December 20, 2011

Tell me again why we use the term "over" weight?

Aside from other sins of the BMI index, the weight of the words we use to classify BMI are absurd:

"underweight"
"normal"
"overweight"

"Under" means shouldn't be this weight, so does "over." "Normal" implies everyone should be in this range. All this is ridiculous. BMI was never meant to assess individuals, anyway, it's a population measure.

But making quantitative and value-laden classifications that indicates someone ought not be in that grouping is bad science

First of all, weight is influenced by many factors -- mostly genetics -- and isn't something one can decide to simply change.

Think of it this way: let's measure everyone's lung capacity and graph it up. There'd be people with readings all over the range, and some on the far edge are very unwell. But others with low capacity may be quite well, and may indeed be doing as well as they possibly could be considering their genetics and medical history. Those people "should" not be considered "under-breathed.

There are many,  but here's yet another reason to look with derision on the "overweight" wording:

Under Or Normal Weight Linked To Raised Risk Of Death Following Surgery

4 comments:

  1. Viewing one's weight as under or over that individuals healthy range is clearly appropriate. A shift away from a child's normal growth curve is quite valuable at identifying an issue that's better addressed early--whether the result of eating disorder behaviors or organic causes. Similarly, an adult who has hovered in their "normal" range and then has an undesired change up or down is best to have it identified. The issue, really, is labeling an absolute weight or BMI unacceptable, even when it has been a normal, healthy place for that individual.

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  2. Oh BMI. My favourite topic.

    The body mass index (BMI), or Quetelet index, is a heuristic proxy for human body fat based on an individual's weight and height. BMI does not actually measure the percentage of body fat.

    While the formula previously called the Quetelet Index for BMI dates to the 19th century, the new term "body mass index" for the ratio and its popularity date to a paper published in the July edition of 1972 in the Journal of Chronic Diseases by Ancel Keys, which found the BMI to be the best proxy for body fat percentage among ratios of weight and height;[3][4] the interest in measuring body fat being due to obesity becoming a discernible issue in prosperous Western societies. BMI was explicitly cited by Keys as being appropriate for population studies, and inappropriate for individual diagnosis. Nevertheless, due to its simplicity, it came to be widely used for individual diagnosis, despite its inappropriateness.

    However, BMI has become controversial because many people, including physicians, have come to rely on its apparent numerical authority for medical diagnosis, but that was never the BMI's purpose; it is meant to be used as a simple means of classifying sedentary (physically inactive) individuals with an average body composition.

    I just get SO riled up about the whole "Average" thing too. What is average/normal weight? It is a mean. Not a scientifically proven fact. Gah Gah Gah.

    The WHO's hysteria about the obesity epidemic. Well, let's put it this way, please show me someone with an eating disorder who is going to volunteer to be weighed and measured by some student with a clipboard as part of a survey.

    Enough already. Tiddlypush and bah humbug to BMI! Charlotte Bevan

    PS I can't sign in as normal because apparently I am an illegal character.....

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  3. Body Mass Index (BMI) does have its problems, but few professionals use absolute values of BMI on their own as an indicator of general or specific health risk.

    As you rightly point out, Laura, BMI is used as a population measure. It does correlate quite well with various indices of health risk in epidemiological studies which employ sample sizes that are adequately large to confer 'good' statistical power. BMI is not useful when applied to individuals, although changes in BMI over time can be useful. BMI also provides little indication of body composition and people with a very high lean/fat mass ratio can register as 'overweight' (which implies 'over-fat') by BMI. Prediction equations employing BMI measures are formulated on the basis of population studies so may be feasibly applied to individuals to provide an index of risk.

    Therefore, I don't think that the use of BMI is 'bad science'.... nevertheless, there have been suggestions that the denominator would be better as a volumetric measure (metres cubed) as opposed to one that links to body surface area (metres squared), especially in taller people. Furthermore, the alleged 'healthy' BMI range is sufficiently large to account for typical variations in the size and density of the skeleton as well as lean/fat ratio. For example, a woman of stature 5ft 5in (165 cm) could weigh anything from 120 - 150 pounds (54.5 - 68.1 kg) and still have a weight that falls in the designated 'healthy' range of 20 - 25 kg.m*m.

    Sure, on its own and in any given individual BMI is not an index of health. But it can be of great value in research. I don't think any professional or academic who uses BMI in their work would argue that when used in isolation it provides a robust measure of health.

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  4. I am laughing that Charlotte Bevan is an illegal character :D

    Charlotte, did you know that Ancel Keys conducted the Minnesota Starvation Experiment? Oh, he also lived to 100, I think...

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