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WEDNESDAY, March 18, 2009 (Health.com) — Obesity shaves two to four years off the average lifespan, while being very obese can shorten your lifespan by 8 to 10 years, according to a new analysis of 57 studies including nearly 900,000 people.

“This is scary and something that we should pay close attention to,” says Ali Mokdad, PhD, a professor of global health at the Institute for Health Metrics and Evaluation at the University of Washington in Seattle. The new findings actually underestimate the true impact of obesity on society because they don’t address the costs of obesity-related illness and other factors, says Mokdad, who was not involved with the current study.

The study, published online March 18 in the journal The Lancet, was conducted in part by the eminent epidemiologist Sir Richard Peto of the University of Oxford. Peto and his colleagues in the Prospective Studies Collaboration, a team of dozens of researchers from around the world, say they did the new study to figure out exactly how body mass index (BMI) relates to mortality. Researchers also investigated how smoking influenced this relationship and how excess weight affected death risk from specific causes.

Their analysis included 894,576 people, mostly from North America and Western Europe. Most were age 46 when the study started and were recruited in 1979; the average BMI for all participants was 25. The researchers eliminated deaths during the first five years of their analysis to avoid including people who were excessively thin due to illness.

Next page: Obesity as bad as heavy smoking