The standard way of assessing weight as it relates to health is the body mass index (BMI) – calculated by dividing an individual’s weight in kilograms by the square of their height in metres. According to conventional wisdom, a BMI of 18.5 ” 24.9 is to be regarded as ‘healthy’, while BMIs of 25.0-29.9 are considered ‘overweight’. Those of 30 or above are considered ‘obese’. While the BMI usually forms the basis of the advice health professionals give to individuals about their weight, there are reasons to be mistrustful of it, I think.
My main gripe with this measurement is that while the BMI tells us something about the relationship between height and weight, it tells us nothing about body composition. It is entirely possible, therefore, to have a heavy, muscularly built, really very healthy individual whose BMI marks him or her out as ‘overweight’ or even ‘obese’. The other side of this, of course, is that someone can have a body composition that is low in muscle and high in fat, but at the same time be classified as ‘healthy’ from a BMI perspective.
The fact that the BMI tells us nothing about body composition means that, in all likelihood, it’s not going to tell us much about health status either. While it might be enshrined in medical lore that a ‘healthy’ BMI is one that ranges between 18.5 and 24.9, there is actually some evidence which calls this wisdom into question.
One way to assess the appropriateness of the BMI categories is to measure the overall risk of death in each category. If BMIs of 18.5-24.9 are deemed as being most healthy, then individuals with BMIs in this range should be at the lowest risk of dying too, right? Well, the biggest and most comprehensive study ever to look into the relationship between BMI and risk of death was published in Journal of the American Medical Association in 2007 .
Some notable findings from this study included:
- Being overweight was not associated with an increased risk of death from cardiovascular disease
- Being overweight was not associated with an increased risk of death from cancer
- Being overweight was associated with a reduced risk of deaths not related to cardiovascular disease or cancer
- Being overweight was associated with the lowest risk of death overall
Recently, some doctors acknowledged this evidence, but then reinforced the usual message that individuals should keep out of the overweight category. You can read about this here.
This week sees the publication of yet more evidence for the notion that ‘ideal’ and ‘healthy’ weights as determined by the current classification of BMI are wide of the mark .
In this study, researchers followed more than 11,000 Canadians over a 12-year period. They then calculated overall risk of death in each category (underweight, ‘healthy’ or ‘normal’, overweight, obese and extremely obese.
Compared to individuals in the ‘healthy’ category (BMI 18.8-24.9), overall risk of death for the other categories was as follows:
Underweight (BMI <18.5): 73 per cent increased risk of death
Overweight (BMI 25.0-29.9): 17 per cent reduced risk of death
Obese (BMI 30.0-34.9): No statistically significant difference in risk of death
Extremely obese (BMI 35 or more): 36 per cent increased risk of death
Here, again, we find that the lowest risk of death was found individuals classified as ‘overweight’. And this result was statistically significant. Perhaps even more surprising than this, though, is the finding that being ‘obese’ did not appear to put individuals at a significantly increased risk of death.
Risk of death is not the only way to measure health. Risk of illness and quality of life should be borne in mind too. However, some would argue that risk of death is probably the best single measure of the impact of a body measurement or lifestyle factor on health. I mean, there is an argument for believing that the most fundamental measure of health is whether someone is dead or alive.
We now have large studies which have found that being ‘overweight’ is associated with reduced risk of death. There is a case for changing the name of the ‘overweight’ category to ‘healthy’ and the ‘healthy’ category to ‘underweight’. And bearing in mind the failing of the BMI in terms of assessing body composition on a personal level, there’s an argument for ignoring it altogether. For information regarding what measurement is a better guide to health status, see here.
1. Flegal KM, et al. Cause-Specific Excess Deaths Associated With Underweight, Overweight, and Obesity. JAMA. 2007;298(17):2028-2037
2. Orpana HM, et al. BMI and Mortality: Results From a National Longitudinal Study of Canadian Adults. Obesity 2009 Jun 18. [Epub ahead of print]