Using BMI to Measure Health Risks

The pros and cons of calculating BMI

Full disclosure, I’ve never been a fan of BMI because I’ve always fallen into the overweight category – until very recently. I played a variety of sports back in college and was in the best shape of my entire life at that time. But according to the published BMI categories, someone six foot one inches tall and weighing 215 pounds is “overweight” and on the verge of being obese.

I’m still six foot one many years later, although with a bit of a stretch, and weigh in just around 180 pounds. That puts me solidly in the “normal” BMI category. Truth be told, I’m sure my body fat content is higher today than it was back in college.

How calculating body mass index got traction

To understand the shortcomings of the BMI a quick history lesson is in order. The concept of calculating a Body Mass Index was first proposed by the mathematician Adolphe Quetelet back in 1832. His passion for probability calculus drove him to study how mathematics could be applied to the physical characteristics of humans. He learned that except for the occasional growth spurts, such as immediately after birth and during puberty, the weight of a person normally increases with the square of their height.

Ancel Keys was the first to dietician identify the relationship between saturated fat and cardiovascular disease. In 1972, Keys and his coauthors would resurrect Quetelet’s work and promote BMI as a measure of obesity.

The pros of BMI as a measure of health risks

Since the metric was based on statistical studies, Ancel Key knew BMI was not perfect. However, he also knew the importance of simplifying concepts when trying to educate the masses. Therein lies the beauty of BMI. With only two inputs it’s possible to develop a rough estimate of someone’s risk of developing several chronic diseases. Gathering weight and height data was already a common practice among physicians, so they could easily calculate the BMI of their patients. Even better, the average Joe Schmo knows their height and weight, so all Joe needs is a handy online BMI calculator.

Intuitively, the calculation makes sense – taller people that are in the same physical condition as shorter persons should weigh more. It’s not hard for us to work this through in our mind’s eye.

The arguments against using BMI

Unfortunately, the simplicity of BMI is also its downfall. Joe Schmo knows his height and weight, but he’s also intimately familiar with his belly button. If he’s got a six pack just north of it, then Joe can set some of those health risk concerns aside. But if he needs to move some fluff around to find it, then Joe might want to reflect on the possible health risks associated being overweight.

In fact, it’s well known that BMI is not an appropriate measure for athletes and bodybuilders. To make this point, let’s use LeBron James as our subject. In case you never heard of LeBron, he is arguably the greatest basketball player of all time. He is in outstanding physical condition and stands in at 6’8″ and 250 pounds. With a BMI of 27.5, he would also be categorized as “overweight.”

Arnold Schwarzenegger is a seven-time Mr. Olympia bodybuilding champion. Arnold is 6’1″ and during competitions his weight was around 235 pounds – which puts his BMI at 31. Arnold was far from obese with a body fat content of only 8%.

The statistical study used to develop the BMI categories was based on a predominantly white group of individuals, which means there can be a bias when used by other races. In fact, studies have found that Blacks / African Americans are penalized by the categories, meaning the upper limit for each category should be higher. Meanwhile the categories are too lenient for Asians, meaning the category threshold values should be lower.

It’s circumference that really counts

Earlier I referred to Joe Schmo’s belly button, and I did that for a reason. In the United States, the Centers for Disease Control has an alternative to BMI that has a lot of merit: waist circumference. Excessive abdominal fat places you at greater risk of developing diabetes, high blood pressure, and coronary disease.

The risk of developing these conditions increases for:

  • Men with a waist circumference greater than 40 inches
  • Women with a waist circumference greater than 35 inches (non-pregnant women)

Taking this measurement is easy too, the process includes:

  • Standing and placing tape measure just above your hipbones
  • Keeping the tape is horizontal around the waist and snug (not squeezing the skin)
  • Taking the measurement just after breathing out

Of course, waist circumference isn’t perfect either, but my waistline has never been over 36 inches so I’m partial to using this approach.

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