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

Researchers from UK analyzed data from 57 studies involving 894,576 adults. The lowest mortality rates, in both men and women, were seen in those with BMIs between 22.5 and 25, the normal range. Above this range, there was a positive association of BMI with death due to specific causes.

With a body mass index (BMI) of 30 or higher - the definition of obesity - median survival is reduced by 2 to 10 years. Similarly, a BMI below 22.5 - the definition of underweight - is associated with higher mortality rates, due primarily to smoking-related diseases. A raised BMI increased the overall risk of death by 30% and specifically affected the death rates due to cancer, blood vessel-related disease, diabetes and those due to kidney or liver disorder.

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.

The researchers concluded that if one is becoming overweight or obese, avoiding weight gain could well add years to his/her life.    
The Lancet March 2009

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