“Dear CDC: BMI ≠ Body Fat”

Happy New Year!  Another year, another study apparently proving that being a little overweight is fine and may even reduce your risk of dying, as compared with being thinner.  This one, from the Centers for Disease Control and Prevention, re-analyzes data from 97 studies involving nearly three million people from various countries.  The study, which is being published in the Journal of the American Medical Association, has been widely publicized.  I counted two articles in the New York Times and one in the Wall Street Journal plus coverage on radio, TV, and the Internet.

This is the most recent illustration of “the obesity paradox,” the fact that, although excess weight is a factor in cardiovascular disease, obese patients with heart disease tend to live longer than those with no excess weight.  I would argue that the root of this “paradox” is in the definition of obesity.  When you used a flawed measuring device, you get incorrect results and BMI, which used as the index of fitness in all these studies, is seriously flawed, as I explained in my post, “What’s Wrong With BMI.”  

BMI is a ratio of weight to height:  multiply your weight in pounds by 4.88, then divide it by your height in inches, squared.  Anyone with a BMI over 25 is considered overweight; anyone over 30 is classified as obese.  This means that people who are fatter than average (unhealthy) are lumped together with people who are more muscular than average (very healthy).  Under this system, a person the same height and weight as Tony Horton would be treated as equally fit even if he had never exercised a day in his life!  Measuring the percentage of body fat for each individual would provide a more accurate picture of the situation.  Lots of people know that there are problems with BMI.  Melinda Beck’s article in the WSJ alludes to this.  A number of her readers know it too, as indicated by some of the comments on the article.  So why did the CDC use it in this case? The answer is that it is easier and cheaper to use existing data and the existing data all use BMI.

As a federal agency under the Department of Health and Human Services, the CDC is looked upon as a leader in the fields of science and health.  When it continues to use and endorse BMI as a measure of good physical condition, it sets an example that others follow, which does a disservice to all of us.  In my doctor’s office my weight is measured on a BMI scale.  I ask my doctor why there is no body fat scale and she says that BMI is the current standard.  As long as this continues, there will be no data on body fat for anyone to study unless they do original studies themselves.  Fortunately, some of this research is being done.  A Mayo clinic study found that people of normal weight can be at greater risk of heart disease if the levels of body fat are high.

The timing of this new study and the ensuing publicity could hardly be more damaging.  Scientists at the CDC are surely aware that fat people who lose weight through appropriate diet and exercise (not illness) will lower their blood pressure, reduce their blood sugar, and improve their cholesterol readings, all of which will contribute to better health.  At the beginning of a new year many people are pre-disposed to turn over a new leaf.  Instead of encouraging people to cultivate better habits, the CDC puts its imprimatur on the perverse message that it’s OK to be fat.