NEW YORK (Reuters Health) - With a body mass index (BMI) of 30 or higher - the definition of obesity - median survival is reduced by 2 to 10 years, according to a new report.

Also, a BMI below 22.5 -- the definition of underweight -- is also associated with higher mortality rates, due primarily to smoking-related diseases, the report in The Lancet medical journal indicates.

BMI is the standard measure of weight in relation to height. It is calculated by dividing weight in kilograms squared by height in meters.

The current findings come from an analysis of data from 57 studies involving 894,576 adults, mostly in North America and western Europe.

The lowest mortality rates, in both men and women, were seen in those with BMIs between 22.5 and 25, the normal range, Dr. Gary Whitlock, from the University of Oxford, UK, and colleagues found.

"Excess weight shortens human lifespan," Whitlock said in a statement. "In countries like Britain and America, weighing a third more than the optimum shortens lifespan by about 3 years. For most people, a third more than the optimum means carrying 20 to 30 kilograms of excess weight."

He added, "If you are becoming overweight or obese, avoiding weight gain could well add years to your life."

SOURCE: The Lancet, Online March 18, 2009.