Healthy Living

Oxford Mathematician Explains the Body Mass Index Flaw

obesity
Image REUTERS/Finbarr O'Reilly

A mathematician from the University of Oxford has said that the Body Mass Index, a standard tool to measure obesity rate, is flawed because it doesn't account for the fact that some people are taller and so might be carrying extra natural weight, according to Mail Online.

BMI or body mass index (developed by Belgian scientist Adolphe Quetelet in the 1830s) is a number calculated according to person's weight and height. For long, BMI has been used as a standard tool to determine if a person's ideal weight. A person is underweight (BMI below 18.5), normal (18.5- 24.9), overweight (25.0-29.9) or obese (30.0 and above). According to many experts, the flaw with this tool is that it does not measure the fat - muscle ratio.

Trefethen found that BMI lets short people believe that they are thin while making the taller people feel that they have a weight problem because it doesn't account for the natural weight that a taller person's height may add to his or her BMI. The fat-muscle theory, he says, accounts for a very small difference in the BMI reading. He also added that many women on the threshold of obesity in the current BMI scale may be diagnosed as obese under the new BMI calculation.

"BMI divides the weight by too large a number for short people and too small a number for tall people," Prof Nick Trefethen from Oxford University's Mathematical Institute, reports The Telegraph. "So short people are misled into thinking that they are thinner than they are, and tall people are misled into thinking they are fatter."

According to Trefethen calculations, people around 5' would gain a point on the BMI scale, making them obese, while people around 6' would lose a point. This, he says, is because the weight (in kilograms), in the new formula is first multiplied by 1.3 and the answer is then divided by the person's height to the power of 2.5, according to a blog from the University of Oxford.

Current formula is weight (kg)/height (m) ^2 = 703*weight (lb)/height (in) ^2.

New Formula is 1.3*weight (kg)/height (m) 2.5 = 5734*weight (lb)/height (in)

Trefethen pointed out this flaw in his recent letter to The Economist.

"In our overweight world, such changes would distress some short people and please some tall people, but the number they'd be using would be closer to the truth and good information must surely be good for health in the long run," Trefethen said.

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