How important is body fat percentage?

Pity the body mass index. It's meant first and foremost to be a tool to assess entire populations, as opposed to a conclusive diagnostic tool for individuals. For that purpose, it's telling us something meaningful when we see that the prevalence of obesity in the U.S., as defined by BMI, grew at an alarming rate from 1986-2000:



For individuals, it's a bit coarse in that it doesn't distinguish muscle from fat, so a very muscular person might well be classified as "obese." For example, superfast former Oregon Ducks running back De'Anthony Thomas probably would have been drafted higher than the fourth round but among other things, there were concerns that he was too slender for regular duty in the NFL.

Thomas' listed stats are 5'9" and 174 pounds, which gives him a BMI of 25.7, considered "overweight." Dude does not look overweight at all:

But of course, very few people are NFL caliber athletes, so pointing out that the ridiculousness of using BMI to classify Thomas as overweight isn't really relevant to its appropriateness for the average person.

Still, there are medical professionals who agree that BMI is a problematic individual diagnostic tool; body fat percentage, they suggest, is a better measure. According to the New York Times, the World Health Organization and the National Institutes of Health "define obesity as more than 25 percent body fat in men and more than 35 percent body fat in women."

The American Council on Exercise suggests the following:

Males: 2-5% (essential fat); 6-13% (athletes); 14-17% (fitness); 18-25% (acceptable); 26+% (obese)

Females: 10-13% (essential fat); 14-20% (athletes); 21-24% (fitness); 25-31% (acceptable); 32+% (obese)

How do you figure out your body fat percentage? The most accurate tests involve going to a specialized clinic but there are quick and dirty calculators that require only basic height and weight stats plus some simple measurements.

I tried all of the variations available at that site and they all put me around 11-12%. Interestingly, this site (which uses only weight and waist) had me around 8.5%. The discrepancy makes sense, if you think about it. Although I do lift weights 2-3 times a week, I'm primarily a runner; for me, resistance training is more for muscle maintenance than it is for muscle development. I'd like to think I'm not totally useless in the upper body (hey, I can do pull-ups), but I'm not steadily increasing the weight I'm lifting or anything, and I clearly look like a distance runner, not a sprinter.

Therefore, any measurement that looks only at my weight (low, since I'm a runner) and waist measurement (low, since I'm lean) is going to overestimate my muscularity. The body fat calculators that ask for bicep, forearm, and chest measurements (and others) are going to be more sensitive to my lack of bulging muscles up top and penalize me accordingly.

Interestingly, I would've thought it would be easier to lower my body fat percentage by adding muscle than by losing fat, even though I doubt that I'm at the edge where I have no more non-essential fat to lose. But it appears not: to get below 10%, I could either lose 4 pounds of pure fat (out of my 15.7 or so), or add 17 pounds of pure muscle.... Well, fortunately I don't have any body image issues and am content with where I am right now. And it's not like I'm a professional athlete or anything, so the sacrifices it would take to decrease my body fat percentage further just don't seem worth it.

In the end, I'm pleased at my relative normality - I'm a right-handed male of average height, and BMI and body fat percentage both agree that I'm lean (but not so skeletal as to be underweight).