If you are moderately overweight, you will probably live two to four years less than if you stick to your ideal weight, according to a major new study of obesity and mortality.

The research may help to resolve a long-standing controversy about whether mild obesity is actually a health risk. While it is accepted that being severely overweight reduces life expectancy, the effects of being slightly overweight have been hotly debated.

A team led by Richard Peto and Gary Whitlock of the Clinical Trial Service Unit at the University of Oxford, pulled together data from 57 studies, as part of a major study called the Prospective Studies Collaboration.

They looked at almost 900,000 people, mostly from Europe and North America, to see whether those with a higher body mass index (BMI) were more likely to die early.

BMI is a very poor measurement of health, but it is still used to measure of how obese a person is, based on their weight and height (calculate yours here). The ideal range is 22.5 to 25 kilograms per square metre.

People whose BMI was higher than 25 kg/m² had shorter lifespans on average.

Those who were moderately overweight, with a BMI between 30 and 35 kg/m², lived two to four years less. This level of mild obesity is now common, particularly among middle-aged people.

People who were severely obese, with a BMI between 40 and 45 kg/m², lived eight to ten years less on average – a reduction comparable to that caused by smoking. This level of morbid obesity is still rare.

The increase in early mortality was caused by a range of diseases, including heart disease, stroke, diabetes, liver disease, kidney disease, some forms of cancer and lung disease.

Whitlock says, "Excess weight shortens human lifespan. If you are becoming overweight or obese, avoiding further weight gain could well add years to your life."

Journal reference: The Lancet DOI: 10.1016/S0140-6736(09)60318-4