More BMI controversy

scaleBMI, or Body Mass Index, is a calculated ratio of height to weight, used by the National Institute of Health to identify people who are overweight or obese.  The BMI for “overweight” is 25-29; for obese it’s above 30.   This supposedly helps medical professionals target recommendations for weight control, and identify people who are normal weight.  The problem has always been that a simple weight-to-height ratio doesn’t address body composition at all.  Until now, the anti-BMI argument has been that it unfairly classifies heavier-but-muscular people as “obese”.
Body builders and football players are always trotted out as examples of why BMI is faulty.  Although judging from the very large abdomens of some football players, I’d say obese is sometimes the right category.

A recent study compared BMI to the World Health Organization definition of obesity for women: more than 35% body fat.  A group of women was measured for BMI, and also with body fat analysis, done with state of the art DEXA scans.*   Result: the simple BMI calculation failed to identify women as obese who were in fact obese according to WHO criteria: 35% or more body fat.

Does this mean BMI is useless and we should all have expensive DEXA scans for complete accuracy?  The study divided the women into different racial groups, and found that, rather than trash the whole BMI measurement, the cut-off points for “obesity” should be lowered, and that these cut-off points were different for different racial groups:

  • 25.5 for white women
  • 28.7 for black women
  • 26.2 for Hispanic women

Quite different from the all-purpose cut-off of 30.  In other words, many women who are in fact “obese” – high body fat, regardless of weight – are not being identified.  The result?  Medical providers might not think it important to advise a white women with a BMI of, say, 27 to work on weight control as a way to avoid chronic diseases.

By the way, this study did not address any BMI issues for men.  The message for women:  don’t get too comfortable being merely overweight.  You could actually have an unhealthy level of body fat.  Why care?  Another study from Europe concludes that obesity could soon become the most significant cause of cancer in women.  Colon, breast and endometrial cancers are already linked to obesity.  Why would obesity increase cancer risk?  That’s not clear.  Researchers speculate that fat tissue increases tumor-driving hormones, or increases inflammation.  Whatever the actual cause, it would make more sense to maintain normal weight and lower your cancer risk.

*Yes DEXA scans, the same scan used to assess bone density for older women, are very accurate for assessing body fat.  Just don’t expect your medical technician to calculate your % body fat when you have a bone scan.  It’s not standard procedure, and insurance companies certainly aren’t going to pay for it.

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