Jillian Michaels says......

Caffeine Confusion
Lately there's been a lot of talk about caffeine and exercise. Studies have shown that consuming caffeine before working out enhances performance and helps to burn fat. Hey, don't down that pot of coffee just yet! You need to hear the whole story behind caffeine and exercise
First of all, coffee and Diet Coke isn't going to do it. As a fitness aide, caffeine is only effective when taken in pill form in conjunction with aspirin. When you drink coffee, tea, and soda, caffeine gives you short-term energy. That's because caffeine stimulates your adrenal glands. These glands dump adrenaline into your bloodstream like it's going out of style. When the caffeine leaves your system, your adrenal glands will be depleted for a while. This will leave you feeling weak and tired from low adrenaline levels.

To stay on top of your game, avoid caffeinated beverages whenever possible. It's fine to have your Starbucks in the morning, but try to keep your overall caffeine consumption to a minimum.