One of the main ingredients in Metabolife is Guarana which is supposed be a lot like caffiene, even inducing the same side effects as caffiene. I myself had been taking the pure guarana pills and felt very awake etc. But after reading the Atkins new revolution diet, and having him mention that caffiene can actually constrict the weight loss I made a decision to avoid the guarana while doing this, only using it from time to time. Hope this helps