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Hi everybody,

So a fortnight ago I put off talking about obesity because it was too big a topic. Like a morning on the crapper after a night at an Indian BYO, though, the subject can’t be avoided, so we might as well load up Angry Birds, settle in and do this shit.

Obesity is defined by Body Mass Index (BMI). Your BMI is number that relates your height and your weight. When it hits 30, you’re suddenly obese. Obese people have an increased risk of everything from A to Z (arthritis to … er … ziabetes), which is worrying because nearly a third of New Zealanders are obese. It may sound simple, but this paragraph alone has raised enough issues to fill about four or five Critic columns, so let’s crack in already.

As a measure, BMI is about as good as Michael Jackson was in his hit song “Bad”. That is to say, it’s bad. It’s far too simple to accurately reflect things like levels of activity, body fat (percent and distribution) and diet, all of which are major players in the link between obesity and death. National sex symbol Dan Carter, for example, has a BMI of 29.7. According to the BMI index, he is therefore flirting with obesity more than a chubby chaser in South Dunedin, despite being one of the healthiest people in the country.

The next problem with BMI, and today’s episode of Sesame Street, is brought to you the number 30. If you plot odds of dying against BMI, the line starts climbing around 25 (meaning you get more and more likely to cark it). Some smart cookie noticed that and defined “overweight” around that point. Then a slightly less intelligent snack said, “well, 25 is a nice number, so let’s make the next category (obesity) at the next nice number: 30.”

Sure, it makes sense to stratify weight levels. It gives public health researchers standardised tools to play with, and it acknowledges that body fat risk profiles are not one amorphous blob. The problem is, though, that people focus purely on “obesity” and the “obesity epidemic,” ignoring the fact that “risk” with BMI sits along a continuum and is not a flat line from 25.1 to 29.9 with a massive step up at 30. People don’t wake up obese having gone to bed a stick figure – getting heftier is a gradual process. The “obesity” epidemic reflects a percentage of the population moving from BMIs of 29.9 to BMIs of 30.0. It isn’t great, but neither is it the tsunami of tubbiness suggested by the media and by many health professionals.

Then there’s the complexity of weight itself. In theory, weight gain and loss is simple – energy in versus energy out. The whole “slow” and “fast” metabolism stuff is a load of nonsense, but there are certainly extra factors that complicate the math.

Despite what you might think while bingeing on Maccas, the body very tightly regulates energy intake by subconsciously controlling appetite. In issue 5 we talked about the fresher five – the extra 5.3kg that people gain, on average, over four years at college. That works out to be 138kJ too much energy each day, significantly less than is contained in the apple a day meant to keep us doctors away (398kJ). The fact that this energy intake is normally so rigidly controlled means that any weight gain must involve a disturbance in the fundamental regulatory process – in the leptin signaling pathway, for instance.

In the tiny bit of space remaining, I want to briefly return to minority stress and stigma. We covered this in issue 17, but I want to touch on it again as fat phobia is one of the few remaining socially acceptable forms of prejudice, and one that has horrific effects on people’s self-esteem and wellbeing. When I introduced this topic two weeks ago I again peppered the column with harsh references to weight. Again it was largely ignored. Sticks, stones and words all hurt like a bitch, and the weight-hate needs to stop.