Why is Body Mass Index such a poor measure of health?
What is BMI?
Body Mass Index is used as a very basic measure of an individual’s health status. It was created by Lambert Adolphe Jacques Quetelet in the 1830s. He was a Belgian astronomer, mathematician, statistician and sociologist, NOT a doctor or health professional. He wanted it to be used to measure the health of a population group in order to help government allocate resources. It was never designed to be used on individuals.
We have all used the online calculator to work out our BMI, simply inputting our height and weight. A person with a BMI between 18.5 to 25 is considered to be healthy. Anything outside of that, you are deemed underweight, overweight, obese or even morbidly obese. If you think that a person’s health can be determined into distinct categories with sharp boundaries that hinge on a decimal place seems nonsensical, I agree. In fact, in 1998 the BMI categories were changed in the US. It meant that millions of Americans woke up and were suddenly overweight without having put on any weight.
It is used in by the medical community, the government and insurers as it is a cheap and easy way of determining perceived health. By using BMI, a doctor avoids having to run any scientifically sound, expensive tests in order to accurately measure our health.
Reasons why it shouldn’t be used
BMI makes no allowance for the relative proportions of bone, muscle and fat in the body. Bone is denser than muscle and twice as dense as fat, so a person with strong bones, good muscle tone and low fat will have a high BMI. This means that athletes who work out a lot tend to find themselves classified as overweight or even obese.
Where you store your fat on your body also needs to be considered when determining whether the fat will be detrimental to your health. People who carry extra weight in the abdominal area tend to be at the higher risk for disease because the fat may be encasing vital organs.
A person whose BMI says they are overweight or obese is often considered unhealthy, while people with normal BMI are often seen as healthy, but research published in 2016 suggested that this was incorrect for 75 million Americans.
Researchers found that 54 million Americans had been classed as overweight or obese, but cardiometabolic measures* showed they were healthy. Another 21 million were classed as “normal” in terms of BMI, but they were unhealthy.
*measures include blood pressure readings and metabolic labs, such as HDL (“good”) and LDL (“bad”) cholesterol, triglycerides, glucose, and C-reactive protein.
This mis-categorisation of people should be viewed alongside data from studies that indicates people in the overweight or moderately obese categories live at least as long— or longer—than people in the normal weight category.
Studies analysing BMI risk for disease rarely acknowledge factors like fitness, activity, nutrient intake, weight cycling, or socioeconomic status when considering connections between weight and disease. When studies do take into account these factors, often increased risk is minimised significantly.
Alternative measures
So, what can we use instead? Well, I personally don’t advocate putting too much investment into the scales (for the same reasons outlined above). You could choose to take measurements but again, this does not take into account many health promoting behaviours.
Instead, we should focus on those behaviours - taking part in joyful movement, finding body acceptance and eating in a way that takes into account gentle nutrition, enjoyment and satiety (intuitive eating).
If we choose to take part in these healthy behaviours without worrying about whether our weight / BMI changes then we will naturally become healthier. If you feel better in your own body then that is the feeling we should be seeking. Always remember that thinness doesn’t not automatically mean you are healthy - the behaviours noted above are what is important.