Globally, Body Mass Index (BMI) is still considered to be the ideal way of calculating a “healthy weight”. And there is no question that the issue of obesity is having a serious impact on a wide range of aspects of modern life. Apart from anything else, the global cost of obesity and its associated health conditions like diabetes and cardiovascular disease runs into trillions of dollars.
The trouble is, on an individual level, BMI on its own has no link to overall health, good or bad.
What is BMI?
BMI is a calculation that divides an adult’s weight in kilograms by their height in metres squared. The number that comes out is then fed into ranges that claim that you are, or are not, a “healthy weight”.
- Below 18.5 is classified as underweight.
- 18.5 – 24.9 is classified as healthy weight.
- 25 – 29.9 is classified as overweight.
- Over 30 is classified as obese.
Where did BMI come from?
Back in the 1830s a Belgian mathematician, Lambert Adolphe Jacques Quetelet, set out to find the parameters for a “normal man”. He had no intention of defining obesity, which at that time was neither widespread, nor recognised as a health concern. And although he did measure men of various ages, the men he measured were primarily Belgian.
It wasn’t until 1972 that Ancel Keys decided that the calculation devised by Quetelet was an ideal formula for defining obesity and renamed it “the body mass index”. It is worth noting here that Ancel Keys was also the man who cherry picked data from the Seven Countries Study to fit his hypothesis – that saturated fat increases cholesterol and causes heart disease (spoiler alert – it doesn’t). That study has informed public health nutrition policy across the globe for decades and opened the door for the food industry to make an absolute fortune from unhealthy “low fat” products. If you want to know the truth about fat, this is a good place to start:
So why is BMI bogus?
To start with, it doesn’t measure levels of body fat, it only measures weight. As muscle and bone both weigh more than fat, anyone with particularly dense bones, or with a large proportion of lean muscle mass is likely to be classified as obese when the BMI calculations are done. This is probably why many athletes show up as obese, when in reality they are very far from being overweight.
On the flip side of that, a substantial percentage of the global population have a “normal” BMI, but they also have a high proportion of body fat and a low proportion of lean muscle mass. Often that body fat is what is called “visceral”, accumulated around internal organs, so it can only be picked up with scans. And it is visceral fat, along with an imbalance between lean muscle mass and body fat rather than BMI, that is most clearly linked with an individual increased risk of developing the diseases associated with obesity .
Average white men…
BMI relates (like so many things in this world) to the average size white man – although in this case, to the average size European man that lived nearly 200 years ago. It does not allow for varying body compositions of individuals from different parts of the world. Even the American Medical Association has recently admitted that BMI is not necessarily the best measure of overweight or obesity in non-white ethnicities (although of course it doesn’t acknowledge that it equally has little or no relevance to women or children).
Women
BMI doesn’t take women into account. Because women are designed to build more humans they stockpile energy in the form of fat, and therefore they generally have a far higher body fat percentage and lower lean muscle mass than men. Nor are hormonal fluctuations and the impact of menopause in women considered. From around 50, women start putting on weight and science is only just catching up with why. And, just like with men, variations in ethnic origins are also not factored in for women.
Children
The BMI is hardly relevant to children, whose rapid growth and change means their body mass index shifts constantly. This is not helped by the fact that, in the UK, children and adolescents are shoved into percentile groups according to their age. So a child who is particularly tall for their age could easily end up in an overweight percentile, despite not actually being overweight. Labelling children as “overweight” or “obese” can have all sorts of unintended consequences. It can follow children throughout their school career and beyond and, in some cases, it can even trigger aspects of eating disorders.
Ageing
The fact that body fat percentage changes with age, regardless of hormone status, is not considered in the calculations. Total body fat in both men and women peaks around the mid-60s and then starts to decrease. At the same time, muscle mass begins to decrease, and both of these changes impact on the outcome of the BMI calculation .
So why do they use it?
It’s quick. It’s easy. And it doesn’t cost anything.
Plus, like so much about allopathic (conventional as opposed to complementary) medicine, BMI is very much a one-size-fits-all approach. Modern medicine loves to stash everything into identikit boxes that lack individuality. Plus it is very convenient to use. Epidemiologists (people who study diseases, how they spread and how they can be controlled) also really like it because it gives them an overview of the population and, like so much of modern medicine overall, individuals are irrelevant in epidemiology.
Many scientists are now of the view that BMI and risk of illness or death should only be applied to large populations (score one for the epidemiologists). It should not be applied to individuals unless a wide range of variables are also considered. Variables that go far beyond what is discussed above and should include things like whether the individual has ever smoked, how much they drink, whether they get any exercise, what their diet is like, what their blood glucose balance and blood pressure are like, when weight gain (or loss) started, and what the individual’s mental health is like all need to be considered.
Are there viable alternatives?
There are other ways to measure obesity. Working out waist-to-hip ratio – but getting the measurements and then doing the calculation isn’t always easy. The good old fashioned calliper test which measures a pinch of skin and the fat underneath in a few areas of the body (this is where the phrase “pinch an inch” comes from) but it isn’t particularly accurate and, of course, it takes time and requires a degree of skill. CT and MRI scans are extremely accurate, but the tests cost money and the number of scanners available is limited.
There are also some amazing and highly technical bits of kit around, like the bioelectrical impedance analyser which sends electrical signals through the body and measures body composition, particularly body fat and muscle mass. But it’s expensive, and BMI calculations are free.
Ideally overall health should be measured by a range of tests that cover physical, mental, and functional health, as well as the BMI. There is a method for doing this, it’s called the Edmonton Obesity Staging System and it has recently been adopted by Canada, Chile and Ireland. Results thus far are looking promising.
The question is, will the NHS do likewise?
