Weighing too much may take as much as a decade off your life, according a new analysis of studies that involved 900,000 people.

Adults who are obese — about 40 or more pounds over a healthy weight — may be cutting about three years off their lives, mostly from heart disease and stroke

Those who are extremely obese, about 100 or more pounds over a healthy weight, could be shortening their lives by as many as 10 years, the study found. Being extremely obese is similar to the effect of lifelong smoking, says Richard Peto, one of the lead researchers and a professor of medical statistics at Oxford University in England.

Study co-author Gary Whitlock, an epidemiologist at Oxford, says, "Obesity causes heart disease and stroke by pushing up blood pressure, mucking up blood cholesterol and triggering diabetes."

The researchers and their colleagues examined the findings of 57 studies involving about 900,000 adults who were followed for 10 to 15 years. Most of the people lived in the USA or Western Europe. The scientists analyzed 70,000 deaths.

Among the findings reported online today and in an upcoming edition of The Lancet:

•Above a healthy weight, every 5-point increase in BMI increases the risk of early death by about 30%.

•People who are overweight but not obese, with a BMI between 25 and 29.9, could be shortening their life span by a year.

•People with the lowest risk of dying early are in the high end of the healthy weight range with a BMI of about 22.5 to 25.

This is a "valuable study that provides a much clearer picture of the risk associated with various levels of being overweight or obese," says Michael Thun, emeritus vice president of epidemiological research at the American Cancer Society.

"What is particularly worrisome in the United States is that more than a third of people now qualify as obese, and a subset of people are becoming progressively more obese. Once you gain weight, it's hard to lose it and easy to gain more. So the goal to stop your weight gain now."

Both obesity and smoking are dangerous to your health, Thun says.

"There has been an artificial horse race between obesity and smoking over which is worse. This is fundamentally silly.

"If you continue to smoke, it takes an average of 10 years off your life. Being very obese has about the same effect."