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Link between weight and mortality is underestimated

4 December 2017

The link between being overweight and having a higher risk of dying has been underestimated, according to a new study led by researchers from the MRC Integrative Epidemiology Unit. The study, published in the International Journal of Epidemiology, analysed body mass index (BMI), health and mortality data in around 60,000 parents and their children, to establish any link between obesity and death rates.

Average BMI in industrialized countries is on the rise causing concern for the implications for public health. Having a high BMI is associated with an increased risk of dying from heart disease or cancer. However, having a very low BMI is also associated with increased mortality. Some 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; indeed it has often been said that it is actually good for health to be mildly over-weight. Many scientists suspect that these studies do not reflect the true effect of BMI on health, because the early stages of illness that increase the risk of death can cause people to lose weight. This would make it erroneously appear as if low BMI caused illness and increased mortality rates. Furthermore, other factors that damage health can also lower BMI, such as cigarette smoking. This 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 observed association, which has received much attention in earlier studies, is distorted by the fact that early stages of illness, health damaging behaviours and other factors lead to both lower BMI and increased risk of death. This study aimed to assess the causal link 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 team 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. BMI of 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 team assessed the health records of around 30,000 mother and child and 30,000 father and child pairs 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 a child’s 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 keep your BMI between 18.5 and 25 is supported by this study, and the widely reported suggestion that being over-weight may be healthy is shown to be erroneous.

Dr David Carslake, the study’s lead author and Senior Research Associate from the MRC Integrative Epidemiology Unit (IEU) at the University of Bristol said: “Correlation is not causation. We need to be cautious interpreting studies based on associations alone."

Professor George Davey Smith, Director of the MRC IEU and Professor of Clinical Epidemiology at the University of Bristol, added: “we are used to seeing daily reports of often conflicting studies purporting to show that something or other is either good or bad for health. These generally come from naïve observational studies, which can produce seriously misleading findings. More robust approaches for identifying the causal effects of factors that influence health, such as the methods applied in this study, are required if we are to make recommendations for public health based on reliable evidence.”

The paper 'Confounding by ill health in the observed association between BMI and mortality: Evidence from the HUNT Study using offspring BMI as an instrument’ by David Carslake et al in International Journal of Epidemiology

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