A big fitness myth that has been floating around forever is that working out on an empty stomach will help you burn more fat. It seems like everyone is spouting this myth, but the truth relies on science. Find out the facts!
The Truth: You should always eat something before exercising so your body has enough fuel to power through your workout.
The rationale behind this widely accepted myth is that forgoing food before exercise will force your body to burn more fat during your workout. This is a big, fat lie: Starving yourself before exercising can actually be detrimental to your body. Let’s get to the bottom of this fitness myth once and for all.
You need sugar to exert energy. Your body needs a certain amount of sugar for fuel when training. When that blood sugar is not there, your body will convert your own muscle tissue into energy. A study published in the Strength and Conditioning Journallooked at cyclists who ate before they trained versus those who fasted before they trained. The amount of fat burn was the same for both groups, but those who had trained without eating first had 10 percent of their calorie burn come from protein — including their own muscle mass. You’re trying to build muscle, not eat away at it!
Your body needs energy to perform at a high intensity. You know I’m always saying that I want you to work out as hard as you can for as long as you can. How can you do that if you haven’t properly fueled your body? Think about it this way: Would you drive a car without gas? Use your iPhone without charging it? Nope and nope. If you haven’t eaten anything, your workout won’t be as intense as if you’d fueled up beforehand, not to mention that you’ll likely suffer from low blood sugar, which will make you dizzy and sluggish.
You don’t need to gorge yourself; a healthy snack will do the trick. I suggest you eat something 45 minutes to an hour before training — you’ll have more energy and endurance to work harder, burn more calories, and improve your muscle tone. Aim for something with carbohydrates and protein. Here are a few quick, healthy snack ideas: a whey shake, low-fat yogurt with berries, or a banana or apple slices with natural almond butter.
The Bottom Line: You should always eat something before a workout. I’m not suggesting you pig out. A small, healthy snack consisting of carbohydrates and protein will properly fuel your body for a killer workout.
source : http://www.jillianmichaels.com/fit/lose-weight/myth-empty-stomach-workouts