Well Being

BMI Formula Flawed, Says Mathematician (Or: Maybe You’re Not Obese/Underweight After All!)

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new body mass index (BMI) formula

The formula used to calculate body mass index is flawed and skewed in favor of shorter people, says Oxford University mathematician Nick Trefethen. BMI is the standard tool used to assess healthy weight and the measure by which almost all obesity statistics are calculated.

The BMI formula — developed by a Belgian scientist, Adolphe Quetelet, in the 1830s — uses weight and height to calculate ideal weight. Those who fall within the 18.5-24.9 range of the index are considered to be at a healthy weight. A BMI of under 18.5 is considered underweight, a BMI of 25-29.9 overweight and a BMI of 30 and above obese.

The BMI has received quite a bit of criticism and scrutiny since coming into widespread use for both statistical analysis and as a proxy for health status. It's that latter part that some take issue with — though the tool was never intended to be used in isolation for determining individual health status.As blogger Phoebe Maltz Bovy commented on Facebook:

“My impression is that BMI is about population-level trends, not whether any of us personally will get heart disease, let alone some kind of official statement about whether we do or do not look fat in those pants.”

In other words, while a BMI over 30 may be broadly associated with high body fat, obesity and/or poor health, not every individual with a BMI over 30 looks or is obese or unhealthy. As many critics note, the formula fails to take into account body composition, and there's a big health difference between pounds of muscle and pounds of fat.

The Trouble With BMI Calculations

Trefethen says the fat-muscle theory accounts for a very small difference in BMI readings. But he believes the current formula also fails to adequately address height differences. In a recent letter to The Economist, Trefethen put the perceived problem quite bluntly:

“The body-mass index that you (and the National Health Service) count on to assess obesity is a bizarre measure. We live in a three-dimensional world, yet the BMI is defined as weight divided by height squared. It was invented in the 1840s, before calculators, when a formula had to be very simple to be usable. As a consequence of this ill-founded definition, millions of short people think they are thinner than they are, and millions of tall people think they are fatter.

In a conversation with another Oxford mathematician published on the university's website, Trefethen said “a better approximation to a complex reality, which is the reform I wish could be adopted, would be weight/height2.5. Certainly if you plot typical weights of people against their heights, the result comes out closer to height2.5 than height2.'

Using the New BMI Formula

Using Trefethen's proposed new BMI formula, millions of people of height around 5 feet would gain a point in their readings while millions around 6 feet would lose a point, he said. For some people, this could mean the difference between being considered a “normal,” healthy weight and being considered overweight/underweight, or between being overweight and being obese.

Obesity conspiracy theorists should hold their applause; it likely means very little in terms of global or American obesity rates and stats. For every taller, “obese” person who would now be merely overweight using Trefethen's model, a shorter, “overweight” person has just become obese.

Trefethen pointed out that many women on the threshold of obesity under current BMI calculations may be diagnosed as obese under his formula (for men, the opposite may hold true).

If I understand correctly, being almost exactly 5 feet and 5 1/2 inches means my BMI calculation would remain more-or-less the same — and that's what I found using Trefethen's new BMI calculator. My “new BMI” comes out about 0.14 points higher than before. If I play with enter my real weight but play with my height, being 5 feet tall would lead to an increase in BMI of over 1 point, while being 6 feet tall would lead to a decrease in BMI of about 0.4 points. You can calculate your own new BMI here.