We’ve said for a long time that Body Mass Index, or BMI, is flawed. According to an article in The Daily Mail this morning, other scientists agree.
Over a year ago we posted a blog discussing, what was then, a new approach to estimating body fat. In case you missed it the first time round, here it is again.
Do we really need a new BMI?
Oxford University mathematician, Nick Trefethen has developed a new calculation which he claims better accounts for the relationship between height and weight.
This calculation still only uses height and weight to estimate how fat or thin someone is which just leaves us wondering if all his work was really worth it when it’s still just an estimate and not accurately measuring body composition. After all, if you don’t know exactly what you’re body is made of now, how can you know what changes you need to make?
Some of you may have heard Dr Mark Porter discussing this topic on BBC Radio 4’s Inside Health. He talked to members of the British public who not only thought that BMI was meaningless to them, but also had no idea where it comes from, how to calculate it or how to read the BMI chart in their GP surgery. For those of you who are interested, BMI is calculated by dividing your weight in kilograms by your height in metres squared (weight/height2).
This out-dated formula was devised over 100 years ago by Belgian scientist Adolphe Quetele, but according to Professor Trefethen it is over-simplistic. He believes it is incorrect because “it divides the weight by too large a number for short people, and too small a number for tall people. So short people are misled into thinking they are thinner than they are, and tall people are misled into thinking they are fatter than they are.” He believes his new calculation goes some way to correcting these problems. Professor Trefethen calculates BMI by multiplying your weight in kilograms by 1.3 then dividing this number by your height in metres to the power of 2.5 (1.3xweight/height2.5). You can use interactive calculator on The Telegraph to find out what this change would mean for you.
But even Professor Trefethen agrees that “BMI is only one of many factors and inevitably not everyone with fit the standard pattern.” He also acknowledged that BMI is “not always a good indicator at an individual level.”
One proposed alternative is A Body Shape Index (ABSI). This is another new measure looking at how our body shape predicts mortality risk. ABSI is just as simple to calculate as both versions of BMI, and takes into account not only weight and height but also looks at where any excess fat is stored using waist circumference (WC). It is well known that fat stored around the stomach area is particularly bad for our health, but any excess weight for our given height also carries greater health risks. Even though it’s still an estimate, by combining these measures the ABSI is a better predictor of an increased risk of premature mortality than either BMI or WC alone. The ABSI equation divides WC by BMI to the power of two thirds (BMI2/3) multiplied by height to the power of a half (height1/2).
We know that accuracy really does matter. If BMI is known to be flawed why is it still used so widely within the NHS? Yes it’s simple to calculate, but so is ABSI which is a better predictor of health risks. Whatever estimate is used, if it’s inaccurate then you may not be getting the advice you need. Obesity is defined as having too much body fat NOT as weighing too much for your height. So, do we really need a new way to estimate body fat when we can accurately measure it?