The authors wrote, as background information in the article:
"Estimates of the relative mortality risks associated with normal weight, overweight, and obesity may help to inform decision making in the clinical setting."
Katherine M. Flegal, Ph.D. and team set out to determine what the relationship between BMI (body mass index) and all-cause mortality might be. They also wanted to provide hazard ratios (HRs) associated with BMI categories.
In this meta-analysis, they found 97 studies that met their criteria, and included over 2.88 million people and over 270,000 deaths. The individuals came from the USA, Europe, Mexico, India, Israel, Brazil, Japan, Taiwan, China and Australia.
They worked out all-cause mortality hazard ratios for grades 2 and 3 obesity (BMI of at least 35), grade 1 obesity (BMI of 30 to 34.99), obesity in general (BMI of 30+) and overweight (BMI of 25 to 29.99), and compared them to normal weight (BMI of 18.5 to 24.99).
The following all-cause mortality hazard ratios were calculated (compared to people of normal weight):
- Overweight - 6% lower risk of death
- Overall obesity (all grades) - 18% higher risk of death
- Obesity grade 1 - no significant difference in overall risk of death
- Obesity grades 2 and 3 - 29% higher risk of death
The authors say that their results are consistent with findings reported in other studies which showed lower mortality among overweight and moderately obese people.
The authors wrote:
"Possible explanations have included earlier presentation of heavier patients, greater likelihood of receiving optimal medical treatment, cardioprotective metabolic effects of increased body fat, and benefits of higher metabolic reserves."