Answer:
Caffiene. And sugar, if you add a lot. Also, sometimes the fat in milks can be bad for you.
OR...
black coffee has very minimal caloric content. Aside from the very young and people with preexisting health problems, caffeine is only a mild stimulant and diuretic. In reasonable amounts (less than say 5 cups/day), it is well tolerated. Studies are showing that coffee may contain antioxidant benefits.