Pow wow, be careful on an empty stomach. Most licensed personal trainers say that exercise on an empty stomach can hinder results as well as being dangerous.
Here is a quote:
Should I do cardio on an empty stomach?
It is never a good idea to exert yourself on an empty stomach. You need food’s energy to drive you through the workout, so a small amount of food in your stomach is best. A lot of people believe that more fat will be burned if they work out on an empty stomach. This simply isn’t true. The majority of fat that is burned occurs in the two hours after your workout as your core body temperature returns to normal. So after a workout, while you’re on the couch watching Oprah, you’re still burning fat. Plain and simple - never exercise on an empty stomach. Eat a light snack about 30 minutes prior to your cardio.
Here is another good read that is explained it much more detail (too long to quote) :
Gotta disagree there. The “experts” will get you in trouble every time. There is no danger in exercising on an empty stomach( unless you are hypoglycemic) and it improves your bodies ability to mobilize fat stores.
These guys come up with new theories all the time, each disagreeing with the previous one. I exercise “best” on an empty stomach, and I lean out the easiest that way…and thats from experience. You only need food in your stomach if you have sent your insulin levels soaring (carbs and sugar) which blocks fat from being converted to glucose. If your insulin is stable, the body mobilizes its fats stores for energy.