General Health Thursday January 24, 2013

The new and improved BMI equation

For all of us who have been using the traditional BMI method to regulate our weight, it’s time we looked for a second opinion. This week the traditional method of calculating BMI came under fire as it fails to take into consideration people’s varying heights. On the 5th January, Professor Nick Trefethen wrote to the Economist who published his letter, which outlined his concerns with using what in his opinion is an outdated method.

What is the problem with the traditional system?

Adolphe Quetelet, who developed the BMI index in the 1840s, actually noticed there was a flaw early on. He stated that it would be a good indication of population trends but not always a good indicator at individual levels. He has turned out to be absolutely correct. It is a wonder, then, why the NHS use it on such an individual level. The traditional system does not account for people’s weight increasing due to them getting taller. Instead it works under the assumption that everyone grows taller before they bulk out, but that is not always the case. Calculating BMI in this way fails to take into consideration the natural bulkiness taller people have, but has allowed shorter people to be heftier than they should be. This has ultimately led to us receiving incorrect body fat results.

What does this mean?

The NHS has relied on the BMI formula to judge whether their patients are overweight and judge their risk of heart disease or high blood pressure according to their results. Until recently it was traditionally calculated by dividing a person’s weight in kilograms by their height in metres squared. By using this method it means that many people could actually be overweight, or indeed underweight, but have not had the problem addressed properly.

What has changed?

Overnight, those who are 5ft have now been afforded a whole extra BMI point, which could very easily push them over the barrier to either “overweight” or even “obese”. Meanwhile, some shorter people may have found themselves fatter than they thought. However, for those who are 6ft, it’s a much better story, as they have lost a whole BMI point which could push them back in to the “healthy” category.

How does the new equation work?

Nick Trefethen, professor of mathematics at Oxford University, has come up with a more reliable method of calculating our BMI which accounts for people’s varying heights. His new formula works by multiplying a person’s weight in kilograms by 1.3 and dividing the result by their height in metres to the power of 2.5. This new method results in a much more accurate reading that takes in to account all the person’s measurements, thus allowing for results on a much more individual level, something that Adolphe Quetelet’s system could not offer.

Reactions to the change

There have been mixed reviews on the back of the proposed BMI equation change. Understandably, women, who are commonly shorter than men and often conscious about their weight, feel they do not need yet another reason to feel fat. Waking up in the morning to the news that they are now a point (if not more) over weight than they were initially happy to believe is quite daunting.

On the other hand, many believe this will be a beneficial change, as it is important for the NHS to have a system that works when they rely so heavily upon it to help advise patients on their diets and lifestyle. Of course it’s also been a good start to 2013 for the taller people among us, who feel they can now afford to loosen their grip on one of their New Year’s resolutions.

To see what your new BMI score is visit Dr. Trefethen’s website:

To read Dr. Trefathen’s letter to the Economist:

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