Why is "oveweight" defined like this anyway?

Factual Questions
1

The CDC definition of “oveweight” for adults is a Body Mass Index (BMI) of 25 to 30 and “obese” is defined as over 30. “Normal” is defined as 18.5 to 25.

Why?

Is the middle third of the distribution in that “normal” range? No. That range represents most of the lower third with a small percentage of the lower third being “underweight” at less than 18.5 (less than 2%). The middle third of the distribution for BMI is 25 to 30.

Well, the middle third can still all be abnormal in one sense if being that way is associated with poorer outcomes. Are people 25 to 30 BMI worse off, even though they are typical and average for this country, than those whose BMI is “normal” and in the lower third of the distribution? No. Those individuals with BMIs 25 to 30 tend to live slightly longer than those with BMIs less than 25. Both are much better off than those with BMIs over 30, especially those with BMI’s over 35, and than those whose BMI is less than 18.5.

So shouldn’t a BMI of 25 to 30 be considered normal and a BMI of 18.5 to 25 considered mildly underweight? How did the CDC originally decide on callling 25 to 30 “overweight” anyway?

2

I personally don’t see how they can justify certain numbers as “normal” for everybody. For instance, take two people who are both 6 feet tall. One may have a small frame i.e. delicate bone structure and little muscle mass, while the other may have a large bone structure and a large amount of muscle mass. So even though these people are exactly the same height, there is no way the BMI chart can accurately determine a healthy weight based on height, since their respective body structures differ.

I wouldn’t rely on the BMI chart at all. It’s far from accurate.

3

I’m obese at 10% bodyfat. Teehee.

4

Most likely, the categories were chosen based on what groups are most similar in terms of risk for various weight-related diseases. There’s a statistical process known as clustering that’s used for this sort of thing.

5

Looking at the CDC website, it seems that CDC is very clear that BMI is but one measurement that needs the context of other measurements of health. Look at the cartoon of the bodybuilder and the fatso who share the same BMI. This page seems to make clear that BMI is not a replacement for things like calipers, underwater weighing, etc, but is simply an easy-to-use tool that gives folks an idea of whether they are at risk of certain diseases.

If I understand the OP right, he’s suggesting that BMI be “centered” around the average weight of Americans. But it seems pretty clear that the CDC is proposing this as a tool to measure where a person might fall into categories of health risks, not as a comparative tool of the body mass among Americans, or as a measure of life expectancy.

6

Ravenman’s explanation is excellent, but I wanted to specifically address this:

The BMI chart is extremely accurate when it’s applied to large groups. Trying to predict an individual’s health from their BMI is not appropriate–although it should certainly inform such a guess–but trying to predict the health of folks across a population from their BMI is a reasonable approach.

7

Not to be nasty, but I didn’t ask for WAGs, but for informed answers.

No, they did not use a cluster analysis and no such clustering exists. Mortality in the US is decreased for the “overweight” compared to the “normal” and more so compared to “obese” and “underweight”. There is an apparent linear increase in some morbidities (incidence of Type 2 DM, hypertension, for examples) from a BMI of 21 on up … but again fewer deaths in the “overweight” group. For some morbidities there is a U-shaped distribution, with best outcomes clustering in the 24 to 27 range.

And while the CDC may intend that clinicians use BMI cautiously, BMI has become the definition of overweight and obesity in all guidelines and nearly all studies done.

Yes, it is a very poor measure on its own, but it is easy to use and reproducible. Skin folds are difficult and give different results by different measurers. Underwater weighing is not available in most doctors offices. Clinically eyeballing for body type is too subjective. So on.

Do we do a service to the public by telling the majority of them that they are fat and setting targets for goal weights that they are near certain to fail to achieve? Especially in the situation when they are less likely to die at a less stringent target? Don’t we instead dilute the importance of weight control and fitness as a message to the truely obese?

Does anyone actually know if there was anything other than an arbitrary “Hey 25 is an easy number for those stupid clinicians to remember” consideration to the choosing of these standards?

BTW, I’m a pediatrician and it is even worse for the pediatric BMI’s … we are now supposed to calculate and chart BMI and BMI percentile for age for kids. The problem is that the BMI percentiles are wrong and were wrong by intent when they were created. If I tell you that your child is at the 85%ile for BMI for age, you’d expect that to mean that in the US today, 85% of kids your childs age are leaner (or at least have a smaller BMI) than your child, right? But you’d be wrong. Your kid is probably nearer the middle of the curve, much nearer. The data was based on pooled studies from the 60’s to 80’s and the most recent of those studies, NHANES III, had its data on kids over 6 yo excluded because it would have produced percentiles that showed fewer kids to be “fat” by BMI percentiles.

I’m not saying that we should not adress the problem of obesity in this country, and particularly the very real and significant problem of pediatric obesity. But creating a generation of scale watchers, telling kids in a very alternate dimension Lake Woebegone way that they are all above average, is counterproductive.