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

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.

5 Responses to Two studies question the validity of the BMI in the assessment of health

  1. a haynes 15 August 2008 at 10:57 am #

    My argument with BMI, and any other simplistic height/weight ratio analysis, has always been that it simply doesn’t measure body composition. Should a muscular person, who may well be categorised ‘obese’ by this criterion seek muscular wastage to fit in the so-called ‘healthy’ mould? Sadly, that’s what a number of sporting types (in sports ranging from rugby to martial arts and weight lifters) of my acqaintance have been told to do by medical professionals. Ridiculous!

  2. Sylvie 15 August 2008 at 12:01 pm #

    Thank goodness that there’s one lone voice in the wilderness actually talking some sense in the storm of health rubbish we see published daily. Of course, being overweight per se isn’t dangerous, millions of people who are overweight have died from natural causes at a good age. My sister has been overweight her entire life, she has low blood pressure and a regular heart beat. She hasn’t had a day off work for about 20 years. Of course, eating good food grown properly or raised properly doesn’t kill you – our ancestors have lived on this stuff for ever and flourished. It’s what we do to our bodies and our minds and what we do to our food that causes the problems and shoving more and more drugs into our bodies with more and more side effects is not the answer. Our food is grown/breed naturally on the earth and our bodies are made to move. We are designed to live in communities and share our lives. We are hard wired for a spiritual life and to grow and learn. If we remember these basic principles we are very likely to live long, happy, well, fulfilled lives. It’s really not that hard……

  3. Dan 15 August 2008 at 4:17 pm #

    I’ve always said that BMI is just technobabble for the old height-weight tables. It’s really nothing new, just a fancy-sounding acronym for an outdated concept.

    There are professional athletes who would be considered “obese” by BMI standards.

  4. Elizabeth 8 August 2010 at 9:22 pm #

    I think I heard that the BMI index was made-up by insurance companies, made unrealistically low, that they could factor in the “risk” — like putting most people in an unhealthy pool, when in fact, many healthy people have a bmi of 25-30

Trackbacks/Pingbacks

  1. Weekend Link Love | Mark's Daily Apple - 17 August 2008

    [...] As if you needed more proof that BMI measures are inaccurate, Dr. Briffa profiles two studies released this week showing you exactly why you shouldn’t get hung up on the numbers. [...]

Leave a Reply