Study : One of the biggest studies ever to look at the effects of weight on longevity concludes that thinner is definitely better at almost all ages. It found that being too heavy seems to shorten life expectancy up to about age 75. After that big doesnt seem to make much difference, unless people are really obese. The study was based on American Cancer Society data on 324,135 men and women who were enrolled in 1960 and then followed up in 1972.