Obese people are much more likely to die prematurely from any cause, while those who are overweight have a lower all-cause mortality risk compared to people of normal weight, researchers from the National Center for Health Statistics, Centers for Disease Control and Prevention reported in JAMA, January 2, 2013 issue.

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 fact that obesity grade 1 is not linked to a significantly higher risk of death compared to people of normal weight suggests that the typically presumed higher mortality rates currently associated with obesity is due to the higher BMI levels (higher obesity grades).

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.”

Written by Christian Nordqvist