Answered by Declan Connelly on Monday, June 1, 2009
Sorry, no. Another diet myth bites the dust. It takes a certain number of calories to perform an activity, regardless of when you do it, so you won't burn more calories if you run a mile at 7:00 A.M. than you would if you ran a mile at 7:00 P.M. The only benefit to working out first thing in the morning is that more of the calories you burn may come from fat: When you first get up, your glycogen stores—your body's preferred source of fuel—are low, so your body may dip into its fat stores for energy. In general, when your body's glycogen reserves are down, you should to exercise at a lower intensity (or eat some carbs)—otherwise you'll feel weak and lethargic as you work out. I wouldn't recommend trying to exercise too much on an empty stomach, since you're susceptible to fainting or dizzy spells. Instead, have a small high-carbohydrate snack, like a glass of juice or piece of fruit, 20 to 30 minutes before your workout.