Report Pension Age Needs To Rise To 72, Urges Think Tank In Independent: in House prices and the economy Posted August 20, 2010 You know the Body Mass Index scale ? its based on morbidity rates of a sample of 100,000 people over a long period of time. Thus if your BMI is an ideal 21, eg if you are 5ft 8 and weigh around 10.5 stones, you are statistlicly likely to enjoy longer life and better health than all those under or over BMI 21. Re low BMI and health: I hope they've done a clever statistical adjustment to allow for the fact that being ill tends to make one lose weight. Otherwise you could find a statistician claiming that being underweight makes you prone to die, simply because terminally ill people become underweight.