My comment (on another blog) about obesity, BMI, and "blame"

Over at Sociological Images, there is a post showing a 1967 public service announcement (PSA) called "To Cure a Fat Child is not a Simple Matter".

One of the commenters there - attentie - wrote the following:

I think the focus should lie on promoting a healthier life style rather than thinner bodies at any costs. The real epidemic is not the fact that people are getting less atractive according to the modern beauty ideal (thin= beautiful) but that people are less active and eat more junk food and candy. Unhealthy diets or excercise routines (too much excercise can be very bad for your joints etc) are no solution. Little things, like walking or taking the bike to work instead of the car or eating vegetables and fruits every day, it all helps to stay healthier. If you lose some weight doing that, that is great, but it should not be the focus point IMHO.

What follows is my response to that comment (and, to a lesser degree, the issue of who is to blame about a fat child).

IMHO, too. Metrics of “thinness” don’t work for me; even though I might think of myself as not being “obese”, that is what I am medically classified as being (6′3″, 243 lbs = 30.4 BMI). (But then again, many athletes have very high BMIs: when he was still in pro-wrestling Dwayne Johnson – aka “The Rock” – was billed as being 6′5″, 275 lbs = 32.6 BMI.)

I was – a year ago – feeling that I was getting a little too chunky for my jeans (was up at ~260lbs), and so (instead of buying new jeans), I bought a new bike, and started cycling my commute instead of busing. I’ve not gone to the gym, except to do yoga (but stopped doing that about 5 months ago), and the only thing I do for “exercise” is ride my bike and go for walks.

I also changed up my diet, and eat out less often than before, but still enjoy a weekly beer night with my friends (don’t worry — I have one or two beers over four hours and never cycle home if I don’t feel safe). The eating out less has not only saved my budget (letting me pay off my bike a lot sooner), but it (I believe) helped slim me down, since I don’t make the heavy cream sauces that I am so partial to when dining out. In addition, since I make a lot of my own food, I am very critical about what goes into meals, and try to be more balanced with my ingredients.

Technically, I’m still “obese,” and in order to move all the way down through “overweight”, I have to either become shorter (not likely happening) or I have to get down to 200 lbs (something that didn’t happen, even when I was in peak physical condition in my early 20s). Despite all of this, however, I still feel that I am “healthy.” I am physically able to do a lot of the things that I like doing, I don’t have to wait several minutes to catch my breath after exertion, and I am happy with my own body image.

Why all that discussion? Well, I wrote up a blog entry about BMI a little while ago, looking at what its implications really mean, and came out with the conclusion that I don’t fit inside the general population parameters for which the BMI works: I’m either too tall (I am much taller than the original subject population from which the BMI was derived), of the wrong body proportions (I have an Asian build of a relatively longer torso and relatively shorter legs, meaning that my weight distribution is different than the original subject population), or more athletic (I’m not really a sedentary person, but tend toward athletic).

Furthermore, although the BMI seems a “scientific” and “objective” measure of obesity because it produces a number (and we tend to have social relationships with numbers that imbue them with an air of objectivity), it really isn’t. It’s based on a generalized trend of height and weight seen among sedentary male Belgians from the mid-19th Century. In addition, that relationship is a description of the general trend of that population, and not a predictive statement about any one person who is either in that population or not in that population.

The BMI still persists, though. Why? Well, because population trends that have a predictive ability at the population level tend to be useful (and BMI still is a good population-level predictor of health risk). It’s also useful because it is easy to calculate, requiring very little in terms of time and money to collect the data (as opposed to lipid panels, %fat calculations, etc.). And (perhaps most annoyingly) it remains a standard, and is thus something that is difficult to overturn.

What does YOUR BMI calculation mean, though? It could be spot-on (e.g., if you are over 30, you may well be obese), or it may be highly deviant (as in the case with Dwayne Johnson). This is called variability, and is one of the reasons why back-calculating population-level statistics to an individual is a statistical no-no (no matter how often we do it).

At the end of the day, is my mother to blame for me being 6′3″ and 243lbs? Well… since I’ve not been living at home for over 14 years, I would say… no. Also, since I’ve (hopefully) shown that BMI categories are socially constructed and have very statistical relevance when assigned to an individual, then I would say that even when I was 18 and going off to college (all 246 lbs of me), she still wasn’t “at fault” for making me “obese.”

UPDATE (11/17/2011 @ 11:28PM): The property of "objectivity" is a little difficult in the case of BMI. BMI is an objective measurement that it isn't subject to personal feelings, interpretations, or prejudice. The measurement of height and weight (if taken properly) is objective. The calculation of BMI (using the formula) is also objective. The collation of height, weight, and BMI data from large groups of people is also objective. The analysis of BMI data as population data is also objective. However, the interpretation of the the population level metric to that of the individual (which is what I'm talking about here) isn't objective, specifically because it is subject to personal feelings, interpretations, and prejudice.