bmi – a conceptual mess, or, andrew perrin i take back what i said earlier
One of the basic measures of health is body mass index (BMI). It is meant to be a simple measure of a person’s obesity which is also correlated with morbidity. Recently, I spent some time researching the validity of BMI. Does it actually measure fatness? The answer is extremely confusing.
If you google “validity of BMI as a measurement of obesity,” you get this article that summarizes a few studies. The original definition of obesity is that you need to have at least 25% body fat for men and 35% for women. This is hard to measure without special equipment, so BMI is the default. Thus to measure BMI validity, you need a sample of people and then compute the BMI and the body fat percentage (BF%). The Examine.com reports that in number of studies compared BF% and BMI. In some ways, BMI survives scrutiny. In non-obese people, as defined by BF%, BMI works well, but it seems to under-report obesity in others. In a few odd cases, mainly athletes with a lot of muscle weight, obesity is over-reported. Thus, you get a lot of mis-classification: “One meta-analysis on the subject suggests that BMI fails to classify half of persons with excess body fat, reporting them as normal or overweight despite having a body fat percentage classifying them as obese.” Translation, we are much fatter than we appear to be.
Then, soon after I read some of these studies, the LA Times reported on a new study from UCLA that examines the BMI-morbitiy correlation. In that study, researchers measured BMI and then collected data on biomarkers of health. This is done using the NHANES data set. See the study here. Result? A lot of fat people are actually quite healthy in the sense that BMI is not associated with cardio-pulmonary health (i.e., your heart stopping). This reminds me of an earlier discussion on this blog, where there were conflicting estimates of the obesity-mortality link and a meta-analysis kind of, sort of, shows an aggregate positive effect.
How do I approach the BMI issue as of today?
- BMI is a rough measure of fatness (“adiposity”), but not precise enough for doctors to be making big judgments about patients on a single number/measurement.
- BMI is not a terribly good predictor of mortality, even if there is a mild overall correlation that can be detected through meta-analysis.
- BMI is probably not correlated with a lot of morbidity that we care about with some important exceptions like diabetes.
The lead author of the UCLA study said that this was the “last nail in the coffin” for BMI. She might be right.