Elite male runners probably have body fat percentages around 8 percent; elite women somewhat higher, maybe 12 percent. When I was in top shape, I hovered around 10 percent. I hate to offer precise numbers for elites, because it varies from person to person. There is no perfect body weight at which to aim.
Determining body fat also can be tricky. Depending on protocol used, measurement of body fat percentage can vary by plus or minus 3 percent, claims Nancy Clark, RD, author of Nancy Clark’s Sports Nutrition Guidebook. During a period of several decades when exercise scientists poked and probed me during their studies, the most precise method of determining body fat percentage was to submerge the subject in a tank of water, weighing before and after immersion. Scientists also used calipers to pinch subjects at different parts of the body, using established formulas to compute percentages. Various electronic devices spit out numbers that seemed to come from Mars. Despite all the scientific devices, I thought the most accurate way of measuring fat was by the bathroom scale.
So I’m not going to draw a line in the sand for you. Yes, you are correct that if you allow your body fat percentage to drop too low it can affect performance as well as good health. Without sufficient fat in reserve, the body will cannibalize itself, consuming muscle. Anorexic women in particular lose their periods and risk long-term bone damage. Men too can suffer long-term health problems if they fail to consume enough calories under the mistaken belief that lightening up will speed them up.
Is there a perfect number for body fat percentage? For you, maybe; a generic number that fits all, no. Over a period of years you can look at your diary and determine what weight your bathroom scale offered when you achieved your fastest performances.