In 1993-1994, demographic data were collected on a sample of 11,326 Canadians over the age of 25. The Cox proportional hazards model was used to determine the effect of BMI on mortality over a twelve year period. It was found that people who were overweight but not obese — BMI from 25 to 29.9 — had lower mortality than people of so-called normal weight (B.M.I. of 18.5 to 24.9).
According to an article in the New York Times:
“Overweight may not be the problem we thought it was,” said Dr. David H. Feeny, a senior investigator at Kaiser Permanente Center for Health Research in Portland, Ore., and one of the authors of the study. “Overweight was protective.”
The abstract is available on-line.