Two studies question the validity of the BMI in the assessment of health

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Variously on this site I have written about evidence which suggests that the standard measure of weight ” the body mass index (BMI) ” is not really fit for purpose. The main issue with it that while it tells us about someone’s weight in comparison to their height, it tells us nothing about the body’s composition. It also tells us nothing about where any excess fat might be distributed in the body. This is important because previous work has suggested that weight that congregates around the middle (either in or around the abdomen) is associated with enhanced risk of chronic disease, where fat elsewhere is relatively benign.

This week, a couple of papers were published which rammed home just how insensitive a measure of health the BMI is. One of these, published in the Archives of Internal Medicine, looked at prevalence of what are termed cardiometabolic risk factors such as raised blood pressure, raised blood sugar, raised levels of blood fats known as triglycerides, raised levels of a substance known as C-reactive protein and low levels of so-called healthy HDL-cholesterol [1]. Individuals with none or only one of these factors was classified as metabolically healthy, where those with two or more of these risk factors were classified as metabolically abnormal.

The percentages of metabolically abnormal individuals in each of the BMI weight bands were as follows:

BMI < 25 (healthy): 23.5 per cent BMI 25.0-29.9 (overweight): 48.7 per cent BMI 30 or higher (obese): 68.3 per cent Put another way, this research found that a considerable percentage of individuals of healthy weight were metabolically unhealthy. In the overweight category, there were more metabolically healthy than unhealthy individuals. And finally, in the obese category, a significant proportion of people (about a third, actually) were deemed metabolically healthy too. The edition of the Archives of Internal Medicine which carried this study also carried another which casts further doubt on the usefulness of the BMI in judging health status [2]. Here, a group of individuals were divided up according to BMI as they had been in the previous study. In addition, those in the obese category were divided according to whether they had evidence of insulin resistance or not (insulin resistance means, essentially, that insulin is not working as well as it should in the body, and this abnormality is believed to be a precursor of type 2 diabetes). The individuals were assessed for a variety of measures of fat (including total body fat, fat around the organs and fat in the liver). They also had the thickness of a major artery (the carotid artery) measured in an effort to assess the cardiovascular health (the thicker the artery, generally speaking, the higher the risk of cardiovascular disease). In this study, individuals in the overweight and obese categories were found, compared to those in the normal weight group to have higher levels of body fat and fat around the organs. However, when the researchers started looking at the results of the obese individuals according to whether they were insulin resistance or not the results became a bit more revealing. Distinct differences were found between these two types of obese person. In particular, compared to those who were insulin sensitive, the insulin resistance individuals were found to have significantly more fat in their livers and thicker arteries. However, most interesting of all to me as the fact that individuals in the thickness of the arteries in the obese group who were deemed to be insulin sensitive were actually not significantly bigger than those of individuals in the healthy weight group. The suggestion here is that an obese individual's arterial and cardiovascular health may not be significantly different from someone with a healthy BMI as long as they are not insulin resistance. What these two studies show very clearly, I think, is that the BMI is, certainly on its own, an inadequate tool for judging health status. We can see that within the pre-determined weight bands there is much individual variation in terms of health status. And what is very clear indeed is that individuals deemed obese according to their BMI may, in fact, be healthy as adjudged by other standard measures of disease risk. References:

1. Wildman RP, et al. The obese without cardiometabolic risk factor clustering and the normal weight with cardiometabolic risk factor clustering: prevalence and correlates of 2 phenotypes among the US population (NHANES 1999-2004).
Arch Intern Med. 2008;168(15):1617-24.

2. Stefan N, et al. Identification and characterization of metabolically benign obesity in humans. Arch Intern Med. 2008;168(15):1609-16.

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