Body fat is essential to keeping our bodies healthy, because not all body fat is bad. The cardiovascular and central nervous system needs it, as well as our skeletal, endocrine, and reproductive systems. It is an energy source, and after about half an hour of exercise, when the carbohydrate reserves are depleted, your body starts to draw energy from fat.

Leaner doesn’t always mean better. About 15-20 percent of body fat is required, so our bodies could function properly, and having fat percentage beneath that number has its negative effects. Male bodybuilders go below 5 percent, which is an alarming amount, but only when contesting (not staying below 5 percent all the time).

Read via the link below and see what happens when body fat percentage goes lower than it should.
http://diet234.com/the-negative-effects-of-a-low-body-fat-percent/