Few things can test your toughness better than working out on an empty stomach. First, you feel the gnawing emptiness of your stomach calling for food. Then, you start to feel the sluggishness that comes from a lack of sustenance.
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Pushing through that wall of hunger can make you tougher but it won't help you burn fat faster. At the end of the day, weight loss is something that happens over time, very gradually, and is a product of how much food you eat every day.
Fasted cardio is the popular term for doing cardio on an empty stomach. If you want to do it properly you eat dinner, go to bed, wake up in the morning, and go for your run. That way, you'll have gone without food for around 8 hours, which is plenty of time for your stomach to burn through last night's dinner.
Read More: Is Cardio Good to Do on an Empty Stomach?
The theory is that your body will have burned through all of its available energy from the last meal and that when you start running you will be dipping into your body's energy reserves. That means that all of the energy for your run should come from the energy reserves in your fat. If you eat a meal before your run then you would theoretically burn through the energy of that meal instead of your body's energy reserves.
What Research Says
When this concept was first being studied, it looked very promising. Researchers would have their study subjects run on a treadmill and connect a machine to them that measures the air that they breathe in and out. With this machine, they can determine whether the subject is using fat or carbs as an energy source.
As it turns out, the group of subjects running on an empty stomach stayed in the fat burning zone longer than the carb burning zone. The group who ate before the run used more carbs for energy. It seems like this would seal the deal and make it known that running on an empty stomach burns more fat, but there was a problem: both groups of people lost the same amount of weight.
Why You Won't Burn More Fat
Even though you might use more fat for energy during a run if you didn't eat before, the effects balance themselves out through the day in a few ways.
The first problem is that you are going to be really, really hungry after your run. You'll be so ravenous throughout the rest of the day that you'll probably end up eating as much food as you would have if you ate before the run.
It's also tough to keep up the intensity during a run when you haven't given your body any energy beforehand. If you eat before your run you'll be able to push a little bit harder and burn more calories that way.
The same thing is true for the number of calories that you burn after you run. If you can run harder you will not only burn more calories during your run but you will use more energy throughout the day as your body recovers.
At the end of the day, it's not worth running on an empty stomach if your goal is to burn fat. The big difference is that you will be more hungry if you run on an empty stomach and it will be more uncomfortable. Another big difference doesn't have to do with fat loss, but muscle mass.
You're more likely to lose muscle mass if you run on an empty stomach. That's because your body is more likely to pull energy from your muscles to fuel your run. If you're trying to gain muscle or even just preserve it, don't take a chance by running while fasted.