We carry two kinds of fat in our bodies, essential fat which is stored in small amounts in bone marrow, organs, the central nervous system and muscles, and is needed for the normal, healthy functioning of all these body systems, and "storage" fat, which is stocked for energy. For men, essential body fat makes up about three percent of total body weight. For women, the percentage is higher - about 12 percent - because it includes amounts in the breasts, pelvis, hips and thighs believed necessary for normal female reproductive function.
For a woman of 25, a healthy range of body fat would be between 21-32 percent. This can increase slightly with age, to 23-33 percent for women between 40 and 59 and to 24 - 35 percent for those over 60. The healthy ranges in men are from 8-19 percent for those between the ages of 18-39, from 11-21 percent for those aged 40 to 59 and 13-24 percent for those over 60.
We now believe that, for women, the minimum body fat percentage should be between 13 and 17 percent although there's no hard and fast "rule" on what is too low for an individual. Keep in mind that there are several ways to measure body fat, and some are notoriously inaccurate, so the ranges and absolute numbers may vary. You may be able to tell when your body fat is too low by whether or not you're menstruating regularly. If your periods stop, nature may be telling you that you don't have enough body fat to nurture a fetus should you become pregnant. By turning off your menstrual cycle, your body is telling you that you are (temporarily) infertile. Please don't think of this as a convenient method of birth control - it isn't. You need normal hormonal function for more than just menstruation and fertility. For example, the health of your bones also depends on circulating levels of estrogen, the principal reproductive hormone in women.
Fitness expert, Dan Bornstein, tells me that when body fat percentage gets too low, energy levels and exercise performance decline. So does tolerance for cold. However, in general, as long as you continue to menstruate and as long as your energy level remains intact, your body fat percentage is probably OK. I would encourage you to gain a few pounds if your periods stop or become irregular or if you lack your usual energy.
Andrew Weil, M.D.