The obesity and BMI divide.

Paul Campos has a Daily Beast article ("Michelle Obama's Let's Move Campaign Is Helping Bullies") that puts it very succinctly what our bovine barnmate was trying to illustrate earlier: that contemporary discourse on weight-lose hinged on the Body-Mass Index (BMI) bordering on pseudo-science of pop-psychology, with all the trimmings of statistical conjuring and arbitrary standards buried in formality to make bullshit sound oh-so authoritative, medical but mostly pompous.

If you didn't get Cow's message earlier, here's Campos laying it out for you, which I doubt substantively differs much from Singapore although written in the context of America:

The Centers for Disease Control website offers these definitions of “overweight” and “obesity” in children: (emboldening by me)

· Overweight is defined as a BMI at or above the 85th percentile and lower than the 95th percentile for children of the same age and sex. · Obesity is defined as a BMI at or above the 95th percentile for children of the same age and sex.

These definitions raise a couple of obvious questions in a nation that has been bombarded with claims that childhood obesity is skyrocketing. After all, by this standard, aren’t exactly 10 percent of children always overweight by definition, while another 5 percent are obese? And what’s the justification for these statistical cut-points, anyway?

The definitions were created by an expert committee chaired by William Dietz, a CDC bureaucrat who has made a career out of fomenting fat panic. The committee decided that the cut-points for defining “overweight” and “obesity” in children would be determined by height-weight growth chart statistics drawn from the 1960s and 1970s, when children were smaller and childhood malnutrition was more common. The upshot was that the 95th percentile on those charts a generation ago is about the 80th percentile today-hence, the “childhood obesity epidemic.”

These definitions are completely arbitrary. The committee members chose them not on the basis of any demonstrated correlation between the statistical cut-points and increased health risk, but rather because there was no standard definition of overweight and obesity in children, and so they invented one. In other words, the “childhood obesity epidemic” was conjured up by bureaucratic fiat.

The committee did this despite Americans being healthier, by every objective measure, than they’ve ever been: Life expectancy is at an all-time high, and demographers predict it will continue to climb steadily. This isn’t surprising given that mortality rates from the nation’s two biggest killers, heart disease and cancer, are at historical lows and keep declining, while infectious diseases are under better control than ever. There’s no reason to think that today’s children won’t be healthier as adults than their parents, just as today their parents are healthier than their own parents were at the same age, continuing a pattern that has prevailed since public health records began to be kept in the 19th century.

(Hat tip Kate Harding.)

What's in a number?

This Cow was uncomplainingly chewing her cud of chocolate croissant and marble cake at breakfast this morning when she read about the Health Promotion Board's National Healthy Lifestyle Campaign: Know Your BMI, Know Your Risks. HPB's press release reveals this campaign to be overwhelmingly about "obesity and overweight issues" and "weight management" and how BMI "is used as a measurement tool to classify a range of health risk categories". There's even a whole website devoted to this magic number.

There are many blogs and websites out there which explore the wider social context of, and experiences arising from the fixation of equating BMI to obesity to health or poorness thereof (Shapely Prose on our link sidebar is an excellent springboard). But this Cow fiddles around with numbers and statistical distributions for a good part of her waking hours, and something about the definition of the risk categories in the BMI, and the definition of obesity in general, strikes her as easy PR without proven improvements in public health outcome.

Definitions of obesity based on percentile measures of a population distribution of height and weight are just that: percentiles. Percentiles are cut-off points which capture an arbitrary proportion of a population that fall above or below the cut-off point. A statement of fact, "X% of people are above Y kg in weight" tells you only that. What makes me healthy or not is a confluence of what genes I was born with and how I live, not my ranking in a statistical distribution that has not been shown to be clearly and simplistically linked to health outcomes. Since health risks and outcomes are affected by more complex factors and interactions than simple measurements of height and weight or the precise mathematical relationship between them that BMI represents, such measures on their own can't tell you much about an individual's health. "Do You Believe in Fairies, Unicorns, or the BMI?" has a more detailed analysis, and is written by an Actual Mathematician(TM)!

Distributions are characterised not just by their mean, but by the variation about the mean. And when distributions measure some characteristics of a population, they change over time. Focusing on one soundbite-y measurement may be convenient for the HPB, but surely an event that promises to "come alive with activities and entertainment for employees and families" has space for ideas on how to be healthy other than trying as hard as you can to not be in that X% that weigh above Y kg?