This is interesting. According to a study cited in the LA Times your activity levels are more important than your body mass index when trying to predict how long you will live. So while having a socially acceptable body mass is mostly determined by diet your actual health is more dependent on how much exercise you get. Fancy that.
Funny since most people eat healthily because they want to be healthy and work out because they want to be sexy bitches.
PS For Christ's sake scientists, get it together! Use BMI all you want but include either bodyfat% or waist circumference. That is all.
PPS Medicalxpress has a better summary. I've nicked a figure: