A new U.K. study suggests the harmful effects of being overweight have been underestimated. The researchers based their assessment on a new study that analyzed body mass index (BMI), health, and mortality data using a robust study design.
University of Bristol investigators reviewed data on around 60,000 parents and their children to establish how obesity actually influences risk of death.
Previous studies have suggested that the optimum BMI, at which the risk of death is minimized, appears to be above the range normally recommended by doctors, leading to claims it is good for health to be mildly overweight.
Scientists were skeptical that the studies reflected the true effect of BMI on health, because early stages of some illness, as well as health-damaging behaviors such as cigarette smoking and other factors, can lead to both lower BMI and increased risk of death.
This makes it difficult to estimate how BMI actually influences risk of death (the causal effect), as opposed to the observed association between BMI and risk of death.
Therefore the new study, which appears in the International Journal of Epidemiology, sought to assess the causal link (cause and effect) between BMI and risk of death.
Using HUNT, a Norwegian population-based health cohort study based in a rural county with 130,000 residents, the researchers were able to see how mortality in the parents related to both their own BMI (the conventional approach) and to the BMI of their adult children.
Because BMI of parents and their offspring is related, due to genetic factors, offspring BMI is an indicator of the BMI of the parents.
The BMI of adult children is not influenced by illness among the parents, therefore using offspring BMI avoids the problems inherent in simply relating the BMI of the parents to their risk of death.
The health records of around 30,000 mother and child pairs and 30,000 father and child pairs were assessed to examine the extent to which BMI may influence mortality risk in a situation that is not biased by “reverse causation” — illness leading to low BMI rather than BMI influencing illness.
The team found that when offspring BMI was used instead of the parent’s own BMI, the apparent harmful effects of low BMI were reduced and the harmful effects of high BMI were greater than those found in the conventional analyses.
Importantly, the results suggest that previous studies have underestimated the harmful effects of being overweight.
The current advice from doctors to maintain a BMI of between 18.5 and 25 is supported by this study, and the widely reported suggestion that being overweight may be healthy is shown to be incorrect.
Dr. David Carslake, the study’s lead author, noted, “An alarming increase in obesity levels across the world, which have risen from 105 million in 1975 to 641 million in 2014, creates concern about the implications for public health.
“This study suggests that previous studies have underestimated the impact of being overweight on mortality and our findings support current advice to maintain a BMI of between 18.5 and 25.“
Source: University of Bristol