There’s been a lot of press about a new paper by Tomiyama et al. reporting that using BMI alone incorrectly classifies some 75 million Americans as healthy or unhealthy. The authors make the fair point that relying solely on BMI to gauge health is a bad idea. Much of the news coverage, impressed by that big number, has taken the view that BMI is pretty useless as an indicator of health*.
The paper didn’t have any figures so I’ve used their results from table 2 to compare how metabolic heath compares with BMI.
Viewed this way, BMI doesn’t look so bad and shows a very clear trend. People who are classed as obese are highly unlikely to be metabolically healthy, while those of normal weight are mostly healthy. People who are overweight but not obese seems to be the grey area, their metabolic health is worse than those of normal weight, but BMI is not very instructive at an individual level. Still, there are over 300 million American’s, which means that even if BMI is wrong about the health of 75 million, it is still instructive about 75% of the time.
I guess this is why people have used and still use BMI, it is extremely fast, cheap and easy to calculate, (you only need to know someone’s weight and height), but you’d also want to know things like blood pressure, smoking status, family history of disease, level of exercise etc. to properly understand a person’s health.
The study was a snapshot of the cardio-metabolic health of people at the time. It didn’t follow people to look at if they got sick (morbidity) or if they died (mortality). A number of studies have done this. Not being a specialist in the field, it seems to be somewhat controversial at the moment with studies disagreeing about when increased weight correlates with increased mortality. One thing these studies do agree on though is that people with a BMI above 35 (Obesity II and III) are more likely to die that those in the lower BMI categories.
*Let’s also dispense with the ridiculous trope that always gets brought up about BMI, something along the lines of BMI is no good because it classes some professional athletes as obese. BMI doesn’t distinguish why someone is heavy, so you can have a high BMI due to large muscle mass. However, what percentage of the population are professional athletes with high muscle mass? Barely enough to be a rounding error. So it’s a bad argument based on nit-picking.