The Pros (and Cons) of Fasted Cardio Workouts

If you paid attention at all in your middle school health class, you know that your body loses weight when it begins using the fat cells in your body, after it has exhausted its supply of calories in your stomach. Logically, it then makes sense that if you work out on an empty stomach, you’ll burn more fat immediately, since your body has no other fuel to work with. Right?

What you should have also realized in that middle school health class is that your body’s inner workings are incredibly complex. As a result, fasted cardio isn’t a surefire boost to your fat-burning goals. Experts have mixed reviews of waking up early to get your miles in before you’ve had breakfast. While there can be a few benefits, there are also some drawbacks.

Generally speaking, if your schedule dictates that you have to be at the gym before the sun rises in order to get a workout in, you might not have much choice about running before you have any food. You might also want to avoid the feeling of your dinner sloshing around in your stomach as you’re working toward your third mile, and prefer to exercise without food in your system. It’s a free world, and these types of choices are solely yours to make. Still, it’s good to know what’s happening in your body when you make those choices. Here are the potential risks and rewards of fasted cardio.

Pro: There is potential to burn more fat in the morning

Man running on treadmill

Man running on treadmill | Source: iStock

Common sense might say that you have a good chance of burning more fat in the morning, and there is some evidence to support those theories. For one, your levels of cortisol are higher in the morning, and cortisol is the hormone that helps to metabolize fat into energy. Your body wakes up a fat-burning machine, in other words. In addition, while you are asleep your body uses up the leftover carbohydrates that are still in your system, leaving you with very little except fat reserves for energy in the morning.

According to, there have been several studies conducted that show people can burn up to 20% more fat if they work out in the morning before consuming any food. Sounds primed to go, right? Unfortunately, that’s not the whole story.

Con: You’re also likely to burn more muscle

Man with muscular back

Man with muscular back | Source: iStock

If you’ve worked hard to build your muscle tone and would hate to see it dwindle, fasted cardio isn’t the method for you. Though some studies suggest you can burn more fat, others show no significant difference in fat loss. What does change, however, is the amount of proteins (and yes, muscle) you burn on top of that unchanging fat level.

In many cases, your body will resort to using amino acids for fuel, the building blocks of protein and what ultimately leads to muscle mass. In that same study mentioned above, researchers found that about 10% more protein was used during fasted cardio workouts, which reduced the amount of overall muscle mass. You can combat that loss by eating proteins like whey after a workout, but it’s not a guarantee you’ll be able to keep all of that hard-earned muscle.

Pro: It can be good for short-term goals

Fasted cardio has long been popular with body builders and other people concerned with their physique for competitions. The reason? For people with low body mass indexes and little fat, fasted cardio can show quick results on performance days. They have low levels of glycogen, the body’s first source of stored energy, according to Men’s Fitness. Instead of burning that in the morning, they’ll burn fat and a little muscle, quickly taking care of the flubbier areas on their bodies.

Unfortunately, this doesn’t work for people who aren’t on extremely strict diets with a normal BMI. Aside from getting your body energized in the morning, pre-breakfast workouts won’t show a marked difference for the typical gym-goer.

Con: It doesn’t promote sustained fat burning

Man laying down after workout

Man laying down after workout | Source: iStock

For most people, the most effective way to make sure your body burns fat instead of muscle when you’re fasting is through low intensity exercise when you can still hold a conversation. These types of workouts can typically burn more calories and fat during your workout than high-intensity alternatives, but they don’t continue to promote that burn after your cool down routine.

Studies have shown that high intensity workouts can be much shorter than low intensity options, but they can end up promoting twice the fat loss over time, since your body continues to burn calories and fat long after you’ve left the gym. To get through a high intensity program without burning muscle, you’ll need some food in your system.

In addition, Ace Fitness suggests that instead of burning fat, fasted cardio might only lead to a fat distribution. In other words, exercising without breakfast might actually contribute to your beer gut. It might be OK for a time or so, but it might not be the best bet for your go-to weight loss strategy.

Follow Nikelle on Twitter @Nikelle_CS

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