(Don't like pesky reading? The Illustrated BMI Project makes most of the points below—in pictures!)

Not trying to start a flame war here, but I have to take issue with what one of my favorite bloggers, Eric de Place, has to say on one of my favorite blogs, the Daily Score, about Alabama's proposed "fat tax":

Seeing as how this year's holiday overeating falls on the eve of a national health care debate, I give you an interesting idea from the land of all things deep-fried:

...the Alabama State Employees' Insurance Board approved a plan that will charge workers an additional $25 to cover their insurance premiums, if they don't take advantage of free health screenings available to all state employees. The program, to begin in January, will require state workers to receive medical screenings for body mass index and health problems such as high blood pressure, high cholesterol and obesity.

[...] At first, you avoid the fee just just by talking to a doctor. You don't have to do or pay anything further, and you don't have to drop 40 pounds right away. Of course, given better medical information some unhealthy employees may want to do something. But that's sort of the point.

Where Alabama's plan becomes "tax-like" is for obese folks who don't show progress. A person determined to be very obese in 2009 would have until 2011 to demonstrate progress. If not, he or she would face the $25 higher premiums [per month]. Apparently, the insurance board has not yet determined what will qualify as sufficient progress to waive the fee. What is clear, at least according to this MSNBC article, is that the threshold for having to consult a doctor, and subsequently show progress, is at a Body Mass Index of 35 or higher. (For reference, that would be for a 5'6" person weighing 217 pounds or more, or a 6'0" person weighing at least 258 pounds.)

Given the serious health consequences of obesity and its related ailments — not to mention the high cost of treatment — it's hard for me to see how this isn't a good idea. In A few simple exams, access to good information, and the opportunity to make a change. What's not to like?

Here's my answer, in (I hope) brief: What's not to like is that BMI is a pretty lousy measure of overall health.

For example, the New York Times' health writer, Tara Parker-Pope—no radical fat activist herself—wrote earlier this year that "there is growing evidence that our obsession about weight as a primary measure of health may be misguided."

Last week a report in The Archives of Internal Medicine compared weight and cardiovascular risk factors among a representative sample of more than 5,400 adults. The data suggest that half of overweight people and one-third of obese people are “metabolically healthy.” That means that despite their excess pounds, many overweight and obese adults have healthy levels of “good” cholesterol, blood pressure, blood glucose and other risks for heart disease.

At the same time, about one out of four slim people — those who fall into the “healthy” weight range — actually have at least two cardiovascular risk factors typically associated with obesity, the study showed.

So if obesity isn't the best predictor of health and mortality, what is? Overall fitness—which, interestingly, does not correspond directly with BMI. And fitness is determined almost as easily as BMI is—by having a person perform a treadmill test. Research has shown that fat people who perform well on treadmill tests are at much lower risk for health problems than skinny people who are out of shape. (A treadmill test could also help get around another potential problem with the Alabama proposal—the fact that, besides the obese, the only people who will pay a premium for health insurance are smokers. It's easy to lie about whether you smoke; it's harder to lie about how long you can run).

Back to Parker-Pope:

Those with the lowest level of fitness, as measured on treadmill tests, were four times as likely to die during the 12-year study than those with the highest level of fitness. Even those who had just a minimal level of fitness had half the risk of dying compared with those who were least fit.

The results were adjusted to control for age, smoking and underlying heart problems and still showed that fitness, not weight, was most important in predicting mortality risk.

Now, in general, are you more likely to be unhealthy if you're morbidly obese? Sure. But if there's a more accurate way of measuring risk—one that acknowledges the fact that it's possible to be overweight and healthy—why not use it? Forcing overweight people to go on a diet isn't going to do anything for someone who's thin but eats junk food, drinks heavily, and never works out. And shaming overweight but healthy people—by mandating they show up for annual weigh-ins or pay a monthly "fat tax"—doesn't make anybody healthier.