BMI: Badly Misleading Information

Metrics can be helpful in tracking progress and measuring adherence to processes. One process that is important to many people is that of losing weight or, more precisely, reducing their level of body fat. A simple metric for that purpose would certainly be helpful. At the same time, such a metric could help private and public agencies assess the prevalence and degree of obesity nationally and internationally. “But such a metric already exists!” you might exclaim. It’s the Body Mass Index (BMI).

For those of us still using bizarre imperial units, the BMI is calculated by measuring your weight in pounds and your height in feet. You then multiply your weight by 4.88 and divide the result by your height squared. For a 6’ 00” person weighing 200 lbs, their BMI is 27.1. So, yes, we have a simple metric, but this BMI is Balmy Metric Idiocy. It’s Badly Misleading Information.

The Proactionary Principle urges us, when making decisions, to strive for objectivity and to use evidence-based methods—not simply methods that are widely accepted and used. The great disparity between the high popularity of the BMI and its low level of objectivity and accuracy serves as an object lesson. It’s not just that millions of dieters use the BMI. It has been used and recommended for years by nutritionists, trainers, and official health, wellness, and fitness organizations. Governments are using it to define many millions of people as overweight and obese for the purposes of crafting health policy. The US National Institutes for Health (NIH) starting use BMI in 1985 to set cut-off points for weight and health.

So, what’s wrong with the BMI? I first realized one of its shortcomings when I ran the calculation for myself. I had been working on regaining some lost muscle mass. In doing so, I had put on a couple of pounds of fat along with the muscle. Despite the small gain, I know that I was still fairly lean. This was confirmed by having the gym staff (on more than one occasion) use their more expensive version of the Tanita bioelectrical impedance scale I have in my bathroom. The result: 12.5% body fat. This was a bit lower than the result on my cheaper Tanita scale at home, but close. Given that result—and the fact that I could easily see my abdominals in the mirror—I should expect the BMI to come out clearly below 25, right?

Using the BMI calculator at MSNBC (and verified by my own calculation), I discovered that my BMI was 27.1 According to that, I was overweight. At the same time, the BMI calculator complained that my waist size was “not typical”. I take it that “not typical” means that I had more muscle than most people. That is one major problem with the BMI: It utterly fails to distinguish between fat and muscle.

Take a slightly more non-typical example (but not at all an unknown one): An athlete or bodybuilder with 10% body fat weighing 225 lbs and standing 6 feet tall. At that body fat level, the BMI should be no more than around 20 (the lower end of normal). In fact, it might well be under 20, since few people have that low a level of body fat. Instead, the BMI comes out as 30.5. The BMI is telling this highly conditioned, wonderfully lean athlete that he is in fact obese!

It’s true that the BMI is a pleasantly simple metric. Simplicity is good, but not at the expense of necessary accuracy and information. Because it considers only height and weight, the BMI doesn’t discriminate between fat, muscle, organ, and water. As such, it’s a foolish way to define normal, overweight, or obese. It doesn’t take into account body frame, making it blind to the differences between men and shorter women. Studies show that BMI does a particularly poor job when applied to children, especially when comparing children of differing ethnic groups. For instance, “Slight Sri Lankan children in Australia have more body fat than white Australian children with the same BMI."

Another fatal weakness of the BMI is that it tells us little about people’s health status or probable future health. One reason for this is that it makes no distinction between the places where fat is stored on the body. It’s now known that abdominal fat is a better indicator of future health problems than fat in other areas, but the BMI is oblivious to this finding. The numbers of the BMI yield a misleadingly precise classification, despite the fact that it’s hard to see any difference in increased risk for premature death or serious illness between those who are of normal weight (BMIs of 20-25), overweight (25 to 30), and obese (over 40).

Risks only go up for those classified as underweight (BMI < 18) or as morbidly obese (BMI < 40). If you have a BMI between 25 and 26, you’re classified as overweight. Yet studies by Flegal at the US Center for Disease Control found this group had the best longevity prospects. A study by Gronniger found that moderately obese men (as classified by the BMI) had the same mortality rate as men of “normal” weight.

The BMI is arbitrary in the way it classifies people as normal, overweight, and obese. No scientific basis has been found for labeling people as overweight or obese on the basis of their BMI. What the BMI really does is to codify someone’s subjective views of overweight and obesity into a pseudo-objective metric. I don’t say this to make things easier for fat people. Personally, I work at staying reasonably lean and I have a strong aversion to body fat in other people. My own arbitrary measures would be at least at strict as those embodied in the BMI—were I to attempt to force my preferences onto everyone else, under cover of science.

As I have argued in the context of critiquing the “precautionary principle”, activists like arbitrariness. Arbitrary measures and principles are easily manipulated by special interests. Politicians can use the arbitrariness of the BMI to hype a “war on fat” and to troll for votes by exaggerating health risks. The weight loss industry and those who sell weight loss drugs can do the same.

The BMI is a simple, slim measure, but it’s too simple to do the job. A better approach will, of necessity, be a little better filled out with information and wisdom. If you hadn’t considered these points before, now you know. Don’t be a Bloody Moronic Idiot by continuing to use the BMI.