Let’s pretend for a second you’re going to drive from Ann Arbor to Chicago. You hop in your car, turn on the ignition, and the gas gauge shoots to E. You smile, say, “perfect!” and jump on the expressway thinking it will be great for your car to take this trip without any fuel. You’d probably think that’s crazy – right? Well, it’s just as crazy as exercising on an empty stomach in hopes to burn more body fat.
I’m sure you’ve no doubt heard of the myth, ‘to lose body fat faster, you should exercise in a fasted state.’ People believe that it taps into your fat stores faster. I mean, makes sense, right? If carbs aren’t readily available, like when you’ve fasted all night long, you have to burn fat – what else is there? Well, there’s fuel you don’t want to burn (like muscle).
Let’s look quickly at 3 reasons not to do cardio on an empty stomach:
1) You burn muscle tissue NOT fat.
At the intensity that nearly everyone exercises at (> 50-60% of maximum heart, or call it above 110 bpm) the predominant fuel source burned for energy is carbohydrate. If carbs aren’t readily available, your body has to turn to muscle tissue as a fuel source, since it’s the only fast-acting fuel available. True, fat is a fuel source, but it takes a very long time to be broken down into energy. As a result, unless exercise intensity is slowed to a crawl (like walking on a treadmill at 1.7 mph) you will burn muscle, not fat, which is very counterproductive.
2) You will burn fewer calories.
That’s right, not only does this result in less fat loss because you’re burning muscle tissue, but it results in lower intensity exercise (because your body is trying in vain, to reduce your exercise intensity, so you don’t burn muscle tissue). Keep in mind that the whole process of fat loss is to create a calorie deficit, by exercising in a fasted state it greatly diminishes your ability to create a significant calorie deficit.
3) You could get sick, throw-up, or pass out.
Any or all of these three can occur because of low blood sugar, which you will almost certainly cause VERY quickly when exercising in a fasted state. Exercise is hard enough for most people without puking or passing out while jogging on a treadmill. Why anyone would intentionally put themselves in a position to do so is beyond me.
Take Home Message:
If you’re looking to get more out of your cardio (from a fitness and a fat loss standpoint) try consuming a little bit of protein and carb before exercise. This process of nutrient timing will diminish muscle breakdown, aid in fat metabolism, speed up recovery, and result in more calories burned. In short – stop at the gas station before jumping on the freeway!