Ok let's hypothetically say that BMI doesn't at all correlate to health.  What should we use?  Bodyfat percentage?  Waist:hip ratio was found to be pretty accurate.  Or do we need a measure of health that tells every hambeast they're perfectly healthy?  Ok so we'll use bloodwork because that's what they seem to cite most often.  What happens when bloodwork inevitably catches up to the already apparent outside signs?  Will we need to find a new measure of health?