The review, published in The Journal of the American Medical Association, looked at dozens of studies on body mass index (BMI) and mortality risks, and found that people who were classified as obese face an overall 18 per cent increased risk of death compared to the risk faced by persons of "normal" weight.
But researchers also found that people who were classified as overweight were found to have a "significantly lower" risk of death than those of normal weight.
"Sometimes that surprises people, but they really should not be too surprised because in our categories of these 97 studies, 80 per cent of them show that there was lower mortality in overweight than in normal weight people," said lead author Dr. Katherine Flegal, of the U.S. National Centre for Health Statistics.
The researchers found that the relative mortality risk is six per cent lower for overweight individuals (with a BMI between 25 and 30) and five per cent lower for obese individuals with a BMI between 30 and 35 when compared against the mortality risk of individuals of normal weight.
Researchers don't know why that is, but suggest that a small amount of fat may provide needed energy during acute illnesses, or have some benefit with a traumatic injury.
Flegal's team said another possible explanation is that heavier patients may seek out medical advice or treatment for new conditions sooner than lighter individuals.
The researchers also found that extremely obese individuals face a 29 per cent increased risk of death.
The study, titled Association of all-cause mortality with overweight and obesity using standard body mass index categories, was co-authored by University of Ottawa adjunct professor and Public Health Agency of Canada researcher Dr. Heather Orpana, and researchers with the U.S. Centers for Disease Control and Prevention and the National Cancer Institute.
The meta-analysis of 97 studies provided the researchers with combined data from just under three million people, and covered more than 270,000 deaths.
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