The reason that caffeine is to be avoided is that, for a lot of people, it can lead to hunger and cravings. Caffeine is also a mild diuretic, a cardiac stimulant and can reduce fine motor coordination.

Caffeine is, perhaps, the most widely-used (and abused) addictive drug in the world. In the body, it works similarly (although more mildly) to amphetamines, cocaine and heroin in stimulating the brain. Caffeine increases neuron firing in the brain, causing dopamine and adrenaline to be released. The adrenaline release causes the following:
* Your pupils dilate.
* Your breathing tubes open up.
* Your heart beats faster.
* Blood pressure rises.
* Blood flow to the stomach slows.
* The liver releases sugar into the bloodstream for extra energy.
* Muscles tighten up, ready for action.

There have also been studies that caffeine may contribute to insulin resistance. Additionally, caffeine has a half life in the human body of six hours. Worst of all, caffeine addicts - like any other addict - can build up a resistance to it, requiring them to consume more and more.