart rate without noshing on a pre-workout meal or snack beforehand. Fasted cardio fanatics claim the practice maximises your fat-burning potential. But, naturally, you might wonder whether working out on an empty stomach is a good (and safe!) idea or just a trend that sounds legit.

The basics of fasted cardio workouts

First things first: How long do you have to go without food for your workout to be considered "fasted"?

Usually, eight to 12 hours. But for some people, it may be just three to six hours, depending on how fast your digestive system is working and how much food you ate at your last meal. Once the body has stopped processing and breaking down food, your insulin levels are low and there's no fuel (glycogen) circulating in your blood. As a result, your body has to turn to another source of energy-usually fat-to power you through the workout.

Typically, fasted cardio happens in the morning (after an overnight fast). But a fasted state can also be achieved later in the day (for instance, if you're doing intermittent fasting or skipped lunch).

Bodybuilders have been using fasted cardio as a fat-loss technique for years, and regular gym goers have recently been adopting it as well. But you may have been doing fasted cardio workouts already without realising it. Technically, any time you head straight to an early-morning workout without eating first, you're doing a fasted workout.

The benefits of fasted cardio

If your primary goal is to lower your body fat percentage and your go-to workout is low- to moderate- intensity cardio, fasted cardio may offer some perks. Research does support that you'll burn more fat when you run in the fasted state than when your body does not have circulating nutrients to use for energy. For instance, one small study found that when people ran on a treadmill in a fasted state, they burned 20 per cent more fat compared to those who had eaten breakfast.

Why? When you don't have readily available energy from food, your body has to look elsewhere.

While running on empty may make you feel sluggish at first, over time, your body will adapt to be more efficient at burning fat for fuel. This may be beneficial if you workout for longer than 30 minutes at a time, four or more times a week (like endurance runners or triathlon-ers). In fact, research published in the Journal of Applied Physiology comparing fasted individuals versus fed individuals over the course of six weeks found that, when training at the same intensity, those who consistently trained in a fasted state showed more improvement in their endurance exercise performance compared to those who noshed before training.

The cons of fasted cardio

Here's the thing: While your body may turn toward the fat stores in your adipose tissue for energy, it doesn't discriminate where it gets the energy from. That means that your body could break down your muscle tissue for fuel.

One study found that one hour of steady cardio in a fasted state resulted in twice the amount of protein breakdown in muscles, compared to non-fasted cardio. The researchers concluded that performing cardiovascular exercise while fasting might not be a good choice for people seeking to gain or maintain muscle mass.

Ultimately, whether your body burns fat or breaks down muscle depends on what kind of exercise you're doing. The idea is to stay between 50 and 60 per cent of your target heart rate, which you can do during a walk, slow run, elliptical jaunt, or yoga class. The easier the workout, the more likely your body will use fat.

So is fasted cardio worth it?

Maybe. The evidence is pretty mixed, so, ultimately, it comes down to your personal preference and goals.

There are absolutely people who love it. In part, because it's something new, and, in part, because it just works with their body. If you're a morning exerciser and don't like eating before your sweat session, it may be worth giving it a try.

If you do decide to fast, make sure to eat after your workout. And if working out while hungry is not for you, there are plenty of other, more effective ways to burn fat.