A recent article in the Journal of the American Medical Association has gotten a lot of attention for claiming that risk of mortality is lower in overweight people than normal weight people. I will write a series of posts critiquing this article, which I think demonstrates many of the problems with modern epidemiology and research in this field.
This first post is a very simple one: We do not correctly classify obesity. The standard classification used by the article is as follows:
BMI < 18.5 : Underweight
18.5 < BMI < 25 : Normal weight
25 < BMI < 30 : Overweight
30 < BMI < 35 : Obese (grade 1)
35 < BMI : Obese (grades 2-3)
I just calculated my BMI for the first time. BMI is calculated as mass (kg) / height×height (m). I weigh 175-180 lbs (80 kg) and am 6 feet (184 cm) tall. My BMI is thus around 23.6 – 24.4, depending on the day. This puts me at the upper end of “normal.” In other words, anyone a bit heavier than me is overweight. Here I am:
While I am not underweight (at least not much), anyone who knows me will tell you I am a bit on the lean/slender side. I have to work to avoid being too skinny. I am not at the upper end of normal, I am at the lower end of normal – a pretty broad range on this scale.
Everyone who works with BMI admits that it is an imperfect measure – that it can classify body builders in heavier categories, for example, and that it doesn’t really incorporate body type. But I’m not a body builder, and I have a standard body type, and the standard scale is telling me that I’m not far from being overweight. According to the scale, I would have to lose 55 lbs (25 kg) to become underweight! Even when I had cancer and became a bit emaciated (to the consternation of everyone who saw me), I only lost about 20 lbs.
I use myself as an example, but I am convinced the standard cut-offs are wrong. Many people who are underweight (anorexic or wasting away due to cancer, and thus at higher risk of mortality) are in the “normal” category. And many people at the perfect weight are in the “overweight” category. So it’s no wonder we will see strange results like that from the JAMA study.
How about you? Is your BMI about right according to the scale above?