Those who are only moderately overweight have a lower relative risk of death than those of supposedly optimum weight, the results show. Modest obesity — a body mass index of 30 to 35 — increases the risks of dying only slightly, leaving only the grossly obese, with a body mass index greater than 35, with a greatly increased risk.

While it may be fashionable to be extremely skinny, it does nothing to prolong life. The risks of dying among people with a body mass index of less than 18.5 are slightly increased.

The results, published in the Journal of the American Medical Association, have been lauded by some specialists as a useful corrective to the national panic in America over obesity. Steven Blair, of the Cooper Institute in Dallas, Texas, said: “I love it. There are people who have made up their minds that obesity and overweight are the biggest public health problem we have to face. These numbers show that maybe it’s not so big.”

However, the study, by a team led by Katherine Flegal, of the US National Centre for Health Statistics, is limited in its conclusions. It looked only at deaths, not at disease or disability, which generally increase with weight.

Earlier studies have shown that being moderately overweight is not necessarily a bad thing and that being grievously thin is a hazard, so the new results will not astonish experts. Nevertheless, the study is larger and more comprehensive than earlier ones.

The team used data from three US National Health and Nutrition Examination Surveys, carried out in the 1970s, 1980s and 1990s. These surveys measured body mass index in a representative sample of the American population, then followed them, recording deaths as they occurred.

The obese categories are linked to 112,000 extra deaths over the time period, which is a fraction of those cited only last year by the Centres for Disease Control and Prevention, which said that 365,000 deaths a year in the United States were caused by people being overweight — making it the second most important preventable cause of death after tobacco. This new study puts the figures at 26,000 per year.

According to the new study, obesity is the seventh-biggest killer behind tobacco, alcohol, germs, toxins and pollutants, cars and guns.

Julie Gerberding, Director of the Centres for Disease Control and Prevention in America, said: “There’s absolutely no question that obesity is a major public health concern of this country (the US).”

The team itself believes that the risks of being overweight are declining as better drugs emerge to control blood cholesterol and blood pressure, and better care is available for a range of obesity-related conditions.

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