Why do we still use BMI?
We’re told the greater our body mass index, the sicker we’re likely to be.
Your risk of heart diseases, type 2 diabetes, a large handful of cancers, and conditions like arthritis are meant to rise the higher that number climbs.
The scale states any number less than 18.5 is underweight, anything higher than 25 is overweight, and 30 and above is obese. Maybe you learnt how to calculate this in a high school health class, or have worked it out using online calculators like the one the Ministry of Health provides.
But of course, it’s not that simple. Fixating on your own BMI in isolation is unhelpful, experts say. When it comes to assessing your health, your doctor should always use other tests alongside BMI – such as checking your blood pressure, cholesterol and blood sugar levels. That’s because for BMI to be useful, it needs context.
The index was founded by Belgian mathematician and astronomer Adolphe Quetelet in the 1800s. He didn’t care about obesity or diets, he was obsessed with statistics.
He found by measuring weight and height he could work out the ‘‘average man’’.
His index was all based on the idea that the more numbers you get, the more accurate your ‘average’ will be.
He used this to measure useful things like the average weight of the Earth’s population and how much we collectively weighed compared with swimming pools.
Fast forward to the early 20th century and insurance companies began seeing a spike in claims from heavier people, so began using BMI to judge a client’s risk of illness.
There is ongoing debate on whether there should be different cutoffs in the scale for non-white people. This is partly because Quetelet’s scale was formed after taking measurements from white, European men.
But the numbers remain useful for getting a quick snapshot of both people and populations, experts say.
In large population studies, a BMI of less than 20 or more than 25 has been linked to increased risk of early death.
Another bonus for BMI is it’s simple to measure, which is helpful when doing research that involves thousands of people.
But at an individual level, BMI doesn’t consider how much muscle you have, or where your body stores its fat.
That’s why it’s ‘‘useless for rugby players’’, as human nutrition expert Sir Jim Mann puts it, ‘‘but very few New Zealanders are similar in build to rugby players’’.
Another number we now track alongside BMI is waist to height ratio. The simple message here is: health risk factors increase if your waist circumference is more than half your height.
Waist to height ratio can give a greater indication of fat, as muscle doesn’t tend to bulk out the stomach.
Some experts argue it better accounts for the way fat is carried by people of different ages and ethnicities.
In New Zealand, the latest data tells us 67.5% of us have a waistline more than half our height. This is about the same as the number of people considered overweight or obese based on BMI.
While debate on the pitfalls of BMI will surely rage for years to come, every authoritative body in the world still accepts it’s a useful tool, despite its limitations. Just remember, it doesn’t tell the whole story.
Reporting disclosure statement: This report was written with expert advice from Professor of Human Nutrition, Sir Jim Mann, Primary Healthcare senior lecturer Lesley Gray and Emeritus Professor of Nutrition Dr Elaine Rush. It was reviewed by The Whole Truth: Te Māramatanga expert panel member Associate Professor Lisa Te Morenga.