How fat are you, really?

Posted: June 3, 2011 in Fitness, Health, Obesity, Uncategorized, Weight Loss
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I realize that scientific studies of things that we consider to be common sense are necessary because sometimes the truth is counter to what we all think we know. But there are times when you still want to say, “Well, duh,” to studies. The poll finding that obese people are more likely to be depressed than their normal to slightly overweight counterparts comes to mind. Really? I can almost guarantee you that obesity starts as eating as a way of dealing with emotional issues, and then the resultant obesity depresses you, making you turn to food more, and the vicious cycle begins.

And so I begin the first of a series of forays into various studies and pronouncements by the experts. Let’s start with the famed Body Mass Index. Slate does a good job of summarizing the problem with this metric, which was based on an equation developed by Belgian Adolphe Quetelet in 1832:

This project had nothing to do with obesity-related diseases, nor even with obesity itself. Rather, Quetelet used the equation to describe the standard proportions of the human build—the ratio of weight to height in the average adult. Using data collected from several hundred countrymen, he found that weight varied not in direct proportion to height  (such that, say, people 10 percent taller than average were 10 percent heavier, too) but in proportion to the square of height. (People 10 percent taller than average tended to be about 21 percent heavier.) …

The first large-scale studies of obesity and health were conducted in the early 20th century, when insurance companies began using comparisons of height and weight among their policyholders to show that “overweight” people died earlier than those of “ideal” weight. Subsequent actuarial and medical studies found that obese people were also were more likely to get diabetes, hypertension, and heart disease …

in 1972, physiology professor and obesity researcher Ancel Keys published his “Indices of Relative Weight and Obesity,” a landmark study of more than 7,400 men in five countries. Keys examined which of the height-weight formulas matched up best with each subject’s body-fat percentage, as measured more directly. It turned out that the best predictor came from Quetelet: weight divided by height squared. Keys renamed this number the body mass index.

The new measure caught on among researchers who had previously relied on slower and more expensive measures of body fat or on the broad categories (underweight, ideal weight, and overweight) identified by the insurance companies. The cheap and easy BMI test allowed them to plan and execute ambitious new studies involving hundreds of thousands of participants and to go back through troves of historical height and weight data and estimate levels of obesity in previous decades…

Gradually, though, the popularity of BMI spread from epidemiologists who used it for studies of population health to doctors who wanted a quick way to measure body fat in individual patients … Keys had never intended for the BMI to be used in this way. His original paper warned against using the body mass index for individual diagnoses, since the equation ignores variables like a patient’s gender or age, which affect how BMI relates to health. It’s one thing to estimate the average percent body fat for large groups with diverse builds, Keys argued, but quite another to slap a number and label on someone without regard for these factors.

So much for listening to the guy who came up with it. That’s one of the problems with studies; the design of double-blind studies shows when there is “statistically significant” differences in large numbers of people when you try to control for all variables but one. Studies demonstrate what happens in groups of people, not in individual patients. Look at the numbers in any study, and you’ll see that though the number of exceptions (or outliers) in any study may be low, they still exist. So BMI, that you see used in all the studies, may have some legitimacy when talking about groups. But when it comes to you specifically, well, it may not be accurate.

Richard N. Bergman of the Keck School of Medicine at the University of Southern California believes he has a better way to estimate an individual’s body fat than BMI, which he shared with New Scientist:

Our goal was to find a simple index of obesity, something that a practicing clinician could use. To do that we looked at a population of 2000 people of Latin American descent who had had their percentage body fat measured directly using dual-energy X-ray absorption, which is an accurate way to quantify body fat. We then asked what parameters we could measure in these people that would best predict the true percentage of fat.

It turned out that hip circumference and height were more correlated with percentage body fat than anything else, including waist circumference and weight. So we designed an equation that could take both of these into account. We call this the Body Adiposity Index. It turns out that BAI is a good predictor of percentage adiposity, so if your BAI is 30, then your percentage body fat is around 30 per cent. It is reasonably accurate — not terribly accurate — but usable as a clinical tool.

I love the qualifier — reasonably accurate but not “terribly”? What the heck does that mean? It’s not great but it’s sure better than the dumbass BMI? So I found a calculator to see what the difference would be for me using the two different measurements (it’s in metric, but they helpfully provide a English measurements to metric converter (Don’t you find it ironic that the English use metric but the colonials still use English?).

According to BMI calculations, I’m 40.2% fat. According to BAI, I’m 45.5% fat. According to my lovely Tanita scale, I’m 49.4% fat. Wow, almost 10 points of difference. Okay, you want to quibble: 9.2 points. Now, for me, any way you slice it, I’m obese. So it doesn’t really matter. But if you’re on the border between healthy and overweight, or overweight and obese, and you want to be sure, guess what? You’re going to have trouble short of getting shot through with radiation for the accurate X-ray test. And, of course, the experts are telling us we’re all getting too much radiation.

So, friends and neighbors, what should we laypeople do when the experts can’t come up with a measurement of how fat we are that’s easy for *them* to use? Don’t sweat it if you’re on a border; if you’re in the ball park, you’re better than most of us. Eat healthy, work out regularly and try to get away from the computer and television once in a while. (And, yes, that last bit was for my benefit more than anyone else’s.)

And thanks, David, for getting me on this science-y kick.


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