The standard way of assessing body weight in Europe and the USA is the body mass index (BMI) which is calculated by dividing someone’s weight in kg by the square of their height in metres. It is generally accepted that BMIs of 18.5-24.9 are ‘healthy’, and that those of 25.0-29.9 mean someone is ‘overweight’, with the implicit suggested here being that they need to lose weight if they are to optimise their health.
Previously on this site, like here I have questioned the basis for these BMI categories. To begin with, for any individual the BMI is a quite useless gauge of their health status. Why? Because it takes absolutely no account of the composition of the body. It is therefore possible for a very healthy individual packing plenty of muscle and very little fat to end up being classified as ‘overweight’. It is also possible for someone with relatively little muscle mass and plenty of fat to be classified as ‘healthy’. Also, even if someone is carry excess fat, where it is found in the body appears to determine it’s likely impact on health (weight around the midriff appears to be particularly toxic). The BMI takes no account of this either.
Even when we forget individuals and focus on populations, there appear to be good reasons for querying the validity of the BMI as it is traditionally used. Proponents say that being ‘overweight’ increases the risk of certain conditions including heart disease and type 2 diabetes. These and other conditions that are more common in the overweight are important, but focusing on them can narrow our focus somewhat.
Some argue, and I would be one of them, that when judging the effect of any lifestyle factor as health, we need to take as broad a look as possible. A risk factor for one condition might be protective for another. For example, sunlight exposure may induce skin cancer, but the evidence appears to show it can reduce the risk of many, many other forms of cancer. It is also associated with a reduced risk of other conditions including cardiovascular disease and multiple sclerosis. Focusing on skin cancer can therefore give us a very skewed impression of the overall impact sunlight has on health.
An article published online in the journal Circulation  this week provides an assessment of the impact of being overweight on health and risk of death. It is an official statement from doctors and scientists acting for the American Heart Association. Some of the paper focuses on the apparent ability of being overweight has to increase the risk of conditions such as heart disease, type 2 diabetes and gallbladder disease. This is used to justify the BMI classification. Plus, the authors also take the line that those who are overweight run the risk of becoming obese, where there are even greater risks for health.
However, as I said, it makes sense to take as wide a view as possible in these matters. And one way to do this is to assess the impact of a lifestyle factor on overall risk of death. When we do this with the BMI, we find quite of lot of evidence which shows that, overall, those in the ‘overweight’ category have a lower risk of death than those in the ‘healthy’ category [2,3]. Notably, one of these studies  found that overweight individuals were at a statistically significant reduced risk of risk of deaths not related to cardiovascular disease or cancer compared to those of healthy weight.
The authors of this week’s paper in Circulation discuss the evidence pointing to the benefits of being overweight, and they also point to the potential deficiencies of these studies. Epidemiological studies such as these look at associations between things, and can’t be used to conclude beyond a shadow of a doubt that being ‘overweight’ is healthier than being of a ‘healthy’ weight. However, it looks like the best evidence we have does indeed suggest that this might be the case.
The authors conclude, however, that effective weight maintenance and obesity-prevention strategies be developed and implemented for all individuals above normal weight. So, the suggestion here seems to be that ALL overweight individuals (who appear to have the lowest risk of death of all, and some would argue, therefore, the best health overall too) need active intervention. This, to me, seems to be an example of treating a problem that doesn’t exist. We have no way of telling if these individuals are going to gain unhealthy weight or not. Why should ALL of them be subjected to some weight-related intervention? Also, what is the evidence for there being any benefit to such an approach, in terms of health, mortality and cost-effectiveness? The authors do not present any.
What seems to be going on here is an attempt by some members of the medical and scientific community to persuade us that people who are well need to be treated as though they are not. I don’t suppose that it will as too much of a surprise to learn that almost all of the authors of this paper have received research grants, speaking fees or consultancy fees by companies that supposedly have weight-related solutions at hand (including food companies, drug companies and Weight Watchers International).
In their conclusions, the authors state that ‘debating the relationship between BMI in this [overweight] range and total mortality misses broader implications’. I’m left wondering what broader implications can be more important than the evidence linking being ‘overweight’ with lowest risk of death.
1. Lewis CE, et a. Mortality outcomes, and body mass index in the overweight range. A science advisory from the American Heart Association. Circulation, 8 June 2009 [epub ahead of print publication]
2. Flegal KM, et al. Cause-Specific Excess Deaths Associated With Underweight, Overweight, and Obesity. JAMA. 2007;298(17):2028-2037.
3. Adams KF, et al. Overweight, obesity, and mortality in a large prospective cohort of persons aged 50 to 71 years old. N Engl J Med. 2006;355:763-778