Seriously though, I didn't just pull that 15% body fat thing out of thin air. First, that is what Dr Barry Sears suggests as optimal for a male, along with 18% for a female. A female has to maintain a higher bodyfat percentage due to fat in her mammary tissue/boobs.

When a female's BF% drops much lower than 18%, her menstrual cycle is screwed up/halted, and many think that can contribute to osteoporosis. The effects on her hormones happen fairly quickly

For a male, this effect may not be immeadiate, but we know for certain that high level endurance athletes, most of whom have quite low BF percentages, have markedly higher rates of several cancers. IE, Lance Armstrong. You can't escape your hormones.