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Body Mass? Irrelevant.

A study of 900 Vancover-area teens found that the BMI is meaningless as an indicator of physical activity and healthy eating.

According to [Dr. Catherine] Sabiston, who is also director of McGill’s Health Behaviour and Emotion Lab, the results showed only a very weak correlation between physical activity and healthy eating, and virtually no correlation between an individual’s BMI and his or her level of physical activity. The study was undertaken to test a comprehensive model of physical activity and healthy eating behaviour in teens aged 15 to 18, partially in response to two perceived problems with existing research in the field.

It's noted that the group studied is traditionally underrepresented in studies. (Got that?) There were some interesting differences between the genders:

The study also found a significant difference in the way boys and girls approach physical activity and healthy diet. Boys, Sabiston said, need to attach value to a healthy diet and feel confident in their ability to follow a healthy diet before they’ll actually do it. Girls, she said, regardless of how they feel about their ability to eat a healthy diet, only need to feel it is important to do so before they’ll eat properly.

I wouldn't be surprised if this was due to the idea that women are "always" on diets and how that's been deemed normal. I found the value requirement for boys to be interesting, because it demonstrates that boys are held to a totally different standard... and in turn, have to have something in mind when undertaking a diet.

Regarding these results:

“A lot of people are surprised,” Dr. Sabiston said, “but when you think about it, BMI doesn’t have a huge impact on physical activity. And in terms of diet, it actually makes sense that someone who is not happy with their body might try to eat more healthily.”

The problem, though, is that many people choose not to think about it. Classification is something we as humans like to do. We've been led to believe for years that the BMI is the Golden Standard when it comes to health, in particular because important things like healthcare eligibility are tied into it... which classifies people into "healthy" and "unhealthy" buckets. "Good" and "bad". Et cetera. And since the idea that the BMI is important is perpetuated by a lot of people, including prominent folks with vested interests, it becomes so. Although the truth is entirely the opposite: the BMI is irrelevant.

There's a great quote at the end of this piece, too. Emphasis mine.

What this study really says, Sabiston explained, is that one cannot assume that someone who is physically active necessarily eats a healthy diet – or the reverse, that someone who is more sedentary or has a high BMI by definition eats a diet of junk food.

“This study drives home the point that as a society, we’re primarily focused on extrinsic things like appearance and weight versus the betterment of health,” Sabiston said. “From a public health perspective, this means we should probably focus on people who are at a healthy weight or even underweight, and emphasize that healthy eating is not just about weight-change.”

Unfortunately, in tandem with the whole "not thinking" thing, it's a hell of a lot easier to use fat people as scapegoats. So here we are in this fabricated crisis. In the interim, we must continue to point out that BMI is meaningless; studies like this make our task just a little easier. [Thanks to LLW!]

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sevendayswonder May 14th, 2008 | Link | Nice to see more work like

Nice to see more work like this!

And in terms of diet, it actually makes sense that someone who is not happy with their body might try to eat more healthily.

Well, this might be true to an extent...but this loosely supports the whole "shame those fatties to be thin" line of thinking, when actually weight stigma is associated with engaging in less "healthy" behaviors because when you hate your body you aren't exactly motivated to "take care of it."

I really hope that this kind of work stimulates longer studies that follow people over the course of their lives and account for dieting behavior and other things. I think the trend is lately for people to say "Well, the fat may not seem to hurt you now...but you just wait. You'll be sorry."

HeatherRadish May 14th, 2008 | Link | They needed a study for this?

the BMI is meaningless as an indicator of physical activity and healthy eating.

Shocked Well, yeah, it's a height-weight chart. There's no inputs for food or activity, how could it possibly measure those things?

it actually makes sense that someone who is not happy with their body might try to eat more healthily.

The report didn't say that those with high BMIs were unhappier with their bodies than those with low BMIs; I wonder where he got that conclusion. My high school experience (admittedly decades ago) was that the size 2 girls were just as unhappy with their bodies as the size 12 girls. The size 12 girls had healthier diets if you defined "1500+ calories, including calcium, protein, fats, regular meals" as healthier than "no food all day, beer at night"...but the larger girls weren't eating lunch out of unhappiness.

rebelle May 15th, 2008 | Link | In response to the study:

In response to the study: Well, duhhhh? We've been saying that for years. You. Can't. Tell. Health. By. Looking. At. Size. Or. Simple. Mass.

But it's good to see at least one study that seems to at least somewhat support this commonsense view getting some exposure. Still, Paul is right. People will still lump us categorically unhealthy — just ask the asshats who said I couldn't have long-term disability insurance through them. No amount of reasoning about BMI (the only reason for their refusal) cut any ice with 'em. It was The Rules.

Then there's that banner ad for some diet or another that is years old and says: "Are you heavy OR healthy? Find out now!" and links you to a BMI calculator. Grrr!!!

Tiana May 15th, 2008 | Link | I don't really like the way

I don't really like the way they represent this. Why say that BMI does not influence behaviour if you could just as well say that behaviour does not influence BMI? Because that's the interesting part of it. The conclusion at the end is awesome, though.

richie79's picture
May 16th, 2008 | Link | This is a great study.

(deleted by poster; sorry for killing a perfectly good thread Sad )

englishqueen01 June 15th, 2008 | Link | Duh

I've argued for years that the BMI is a faulty indicator of health and fitness.

All the BMI measures is your weight and your height - it does not take into account things like bone structure, muscle mass, body type, etc.

In other words - a bodybuilder with 1-2% body fat could (and have been) classified as "overweight" or "obese" because he weighs too much. Here's a link to a group of pictures of people with their height, weight, and BMI - proof of how faulty the BMI is (

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