I make no secret of the fact that I think a lot of health information and advice that comes at us is not just unsupported by the science, but also seems to have been contrived with profit in mind. For example, despite the fact that taking dietary steps to reduce cholesterol have not been shown to save lives, there remains considerable appetite for supposedly cholesterol-quelling foods including specially formulated margarines and yoghurt drinks. And the dental profession continue to trumpet the tooth-preserving ‘benefits’ of water fluoridation, despite the fact that the biggest review of the evidence ever conducted concluded that this practice only helps one in six people, but causes dental disease (in the form of something known as ‘dental fluorosis’) in almost one in two. I could go on.
Another major myth that some seem keen to propagate is that having a body mass index (calculated by dividing weight in kg by the square of one’s height in metres) of 25 or more is putting us in some sort of mortal danger. However, as I’ve written before, the BMI seems to be virtually useless for adjudging an individuals health status. In a previous post, I outlined the evidence which suggests that BMI has practically no relationship to disease risk, but what seems to be far more important is any fatty accumulation around the midriff (formally known as ‘abdominal obesity’). Abdominal obesity is often assessed by comparing the waist circumference with the hip circumference. Basically, the lower your ‘waist-to-hip ratio’, the better with regard to risk of conditions like diabetes and heart disease.
The reason that I’m writing about this now is because this week a study which assessed the relationship between BMI, waist-to-hip-ratio and mortality in middle-aged women [1]. One reason that I was drawn to this particular piece of research, published in the Archives of Internal Medicine, is because it focused on mortality (overall risk of death). Assessing the effect of a factor on this allows us to make much more useful judgements about the possible health effects of some factor than looking at homing in on cancer of the gallbladder or some such rarity.
In this study, about 18,000 women were followed for some 9 years. In keeping with previous research higher waist-to-hip ratio was associated with an increased risk of death. Compared to women with the lowest waist-to-hip ratios, those with the highest were some 30 per cent more likely to die.
However, also in keeping with previous research, an elevated BMI was not associated with an increased risk of death. Not only that, but this study found that, if anything, having a raised BMI might be a bit of a life-saver: compared to women with a BMI of 18.5 ” 25, those with a BMI of 35 or more were 30 per cent less likely to die. Bear in mind now that according to official tables, a BMI of 18.5-25 is labelled as ‘healthy’, while individuals with a BMI in excess of 30 are branded ‘obese’.
In the light of this and other existing evidence, the BMI is looking increasingly obsolete as a useful body measure. My guess, though, is that certain factions will continue to cling to and promote the BMI. Why? Because using the BMI, rather than the waist-to-hip ratio, will ensure that more people will be under the impression that they need to lose weight for the sake of their health. And that means good business for the diet, food and pharmaceutical industries.
Not uncommonly in practice I see individuals who are genuinely comfortable with their weight, but have nonetheless been advised to lose weight on the basis of their BMI. If this sounds like you, then you can almost certainly ignore this advice knowing that the weight of evidence is on your side.
References:
1. Tice JA, et al. Risk factors for mortality in middle-aged women. Archives of Internal Medicine 2006;166(22):2469-2477
