Flaws In Body Mass Index

Body Mass Index (BMI) is a metric used for measuring health. It's calculated by dividing a person' weight in kilograms by the square of the person's height in meters. If you prefer to use English units, it's your weight in pounds divided by the square of your height in inches, then multiplied by 703. BMI doesn't scale well. A tall man with the exact same build and body composition as a shorter man will have a higher BMI. Secondly, the measure ignores variation in body shapes. Some people are slender, others are stocky. Moreover, people carry fat in different places. Subcutaneous fat just below the skin is generally not associated with a steep rise in mortality, while abdominal fat is. Finally, BMI does not differentiate between fat and muscle mass. This glaring drawback means that many muscular athletes are considered overweight, or even obese. ¹

BMI is one of the most widely used measures of obesity. But it's apparent that it is flawed in more than one way. Being overweight may not be as unhealthy as it was 40 years ago. Researchers have found a BMI of 27 is linked to the lowest rate of death, but someone with a BMI of 27 is currently classified as being overweight. Here are the CDC numbers for BMI:

Below 18.5--underweight 18.5-24.9--normal or healthy weight 25.0-29.9 --overweight 30.0 and above--obese

Researchers looking at 120,528 people from Copenhagen found the lowest risk of having died from any cause was a BMI of 27 in people in the 2003-2013 group. The study showed that in this time frame, there was no difference between the death rates of people with a BMI of 18.5 to 24.9 (healthy), and those with a BMI of 25 to 29.9 (overweight), which were 4 per 1,000 per year for both groups. ² In what could be the death knell for BMI, research out of UC Santa Barbara and UCLA reveals that millions of Americans labeled overweight or obese based on their BMI are, in fact, 'perfectly healthy.' The findings suggest that 34.4 million Americans considered overweight by virtue of BMI are actually healthy as are 19.8 million who are considered obese. According to Jeffrey Hunger, a doctoral student in UCSB's Department of Psychological & Brain Science, and a co-author of the paper, BMI is a deeply flawed measure of health. “In the overweight BMI category, 47 percent are perfectly healthy,” he said. “So to be using BMI as a health proxy, particularly for everyone within that category, is simply incorrect. Our study should be the final nail in the coffin for BMI.” ³ More recently Alan Finkel says his concern is that BMI ignores elementary physics. He reports, “The problem traces back to Lambert Adolphe Quetelet, the Belgian statistician who invented the BMI in 1830. Quetelet failed to consider the mathematics of scaling. He defined the BMI as weight divided by height squared. Note, however, that weight is proportional to volume, which is proportional to height cubed. The upshot of this is that, all other things being equal, BMI varies directly with height, which it clearly should not.” ⁴

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What are alternatives to BMI?

There is the Body Adiposity Index which multiplies hip circumference by height and is more accurate than BMI reports Lila Abassi. She adds there is also the Waist Circumference Measurement which can help determine heart disease risk (at risk women > 35 inches and men >40 inches waist circumference). Another calculation is the waist to hip ratio where a value of over 0.9 for men and 0.85 for women deems the presence of abdominal adiposity. Two other methods are Hydrostatic Weighing and Body Fat Measuring which uses calipers to measure skin and fat from the waist, shoulder blade, biceps and triceps. ⁵ If only we could go back to 1998. The present problem goes back to a rule change that year. In 1998 the National Heart Institute redefined overweight to be a body mass index of 25 or more for both men and women as being overweight. Before 1998, a man was officially overweight with a BMI of 27.8 and a woman at 27.3 ⁶ Why not simply go back to the 1998 listing? It would make so much sense, and be in agreement with many research studies.

References

1. Ross Pomeroy, “A new potential replacement for body mass index,” realclearscience.com, December 30, 2015
2. “BMI categories may need adjusting, argue researchers,” [url=http://www.nhs.uk]http://www.nhs.uk[/url], May 11, 2016
3. Julie Cohen, “A flawed measure,” news.ucsb.edu, February 4, 2016
4. Alan Finkel, “Body mass index miscalculation,” cosmosmagazine.com, June 26, 2018
5. Lila Abassi, BMI is bologna,” acsh.org, February 5, 2016
6. Gina Kolata, “It isn't flab, it';s just a scale that's off kilter,” The New York Times, November 29, 2004