Don’t underestimate the mortality risks of being overweight or obese

A new research suggests that many obesity studies substantially underestimate the mortality risks associated with excess weight. The scholarly community is divided over a large meta-analysis that found that overweight is the optimal BMI category and that there are no increased risks associated with obese class.

Skeptics of the meta-analysis argue that the findings are likely driven by biases, especially by illness-induced weight loss. Using BMI at the time of the survey to assess the mortality risks of overweight and obesity is problematic, especially in older populations, because slimness can be a marker of illness.

Researchers have attempted to address this bias by eliminating ill people from their samples; however, such measures are inadequate because information on illness is ascertained by self-reporting and not everyone with an illness has been diagnosed.

Here in the new study researchers use highest BMI in life to predict mortality rates. They said that in the previous literature, the normal weight category combines data from low-risk, stable-weight individuals with high-risk individuals who have experienced weight loss. Use of weight histories makes it possible to separate the two groups and redefine the reference category as people who were a consistently normal weight throughout their lives.

They found that the percentage of mortality attributable to overweight and obesity in this group was 33 percent when assessed using maximum BMI. The comparable figure obtained using BMI at the time of survey was substantially smaller at 5 percent. The source of the discrepancy became clear when they started looking more closely at peoples' weight histories.

A considerable fraction of individuals classified as normal weight using BMI at time of survey were formerly overweight or obese. This group had substantially elevated mortality rates compared to individuals that were consistently normal weight throughout their lives, suggesting that for many of them the weight loss was related to an illness.

The researchers concluded that the findings provide simple and compelling evidence that the prior literature underestimates the impact of obesity on levels of mortality. But the current results need corroboration in future studies because maximum BMI was calculated from peoples' recollection of their maximum weight, which may be subject to recall error.