Sunday, September 16, 2007

He weighs, she weighs

Who hasn't heard of the BMI these days?

It's all over the place: the evening news, magazines, actuarial tables, notes being sent home from schools.

BMI, or Body-Mass Index, is a number derived from height and weight. There are plenty of internet calculators to help you come up with yours. You are classified as being underweight, normal weight, overweight, or obese based on your BMI.

There are several reasons not to like the BMI, detailed in any number of places. My objection today is to it's gender-neutrality. There is no difference in BMI classification for men and women. Putting aside issues of muscle mass, or changes in body composition based on age, we're still left with the same chart for men and women. An average build, in-shape male always weighs more than an average build, in-shape female. Why should this not be reflected in the BMI charts?

This is where the BMI loses it's mainstream credibility, at least my experience. Mildly obese middle-aged males who look at their 'goal weight' on a chart are generally dumbfounded - they haven't seen a BMI of 25 since they were 17 years old. But when they ask me "I'm supposed to weigh the same as my wife is supposed to weigh?" it's hard to answer yes.

A woman with a BMI of 18-20 looks very trim. A man with a BMI of 18-20 looks sickly. I suppose one could claim that these perceptions are based on misogynistic ideas about what makes men and women attractive, but I don't think so. We have different growth charts for boys and girls; why the same weight chart for men and women?


Rob said...

Agreed. Plus, BMI has little correlation with anything significant. Hip to Waste ratio is the best. I always laugh with patients when we look at goal weight. Those numbers are actually really misguided as far as numbers to put in front of people. They don't really reflect health, in my opinion.

rlbates said...

I always (and still do) liked the formula for a healthy weight(I think from old insurance standards)-- a woman 100 lb for the first 5 ft of height plus 5 lb for each addition inch of height [range is +/- 10%]. For men, it's 100 lb for the first 5 ft of height plus 6 lbs for each additional inch of height [range is +/1 10%]. So for me at 5'4" the range is 108-132 lb. But a man at 5'4" would be 119-145 lb.

Dr. Smak said...

Rob - do you measure waist to hip? I tend to ignore the BMI except for patients somewhat close to it. For those who are very overweight, I try to set a reasonable weight loss goal instead of shooting for the impossible.

RLB - I like that rule as well. Much simpler, too!