As a companion piece to a recent sizable study on weight and risk of age-related disease, here is another set of data to suggest that the existing consensus on the harms done by excess visceral fat tissue are, if anything, an underestimate. There is a large body of research that covers the many mechanisms by which the visceral fat packed around internal organs causes damage, such as through inflammation and immune dysfunction, the presence of raised numbers of senescent cells, the metabolic disarray that leads to diabetes, and so forth. Collectively it is a lengthy cautionary tale for those living far enough along the upward curve of technological progress to have reliable access to cheap calories, but not far enough to have reliable technological means to prevent the consequences of consuming those calories.
The harmful effects of being overweight have been underestimated, according to a new study. Previous studies have suggested that the optimum body mass index (BMI), at which the risk of death is minimised, 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 behaviours, 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. This aim of this study was 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, 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.