BMI and the Inaccurate Measuring of People
BMI is an estimate of body fat and a good gauge of your risk for diseases
that can occur with more body fat. The higher your BMI, the higher your risk
for certain diseases such as heart disease, high blood pressure, type 2
diabetes, gallstones, breathing problems, and certain cancers.
Americans keep putting on the pounds — at least according to a report released this week from the Trust for America’s Health. The study found that nearly two-thirds of states now have adult obesity rates above 25 percent.
That seems reasonable and a good place to start in getting your baseline measures as you move forward in making decisions about your health but what if this measure was wrong? What if you learned that this basic idea was based in incorrect ideas about the human body and was a leading means to both discourage good health and punish people who have the best intentions to improve themselves?
Health professionals have been using body mass index, or BMI, to help decide whether people are overweight or underweight for more than 100 years. Researchers in population studies, doctors, personal trainers, and others use the BMI in their work.
The use of body mass index to determine individual health should be scrapped immediately, Reports have long warned that obesity strategies failed to promote healthy behaviors and could be contributing to body-image pressures.
There are a myriad of reasons why BMI isn’t as accurate a measure of health as many of us were brought up to believe, but one of the main ones is that BMI can’t distinguish between fat and muscle. A pound of fat takes up a much larger volume on the body than a pound of muscle, even though they of course weigh the same. The drawbacks of using BMI is that some people are more muscular and this will make your BMI higher, even in a healthy person. Even something as benign as your framesize (this is the idea that some people are skinnier than others and some people are broader than others, the classic ‘ectomorph’ and ‘endomorph’ comparison). A broader person will have a higher BMI which will result in a higher BMI.
Gender is another pitfall of BMI. At the same BMI, women will usually have more body fat than men.
What is wrong with BMI has been the same problems that have plagued it from the start.
1. The person who dreamed up the BMI said explicitly that it could not and
should not be used to indicate the level of fatness in an individual.
The BMI was introduced in the early 19th century by a Belgian named Lambert
Adolphe Jacques Quetelet. He was a mathematician, not a physician. He produced
the formula to give a quick and easy way to measure the degree of obesity of
the general population to assist the government in allocating resources. In
other words, it is a 200-year-old hack.
2. It is scientifically nonsensical. There is no physiological reason to square a person’s height (Quetelet had
to square the height to get a formula that matched the overall data. If you
can’t fix the data, rig the formula!). Moreover, it ignores waist size, which
is a clear indicator of obesity level.
3. It is physiologically wrong. It makes no allowance for the relative proportions of bone, muscle and fat
in the body. But bone is denser than muscle and twice as dense as fat, so a
person with strong bones, good muscle tone and low fat will have a high BMI.
Thus, athletes and fit, health-conscious movie stars who work out a lot tend to
find themselves classified as overweight or even obese.
4. It gets the logic wrong. The CDC says on its Web site that “the BMI is a reliable indicator of
body fatness for people.” This is a fundamental error of logic. For
example, if I tell you my birthday present is a bicycle, you can conclude that
my present has wheels. That’s correct logic. But it does not work the other way
round. If I tell you my birthday present has wheels, you cannot conclude I got
a bicycle. I could have received a car. Because of how Quetelet came up with
it, if a person is fat or obese, he or she will have a high BMI. But as with my
birthday present, it doesn’t work the other way round. A high BMI does not mean
an individual is even overweight, let alone obese. It could mean the person is
fit and healthy, with very little fat.
5. It’s bad statistics. Because the majority of people today (and in Quetelet’s time) lead fairly
sedentary lives and are not particularly active, the formula tacitly assumes
low muscle mass and high relative fat content. It applies moderately well when
applied to such people because it was formulated by focusing on them. But it
gives exactly the wrong answer for a large and significant section of the
population, namely the lean, fit and healthy. Quetelet is also the person who
came up with the idea of “the average man.” That’s a useful concept,
but if you try to apply it to any one person, you come up with the absurdity of
a person with 2.4 children. Averages measure entire populations and often don’t
apply to individuals.
6. It is lying by scientific authority. Because the BMI is a single number between 1 and 100 (like a percentage)
that comes from a mathematical formula, it carries an air of scientific
authority. But it is mathematical snake oil.
7. It suggests there are distinct categories of underweight, ideal,
overweight and obese, with sharp boundaries that hinge on a decimal place.
8. It makes the more cynical members of society suspect that the medical
insurance industry lobbies for the continued use of the BMI to keep their
profits high. Insurance companies (which are private companies who must answer to shareholders and make the largest profits possible) sometimes charge higher premiums for people with a high
BMI. Of course they do – it is ‘science’ and it is a good excuse to squeeze more money out of consumers, which is the heart of ‘capitalism’. Among such people are all those fit individuals with good bone and muscle
and little fat, who will live long, healthy lives during which they will have
to pay those greater premiums.
9. Continued reliance on the BMI means doctors don’t feel the need to use
one of the more scientifically sound methods that are available to measure
Those alternatives cost a little bit more, but they give far more reliable
10. It embarrasses the U.S. Do I need to mention that it is embarrassing for one of the most scientifically, technologically and
medicinally advanced nations in the world to base advice on how to prevent one
of the leading causes of poor health and premature death (obesity) on a 200-year-old
numerical hack developed by a mathematician who was not even an expert in what
little was known about the human body back then.
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