People often get into a lather about BMI. Body Mass Index is a measure of the relative ratio of weight to height (and yes, it’s metric). It’s the basic yardstick by which obesity is measured. It’s easy to calculate. Normal BMI is considered to be between 18 and 25. This can cause much consternation for people who consider themselves otherwise healthy, though if you are reading here you’re probably not looking to rationalize away a weight issue.

The realities are that these old measures stick and stick hard. In January, the International Journal of Obesity published a paper that showed beyond doubt that normal BMI is an important predictor of risk of death. The study was simple and the numbers were large (over 50,000 women). Using health records from the 1980s through 1997 with measured BMI, the risk of death at various levels was compared. Turns out they found some interesting things.

Being outside of normal BMI at all increased the overall risk of death over the next 10 years. Whether with or without chronic disease, underweight women were at less risk than overweight women though women with a BMI of 20-24 were at the lowest overall risk. Underweight gets away with less risk than overweight until breast cancer comes into play. Regardless of weight, BMI outside the normal range was associated with a roughly equal risk of death for women with breast cancer detected by screening.

The study goes on to correlate BMI at age 40 with overall life expectancy. The results are unsurprising. No matter how this numbers get diced up the results are the same. The human frame lives longest and healthiest within a certain range and we know what that range is.