A lot of theories get thrown around in the fitness community on different ways to optimize training and get better results.
One that’s talked about a lot is fasted cardio. Some corners of the internet will tell you that fasted cardio helps you burn fat quicker, and get more benefits from your cardio workouts. Yet others claim this is totally false.
So what is it? Is fasted cardio really a better way to train? Are there any benefits? Is it safe? Keep reading and we’ll explain everything you need to know.
What Does Fasted Cardio Mean?
Fasted cardio means working out while ‘fasted’ – aka on an empty stomach.
‘Cardio’ refers to cardiovascular exercise, such as running, biking, or swimming, but the term is generally used to broadly describe any kind of exercise done on an empty stomach.
Most people do fasted cardio first thing in the morning, when you’re typically in a fasted state having just woken up from an ~8ish hour sleep.
However, there’s no requirement that fasted cardio be done in the morning, it just matters that you’re fasted, which is usually at least four to six hours after eating.
Benefits of Fasted Cardio
The benefits of fasted cardio are largely theorized, and not clearly proven by scientific consensus at this time.
There are, however, some non-scientific benefits to working out on an empty stomach, which may make it worthwhile, regardless of what the studies say.
Let’s run through the potential benefits now.
May Burn More Fat
Proponents of fasted cardio claim that it helps you burn fat at a faster rate.
The idea is that, having not eaten, the body has no readily available glucose to use as energy. As a result, the body is forced to use fat stores to power your workout.
If this holds true, fasted cardio would be a more effective way to cut down body fat and lose weight.
Some studies [1, 2] appear to support this, showing greater fat oxidation from fasted workouts.
However, other studies have come up with inconclusive results.
One review, looking at five separate studies, found little to no difference in body mass from people working out post-fast vs those eating normally.
Less Stomach Discomfort
While it can be debated whether fasted cardio helps you burn more fat, it does have some other potential benefits.
For one, you’re less likely to feel stomach discomfort during your workout.
It can be awkward working out while there’s still food in your gut, particularly exercise where you’re moving around a lot, like running or swimming or HIIT circuits.
Fasting removes this possibility, and for some (particularly if you’re prone to digestive issues), it may feel better.
More Convenient
Relying on being fully fueled prior to your workout can make it harder to fit workouts into your day.
For example, if you want to work out in the morning, you might have to get up, eat something, then wait a little while for it to digest before you work out, meaning you might need to get up earlier to fit everything in.
Or it could be difficult to squeeze a workout into the middle of your day, such as your lunch break, as you need to plan ahead to ensure you get a meal at the right time.
If you’re used to working out fasted, though, it’s easy to fit a gym sesh in whenever you have little free time in your schedule.
Supports a Caloric Deficit
Obviously, if you’re trying to maintain a caloric deficit, then it benefits you to be able to work out while fasted.
You’re burning calories from your workout, and you’ll likely end up eating less. So overall, the difference between calories in and calories out will be bigger, and you may end up losing more weight.
Is Fasted Cardio Safe?
While there are some potential benefits to fasted cardio, there are some risks.
Generally speaking, it’s safe to work out on an empty stomach. But it depends on the type and intensity of exercise, as well as how long you’ve been fasting.
Low to moderate-intensity exercise, done after a moderate fasting period (roughly 6-16 hours) is safe for most adults.
However, more intense workouts can become dangerous if your body is running low on energy. You can pass out, or get injured as a result of having reduced strength and coordination.
The longer you’ve been fasting, the more you need to be mindful of intensity. If you’re on an extended fast (e.g. 24+ hours), it’s best to avoid exercise, or do only very low-intensity exercise.
On top of safety risks, you may end up losing muscle mass, as if your body doesn’t have enough body fat to burn for energy, it will start burning protein, reducing the amount of protein available to support muscle growth and repair.
Should You Do Fasted Cardio?
Studies are inconclusive on whether fasted cardio has any real benefits.
But that’s not to say you shouldn’t do it. If you feel better working out while fasted, or it makes it easier to fit workouts in, go for it.
Just be sure you do it safely, and don’t expect that fasted cardio is going to completely change your body composition overnight.
How to Do Fasted Cardio Safely
If you’re going to work out fasted, just make sure you’re aware of the risks, and take the following steps to stay safe:
- Stick to low or moderate-intensity exercises.
- Be aware of how your body feels during the workout, and stop if you’re feeling dizzy or lightheaded.
- Stay hydrated during your workout.
- Refuel afterwards with a balanced meal, featuring protein and carbs.
- Take adequate rest between your workouts, and avoid pushing yourself into too much of a sustained calorie deficit.
Final Thoughts
Ultimately, cardiovascular exercise is almost always good for you. These benefits apply whether you’re fasted, or fully fueled.
There’s no strong evidence that fasted cardio is a better way to train, than fueling your body normally.
It may help you burn more fat, and for some, it will be more convenient to work out first thing, on an empty stomach. But there’s nothing to suggest that you should go out of your way to be in a fasted state when you do your time on the treadmill or the bike.