BMI is everywhere- online calculators, posters in public spaces, talk of it's importance at the doctor's office... Well, ya know what? I'm calling bullshit.
BMI was originally called the Quetelet Index, after the man who devised it in the 1800s, Lambert Adolphe Jacques Quetelet. He thought that if men grew in weight in proportion to their height, they would be healthy. The term was updated by Ancel Keys (you may remember him from my blog about the Mediterranean diet) in the 70's, and it is worth noting that Keys himself hoped for the index to be used in population studies, rather than as a tool for individual health evaluation.
But, because BMI is easy to measure and calculate, it has become the primary assessment tool for health. It must be said that although it is the primary tool, it is not fit for purpose. Here's why:
It tells us nothing about body composition.
It only tells us weight and cannot separate fat mass, muscle mass and bone mass. Worth noting that muscle weighs more than fat, so many people who've been spending hours in the gym get classified as overweight or obese. Equally, someone carrying a high % fat mass could be classified as "normal" weight. In fact, it is thought that BMI misclassifies large percentages of the population, with one study indicating that nearly 40% of the cohort were misclassified using BMI. Many academics and professionals have stated that a visual inspection could give a better idea of body composition that BMI.
It gives us an indication of size in relation to height, but what does that tell us about the person?
Not a lot. Not about their lifestyle- sleep, stress, activity levels, nutritional status. Not about their metabolic health. Not about their family history. Not whether they are a good person. Weight V height provides limited information to make assumptions about health and certainly shouldn't be used as a diagnostic tool, yet here we are...
People are not built the same and are not built with this height to weight ratio in mind.
And the idea that if every body fit into the "normal" BMI category we would all be healthy is ludicrous. Bodies are diverse in all aspects from height and weight to hair and skin colour. That's part of what makes humans cool. We accept animals in the size bodies they come in- we don't down on dachshunds because they're shorter than other dogs. Why can't we just accept that all humans are unlike any other?
BMI doesn't consider weight distribution.
i.e. whether someone carries all their weight in their stomach or whether it is evenly distributed. Trunkal or central fat mass is often what is considered "high risk", as it could affect our organs. BMI cannot tell where we carry weight and therefore is limited in its ability to indicate disease risk.
The category cut-offs are arbitrary and are not standardised.
Different countries have different category cut-offs- in the UK overweight=25+, but in Japan=23+. In fact the States reclassified their categories a few years ago, so that huge percentages of the population changed categories (+ therefore "risk status") overnight. People went to bed "normal" and woke up "overweight". How ridiculous is that!
BMI is VERY difficult to change.
Well first of all, we can't change our height so.....
And of course, weight loss attempts are futile (see my blog on that here).
BMI doesn't account for age-related body composition fluctuations and changes.
Our body composition changes a lot over our life time. Our bone mass is at its peak in our 20's and it declines from about age 30. Our muscle mass also depletes with age (for several reasons). Add to those factors changes to diet and activity levels and BMI is pretty unhelpful for the ageing body.
So why is it so commonly used in healthcare?
Because it is easy to test- no fancy equipment needed, no patient fasting required, no waiting for results. Healthcare wants to be seen to do something about weight so it looks at BMI and therefore can tick a box saying "we're keeping a tab on it, we care". And that's the reality- checking BMI for healthcare professionals is ticking the box and reaching pointless targets. It is not helping. It is not improving health. In fact, it is creating more problems.
Summary: BMI tells us nothing about health and everything about weight. It is a tool that although is easy to measure, adds little to our knowledge of people.
BMI is an old, unproven theory, please don't put value on your BMI number or category.
This covers some of the many reasons why I DO NOT weigh any of my clients. We spend so much time focusing on these numbers that literally mean nothing. They do not measure health.
Once more for the people in the back-
BMI/WEIGHT IS NOT A MEASURE OF HEALTH.
Lots of love,
Little O x