Why Are We Using the BMI?
The body mass index (BMI) was devised in 1832 by Adolphe Quetelet. It equals weight divided by height squared. The National Institutes of Health defines 25+ as “overweight” and 30+ as “obese.” But:
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.
Interesting.
Good question.
Interesting post.
Why BMI? Cynics might say it makes it easier to talk about an obesity epidemic, rustle up grants, and have an excuse for telling people what to eat and how to live.
The use of BMI is relatively new in the US. In 1985, an NIH Consensus Development Conference on the Health Implications of Obesity defined obesity as BMI greater than or equal to 27.8 for men and 27.3 for women. There was no “overweight” category as there is now. The numbers were taken from the sex specific 85th percentile of BMI distribution for 20-29 years olds in NHANES II.
This produced population overweight estimates of a little over 27%.
The government then changed the definition of overweight. In “Healthy People 2010, Health Promotion and Disease Prevention Objectives for the Nation,” healthy weight was defined downwards to a BMI of 19 to 24.9. Anything 25 and over was then considered overweight. [See Kuczmarski and Flegal,2000, American Journal of Clinical Nutrition, http://www.ajcn.org/cgi/reprint/72/5/1074%5D
By changing the overweight cutoffs, the estimated number of overweight adults went from 61.7 million to 97.1 million with the stroke of a pen. In fact, there does seem to have been a rightward shift in the weight distribution, one that may have stopped. The people who are getting heavier are the already very overweight.
A 2005 article in JAMA provides data for 2168 NFL players, average age 26, age range 20 to 44. Ninety-seven percent had BMIs that where higher than the healthy weight range, 56% had BMIs that classified them as obese. [http://jama.ama-assn.org/cgi/content/full/293/9/1061-a]
Rather than addressing problems with the BMI measure itself and the subset of people who have severe weight problems and/or metabolic syndrome, public health officials have chosen to monkey with the healthy weight definitions.
The result? An instant obesity epidemic and a really useful public health scare.
I am a regular weightlifter who has more muscle mass than the average person. According to the BMI, I’m obese with a 10% bodyfat level.
BMI is junk.
I’m not sure I know of one physician or health care provider that makes a diagnosis or treatment plan for a patient based on BMI, alone.
We use many other factors and yes, BMI may be one of them. We are increasingly using waist circumference as an indicator of when someone is at higher risk for vascular and diabetic related disease processes. We
look a each patient differently; obviously a weight-lifter is not going to fall into conventional BMI ranges. It is a rough estimate.
BMI gives us a measurement that the general public is more familiar with than LDL, HDL, HgbA1c, BNP levels.
BMI is not a great indicator for everyone, but it’s not completely useless in the clinical setting.
The comment by LInda Gorman is fascinating. I never thought about the possibility that our obesity epidemic has been created with pen and ink.