Understanding BMI: Doublethink From the CDC

Compare the following statements from CDC's FAQ about the statistic known as "Body Mass Index" or "BMI".

Calculating the BMI is one of the best methods for population assessment of overweight and obesity.

versus

Most studies have examined the relationship between BMI and risk of disease. Therefore we do not know whether two people with the same BMI but different amounts of fat have different risks for disease.

So while they overstate the value of this statistic in the first statement they hedge by the time they get to question 3.

Dare I ask how both of these statments can be true?

I'm working on a table to show how silly this is, but here is an example:

A man is consdered overweight with a BMI higher than 24.9.

If he is 6'0" tall that means he can weigh no more than 183.3 lbs. without being classed as "overweight" by the CDC. He is "obese" at or above 220 lbs.

Yet to weigh as little as 136.9 is considered "normal". Normal?!? that's death-camp survivor range!

A man I know is a shade over 6 feet tall and his last reliable independent body composition measurement said he had 176 lbs of lean mass. A truly healthy body-fat proportion would be 10%. That would put his net weight at roughly 195.5. Thats well in to the CDC's "overweight" range. To be at the upper borderline of CDC "normal" he'd have to be at 4% body fat. Sorry folks, that really would be unhealthy.

That was three years ago. The subject has been working out since and has put on some muscle mass, what then? This isn't a pro athlete and this isn't a guy who pumps iron for 2 or more hours a day. This is an office-worker who does 10-20 minutes of daily calesthenics and can't stand to do anything physical outdoors from April through September [in central Arizona].

The answer is that the CDC and other BMI advocates are just crazy, and they're able to hide their insanity behind a calculation for which they know they dare not publish tables.

BMI: The Lie That Keeps on Giving.

Junk science fads come and go. This particular piece of trash is linked over at Hugh Hewitt's blog.

It is a pretty snazzy presentation purporting to document an "obesity epidemic" based solely on the rediculous statistic known as "Body Mass Index" or BMI. According to Mark Twain there are three kinds of lies: Lies, Damned Lies and Statistics. BMI is the latter two.

BMI is a ratio of Mass to the square of Height done in [of course] SI "units" kg/meter^2. That's all it is, Mass over Area. What does that tell us about obesity? Nothing at all. Really, it makes no discrimination between fat mass and non-fat mass. Someone please explain to me how a statistic that contains zero information about the subject's quantity of body fat can be used to deduce obesity? [Don't bother, that's rhetorical.]

Let me give you an example: when Arnold won Mr. Universe he was 6'0" tall, weighed in at roughly 225 lbs. and had approximately 4% body fat. That gives him a BMI of 30.5 at the time. Was he obese? According to the proponents of BMI, emphatically YES.

See the disconnect? Ever seen the Odessa/Permian football team? Guess what? Every strapping young man in their entire line count in the alleged "obesity epidemic". I call shenanigans!

I forgive Hugh for not being a scientist, but enough with this horse dung already. It doesn't take a professional medical researcher to see what a crock it is as an obesity statistic.

The obesity crisis has become the bandwagon for people who expect the government research grant spigot to be open wide over their mouths. Worst of all there are some who use this debate to advocate that the government impose fad pseudo-science "healthy eating" uniformly across our society. Such authoritarianism goes hand in glove with socialised medicine.

Such advocates are happy to blur the reality with misleading statistics because if they didn't they couldn't credibly claim any kind of emergency (national or otherwise) exists.