Based on data collected from policyholders, life insurance companies have developed tables that list standard weight ranges for various heights  Policyholders whose weight fell within the range given for their height lived longer than those whose weight fell outside their given range
Ttherefore, if people whose weight falls outside their given range modified their weight to fall within that range, their overall life expectancies would improve
People's efforts to modify their weight to conform to a given range would not damage their health enough to decrease their overall life expectancies.