04 Dec 2017 --- The harmful effects of being overweight have been underestimated, according to a new study that analyzed body mass index (BMI), health and mortality data in around 60,000 parents and their children to establish how obesity actually influences the risk of death. The University of Bristol, UK study is published in the International Journal of Epidemiology.

Previous studies had 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. However, scientists suspect these studies do not reflect the true effect of BMI on health, because early stages of illness, health-damaging behaviors such as cigarette smoking, and other factors can lead to both lower BMI and increased risk of death.

Indeed, more recent studies – including one from Imperial College London and the University of Cambridge and an additional study from the University of Birmingham – have suggested that the idea of “fat but fit” is a myth.

“These data confirm the importance our lifestyle plays in our health and the need for appropriate nutritional advice to help, ideally, prevent the development of overweight and obesity, but also as part of an approach to treating the problem,” Professor Neil Thomas, senior author of the University of Birmingham study, told NutritionInsight at the time. “The earlier it is addressed, the more likely it is to be successful.”

The BMI-lowering qualities of factors such as illness make 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. The aim of the University of Bristol study was to assess the causal link between BMI and risk of death.

Norwegian-based study
The Bristol Medical School team, with co-workers from the Norwegian University of Science and Technology, were able to see how mortality in parents related to both their own BMI (the conventional approach) and to the BMI of their adult children. They did this using HUNT, a Norwegian population-based health cohort study based in a rural county with 130,000 residents.

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.

Call for action based on reliable evidence
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, the University of Bristol notes.

“An alarming increase in obesity levels across the world – which have risen from 105 million in 1975 to 641 million in 2014, according to a recent Lancet study – create concern about the implications for public health,” says Dr. David Carslake, the study’s lead author and Senior Research Associate from the MRC Integrative Epidemiology Unit (IEU) at the University of Bristol.

“This study demonstrates that correlation is not causation and that when it comes to public health recommendations we need to be cautious interpreting data based on associations alone,” Dr. Carslake adds. “We found 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.”

“We are used to seeing conflicting studies purporting to show that something is either good or bad for our health,” adds Professor George Davey Smith, Director of the MRC IEU and Professor of Clinical Epidemiology at the University of Bristol. “These generally come from naïve observational studies, which can produce seriously misleading findings.”

“More robust approaches for identifying the causal effects of factors influencing health, such as the methods applied in this study, are required if we are to make recommendations for public health based on reliable evidence,” concludes Professor Smith.