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BMI weight guide: misused, outdated — but not out of use

Do you know your body weight measurement? Turns out the BMI formula was never intended to be used as an individual guide — and waist circumference is considered a better gauge.

Do you know your body weight measurement (BMI)?

Most likely you do. For two decades, the rather simple BMI equation (your weight in kilograms divided by the square of your height in meters) has been the medical community’s preferred way of determining who is “underweight,” “normal weight,” “overweight” and “obese.”

Yet, as an article published earlier this week in the online magazine Slate points out, the researcher who developed this formula in the early 1970s — University of Minnesota physiology professor and obesity researcher Ancel Keys — never intended it to be used for this purpose. He envisioned BMI only as a useful tool to help epidemiologists estimate and compare overall obesity levels of large populations from huge pools of historical height and weight data.

“His original paper warned against using the body mass index for individual diagnoses, since the equation ignores variables like a patient’s gender or age, which affect how BMI relates to health,” writes Slate reporter Jeremy Singer-Vine. “It’s one thing to estimate the average percent body fat for large groups with diverse builds, Keys argued, but quite another to slap a number and label on someone without regard for these factors.”

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Yet slapped on us it’s been, as Singer-Vine points out:

[H]ealth professionals have increasingly used body mass index to justify lifestyle recommendations for their patients. And online BMI calculators — there’s even one hosted by the NIH — invite people to diagnose themselves without any medical supervision whatsoever. Faulty readings could promote a negative self-image among healthy people and lead them to pursue unnecessary diets. Or the opposite problem: People with a little too much body fat might be lulled into a false sense of complacency by a misleading BMI.

During the past decade or so, a growing number of studies and journal reviews have begun to caution against relying too heavily on BMI as an indicator of a person’s risk of disease. Even its use in research, notes Singer-Vine, is being challenged:

A recent critique of the body mass index in the journal Circulation suggests that BMI’s imprecision and publicity-friendly cutoffs may distort even the large epidemiological studies. (There’s no definitive count of how many people are misclassified by BMI, but several studies have suggested that the error rate is significant for people of certain ages and ethnicities.) It’s impossible to know which studies have been affected and in what direction they might have been skewed.

As BMI’s critics have pointed out, there’s a better alternative for figuring out if you’re at a healthful weight. Writes Singer-Vine:

It turns out that the circumference around a person’s waist provides a much more accurate reading of his or her abdominal fat and risk for disease than BMI. And wrapping a tape measure around your gut is no more expensive than hopping on a scale and standing in front of a ruler. That’s why the American Society for Nutrition, the American Diabetes Association, and other prominent medical groups have lately promoted waist circumference measurements as a supplement to, or replacement for, the body mass index.

In fact, a European study recently found an association between having a large waistline and a doubling of the risk of dying prematurely from heart disease, cancer and other chronic medical conditions. And, as I reported last year, other research has found a link between abdominal obesity and an increased risk of Alzheimer’s disease.

So get out the tape measure. Abdominal obesity is currently defined in men as having a waist circumference greater than 40 inches and in women as having a waist circumference greater than 35 inches. Some experts, however, believe those numbers are too high. Stay tuned.