Part 1: What is BMI?
There is some notion that a person's weight relative to a person's height can predict health problems and perhaps, early death. Besides ourselves, who else really cares about a measurement that predicts health problems and death?
Answer: Insurance companies!
The search for an index of relative body weight happened when insurance actuaries in the late 1940s noticed that overweight policy holders had an increased mortality rate. The relationship between weight and cardiovascular disease became the subject of many epidemiological studies.
For reasons that are not clear, the insurance companies reached back to 1832 and found the work of a Belgian mathematician, astronomer and statistician named Adolphe Quetelet. This guy had developed a ratio of weight in kilograms divided by the square of the height in meters, which he modestly called the "Quetelet Index".
Adolphe Quetelet's cross-sectional studies of human growth led him to conclude that weight increases as the square of the height. In 1972 the Quetelet Index was renamed to Body Mass Index or simply "BMI".
Okay, Quetelet was smart guy. He is greatly renowned for his application of comparative statistics to social conditions. As matter of fact, Adolphe Quetelet is consider the father of social sciences.
As a mathematician myself, I am trying to get my head around an 184 year-old index that was developed by a study of human growth that is now applied to adults that have stopped growing!
Part 2: BMI Tables - Making Dubious Mathematics Work
Mathematicians are really good humored people and have a saying: "Never let reality interfere with a good theory!" So what happens is that you come up with a way to make the theory appear to work. Hence, the creation of BMI scoring interpretation!
Below 18.5 Underweight
18.5 to 24.9 Normal
25.0 to 29.9 Overweight
Over 30 Obese
Over 35 Severely Obese
Over 40 Morbidly Obese
Over 50 Super Obese
There are some assumptions built into these definitions:
Assumption 1: These BMI classifications apply to sedentary people.
Assumption 2: We are all sedentary.
Assumption 3: BMI predicts fat percentage.
BMI is often inaccurate when applied to an individual. I will get into that later. However, when trying to assess the health of a population, BMI works great because that how Adolphe Quetelet developed the calculation in the first place!
In the United States, Colorado has the lowest obesity rate of 21%. Mississippi has the highest obesity rate at 35.1%. BTW, I don't think Colorado has much to crow about because, over the years, obesity rates have been climbing, even in the relatively "lean" states.
Part 3: Why is BMI Stupid?
Why does a particular measurement, such as BMI, apply to a population but not to a single individual? The particular phenomenon is called "The Law of Large Numbers", which means a large sample will converge to the mean, which is also called "average".
A few samples give you highly variable results. A sample size of one individual, will be all over the map.
Real obesity is a bad thing. Too much fat puts a strain on your heart and other vital organs. Here is a list of obesity related problems:
Heart disease and stroke
High blood pressure
Gallbladder disease and gallstones
Breathing problems, such as sleep apnea
This is pretty grim stuff and comes from the National Institute of Health. However, BMI can be misleading. As a result, health practitioners have developed ways to determine body fat percentage.
For men, a waist size of 40+ inches is an indicator of too much belly fat. For women, that number is 35+ inches. Fat measurements, such as calipers, electric impedance, water weighing and x-rays, are more accurate than BMI.
Part 4: Personal Experience with BMI
My company provides, at no expense to employees, certified health and wellness coaches.
There is a yearly health screening that measures blood sugar, heart rate, blood pressure, cholesterol, triglycerides, waist diameter, height and weight. Of course, the height and weight is used to determine BMI. If your BMI indicates you are overweight or obese, you don't pass. If you don' pass any test, you have to visit a health coach to get your health insurance discount.
I failed the BMI calculation. I am 5' 11" and weigh 210 pounds. My BMI is 29, which is overweight and borderline obese.
What about other measurements? I have a 36-inch waist so I don't have much belly fat. But most importantly, my total body fat is 17 percent! At my age of over 55, that puts me in solidly in the "athlete" range. As a matter of fact, my percentage of body fat puts me in the "athlete" range for someone 15 years younger than me.
But because I failed the BMI calculation, I have to visit the health coach once a month. My health coach is a nice young lady that genuinely cares about other people. Visiting my health coach once a month was a nice break from my normal hectic day.
I am just afraid, as a society, we are overly dependent on a 184 year-old mathematical formula that has the great virtue of being easy to calculate before the advent of digital computers.
End of Rant!