Caffeine is a diuretic, so you have to drink MORE water when you consume it.

Also coffee has carbs in it -- not a lot, but enough that it's not "free."

Atkins disallows coffee because he found that it triggers an insulin response in some people. Thus it has the same bad effects (weight gain, energy fluctuations) as carbs.

But others since then have done research calling into question that finding.

If you do drink coffee, watch the carbs, drink MORE water, and, if you notice a stall or dips in your energy, try cutting it out.