Mathematicians Find We've Been Calculating Body Mass Index Wrong

Being short in stature has its difficulties but Oxford University researchers have found another drawback – they have been made fatter by a new weight index

Mathematicians found the formula used to calculate body mass index (BMI) has made tall people more overweight and those vertically challenged not fat enough.

They argued it did not take into account a person’s weight tends to grow with their height, giving taller people more room to bulge.

Consequently Prof Nick Trefethen, a leading mathematician, has devised a new formula after finding the current BMI divided weight by too large a number for short people and by too small a number for tall people.

“The NHS relies on the BMI pervasively in all of its public discussions of obesity,” he said.

“We deserve an explanation of what justification they have for using this formula.

“BMI divides the weight by too large a number for short people and too small a number for tall people.”

He added: “So short people are misled into thinking that they are thinner than they are, and tall people are misled into thinking they are fatter.”

The BMI formula is used by doctors to work out if someone is overweight or obese and so at risk of problems from high blood pressure to heart disease.

It is traditionally calculated by dividing a person's weight in kilograms by their height in metres squared with the aim of giving a measure of an individual's body fat.

But this assumes people scale up according to a model of growth, in which they get taller more quickly than they bulk out.

A BMI of between 18.5 and 24.9 is normal, less than 18.5 is seen as underweight while 25 to 29.9 is considered overweight. A mark of 30 or above means a person is obese.

Prof Trefethen started investigating the problem after realising conventional BMI calculation methods failed to take into account that taller people tended to be bulkier than those who were smaller in height.

He found short people were misled into thinking that they are thinner than they are and tall people are misled into thinking they are fatter.

For those who are 150cm tall [five feet], the new formula would add a whole BMI point, enough to topple people lurking on the borders of the "normal" weight into "overweight" territory and a similar number would become "obese".

Those of 180cm or above (six feet), lose a BMI point. Only those of average height (170cm) will remain the same BMI.

The mathematical formula was devised by the Belgian scientist Adolphe Quetelet in the 1830s. But the professor insisted his formula was far from simply an academic exercise.

“BMI is only one of many factors and inevitably not everyone will fit the standard pattern,” he said.

“We know that BMI is a good indicator of population level trends, but not always a good indicator at an individual level."