Overweight Carries Higher Mortality Risk Than Previously Thought

A new study has suggested that the association between overweight and the risk of death may have been underestimated in previous studies.

In the Norwegian HUNT study, a prospective study, the researchers had aimed to assess whether an increased risk for mortality is caused by low body mass index (BMI) or other confounding factors, such as concurrent ill health. They evaluated 32,452 mother-and-child pairs and 27,747 father-and-child pairs until 2009.
________________________________________________________________________________

RELATED CONTENT
Overweight, Sedentary Children Benefit from “Interruption” Intervention
Asthma Symptoms Are More Frequent in Overweight Children
________________________________________________________________________________

Cox regression was used to estimate conventional hazard ratios for parental mortality per standard deviation of BMI, and results were adjusted for behavioral and socioeconomic factors.

The BMI of parents’ children was also used as an instrumental variable for parents’ own BMI in order to estimate hazard ratios. Furthermore, cubic splines were used to analyze the shape of associations between mortality and BMI.

A total of 18,365 parental deaths had occurred over the course of follow-up. Findings from the study showed that conventional associations between all-cause, cardiovascular-, and cancer-related mortality, and parents’ own BMI were substantially nonlinear. The researchers noted that the risk of mortality was elevated at both extremes and minima (21-25 kg m⁻²). Results also demonstrated that equivalent associations with offspring BMI had been positive.

Ultimately, there was no indication that low offspring BMI was associated with increased mortality among parents. However, the researchers found that the association between overweight and mortality may have been underestimated in previous studies. The linear instrumental variable hazard ratio for all-cause mortality per standard deviation increase in BMI was found to be 1.18 vs compared with 1.05 in the conventional analysis.

“Elevated mortality rates at high BMI appear causal, whereas excess mortality at low BMI is likely exaggerated by confounding by factors including concurrent ill health,” the researchers concluded. “Conventional studies probably underestimate the adverse population health consequences of overweight.”

—Christina Vogt

Reference:

Carslake D, Smith GD, Gunnell D, Davies N, Nilsen TIL, Romundstad P. Confounding by ill health in the observed association between BMI and mortality: evidence from the HUNT Study using offspring BMI as an instrument [Published online December 1, 2017]. Int J Epidemiol. https://doi.org/10.1093/ije/dyx246.