London ? Being obese can take years off your life and in some cases may be as dangerous as smoking, a new study says.

British researchers at the University of Oxford analyzed 57 studies, mostly in Europe and North America, following nearly 1 million people for an average of 10 to 15 years. During that time, about 100,000 of those people died.

The studies used Body Mass Index (BMI), a measurement of body fat based on height and weight, to determine obesity. Researchers found that death rates were lowest in people who had a BMI of 23 to 24, on the high side of the normal range.

Health officials generally define overweight people as those with a BMI from 25 to 29, and obese people as those with a BMI above 30.

The study was published online today in the medical journal Lancet. It was paid for by Britain’s Medical Research Council, the British Heart Foundation, Cancer Research UK and others.