The Fat Fixation Fallacy (Body Fat Percentage)

My daughter told me a tale, told second hand, from her gym. According to this account, a young woman went to the gym with her (idiot) boyfriend. At the gym–a much swankier place than mine, apparently–she had her body fat percentage tested. She came out at 8% body fat. When this was announced, the boyfriend supposedly said, “You have a lot of work to do.”

That’s how eating disorders and neuroses get started, I think. As we continue to consider methods for answering the question, “How much should I weigh?” we’ll take up body fat percentage next.

Body fat percentage is a fairly straightforward metric for your weight, involving a good deal less arithmetic than BMI. How heavy should you be? Body fat percentage can give you a pretty good idea.

How do you calculate it. It’s simple: First, weigh yourself. Let’s say you weigh 150 pounds. Then take all of your fat out and weigh it. We’ll say you have 45 pounds of fat. Divide the fat pounds by the total weight (45/150) and you get 30% body fat. Now be sure to put that fat back in. Leaving it out is cheating!

Obviously there’s a problem there. We can weigh ourselves easily enough, but weighing our fat is a bit tougher. The more accurate methods for doing that (at least on a live body) are also the more difficult or expensive. For example, you could use a body fat caliper for less than \$10, but its results will not be nearly as accurate as underwater weighing or biometric impedance measurements. But can you manage to do either of those methods on your own? No.

Let’s assume that you can accurately measure your body fat percentage. Where should it be? The number for men should be considerably lower than that for women. Female athletes will typically have 14-20% body fat while male athletes will be in the 6-13% range. That suggests that the bonehead boyfriend at my daughter’s gym didn’t have a clue what he was talking about. A woman with 8% body fat is probably in an unhealthy weight range.

So how heavy should you be using body fat percentage as a guide? Let’s assume that want to have an athlete’s build. You could determine a weight at which your body fat would come in at, say, 16% for a woman or 10% for a man. For example, maybe a man discovers that 162 pounds yields that magical 10% figure. But what’s magical about 10% for a man or 16% for a woman. The “athlete” percentages might cover a weight range of 8 to 15 pounds for an average woman or man. That’s something like 98 to 106 pounds for an athletic woman or 160 to 175 for an athletic man. You can see that we’re not getting a great deal of certainty with this measurement.

Body fat percentage is a useful tool and certainly far superior tool to BMI, but it does not give us that magical number of “How much should I weigh?” At the same time it can lead us into making an idol out of the number, which we can already do on the bathroom scale.