BMI Is the Best Metric for Weight Loss and Health

A better metric would factor in the type of tissue you have. Is that weight fat or is it muscle? In order to determine that, you have to do things like fully submerge a person in a giant water tank. They're simply not going to do enough of the better body composition tests on enough people for large-scale observational research about weight and health. The other good thing about BMI is that we've been tracking it in people for a long time.