Topic: The (Lack of) Science behind the Body Mass Index (BMI)

[Pandora]

Are you fat? The answer may depend on which test you take.

If you’re going by your body mass index, or BMI, a measure that factors in your weight and height, you are considered overweight if that score is 25 to 29, and obese if it’s 30 or higher. But a surprising new study finds that some people with a BMI pushing 28 actually have little body fat — and some folks with a BMI as low as 24 have too much.

The results question the validity of BMI, the most common measure for determining who needs to shed some pounds, says study author James Pivarnik, a professor of kinesiology and epidemiology at Michigan State University in East Lansing.

“If you’re going to classify a person as overweight by BMI, depending on who you’re working with, that may not be the best way to do it,” he says.

While prior research has found that BMI isn’t always an accurate indicator of fatness in athletes, who may be more muscle than fat, the new study is one of the first to show that BMI may not necessarily work for the general population either.

Other experts say they’ve seen this firsthand with clients, and that clearly BMI isn’t the best test for everyone. “I don’t think it’s accurate enough,” says Dr. Kenneth Cooper, founder of the Cooper Aerobics Center in Dallas.

Pinch an inch?
At his facility, trainers prefer to rely on skin-fold tests that use fat calipers to measure body fat at various points, such as the back of the arm, abdomen and thighs. In addition, they use underwater weighing, a common lab test that determines how much of a person’s body is fat and how much is muscle. Other centers also use a measure called the waist-to-hip ratio , which assesses abdominal fat. Some fat is worse than others, and that around the middle is among the deadliest.

But researchers studying large populations of people rely on BMI because it’s an easy figure to calculate — they just ask people how much they weigh and how tall they are, and then do the math (weight in kilograms divided by height in meters squared). Averaged across many people, BMI is a good indicator of morbid obesity, Pivarnik says. People with a BMI pushing 40, for instance, are bound to be carrying too much around the middle, and elsewhere.

But when you look at certain individuals, BMI may be way off the mark.

In the new study, published in Medicine & Science in Sports & Exercise, the researchers calculated both BMI and body fat percentage for 439 college students.

To determine body fat, they used a BOD POD — a laboratory test that mimics underwater weighing but requires subjects to sit in a dry chamber rather than getting in water.

Study results showed that male and female college athletes, as well as male non-athletes, could have a BMI suggesting they were overweight yet still have healthy levels of body fat, defined as less than 20 percent fat in men and 33 percent in women. On the other hand, non-athlete women with a BMI indicating a normal weight could have too much body fat.

Pivarnik says large amounts of heavy muscle mass in the athletes accounts for the higher BMI, yet the athletes had low body fat because they were in shape. Even young non-athlete men could be muscular and fit yet not overly fat.

For women, the study shows, thin isn’t everything. Those who were slim yet didn’t work out to build muscle still could be quite fatty.

Pivarnik says he worries that some people, particularly young women, may find that pumping iron puts them into the overweight category per BMI, so they skip weight training altogether.

“Don’t worry about the thinness,” he says. “Worry about the working out part.”

BMI formula was derived by a Belgian "between 1830 and 1850".

... the inventor of BMI is the Belgian (Flemish) mathematician Adolphe Quetelet (1796-1874). I also found several biographies of his on the internet (see list at the end of this article). Interestingly, the BMI formula did not become popular amongst physicians until well after 1970.

What is the scientific basis for the formula? We have to go back to Quatelet and read that "in 1846 he published a book on probability and social science that demonstrated as diverse a collection of human measurements as the heights of French conscripts and the chest circumferences of Scottish soldiers could be taken as approximately normally distributed." (sites 2 and 3 listed below). Therefore the most likely explanation is that both the BMI and the linear expression of Equ. (2) were derived as a gross approximation to a set of observed weights and heights of recruits in a Western European army.

...

I find very disturbing to use a statistical measure derived over 150 years ago, most likely, from a group of young men to determine the proper weight for men and women of any age. There have been several publication critical of the BMI (see below) and if your doctor follows the literature, she/he may not mention BMI at all.

There is another disturbing fact in Equ. (1). Why is the weight expressed as the square of the height? Elementary solid geometry tells us that the volume of an object is equal to the third power of its linear dimensions, so the weight should be proportional of the cube of the height. However strength is proportional to the cross section of your arms and legs, so it is proportional to the square of the height. As a result, the ratio of strength over volume is declining function of height and that is why insects are far more mobile than dogs and dogs are more mobile than humans. Because for any given person height is determined by genetics and childhood nutrition, it must be taken as a given. Then the formula for proper weight must contain both a cubic term (to account for the volume) and a quadratic term (to account for the need for strength). I have not seen any such formula, but using the BMI expression because we do not have an accurate expression does not seem to be the smart thing to do.

There have been articles pointing out that BMI gives too low weights for tall people but the public is not aware of them. Another flaw of the formula (also recognized in the literature) is that it does not distinguish between weight due to muscle and weight due to fat. Again that knowledge is not widely available. If you have the patience to read the Wikipedia article to the end you will find a good critique of the BMI in the section "Limitations and shortcomings" where there are also citations to the literature.

...

There are several postings that provide critiques to the BMI index. Some of them are listed below in no particular order.

    BMI: A critique of its use in human biology and the health professions by William D. Ross and Ottó G. Eiben, 2002. Quote from the abstract: "We contend that the common BMI scale for men and women to ascribe a health weight range is a mathematical artifact."

As I said, the best "nonprofessional"  ::snort::  method for "calculating" a proper weight is your scale and your mirror.

[IronDioPriest]

I have known that the BMI is bullspit from the moment I was measured by it. According to the BMI my ideal weight is 175. But I am comfortable, and look and feel healthy when I weigh 200-205. I could not achieve 175 without losing all my fat and allowing muscle mass to atrophy. No possible way. Several years ago I worked out and watched my diet like a madman for several months and basically burned away all my fat. I got down to 190. My wife thinks I am unattractively skinny at 190, and my body did NOT want to stay at that weight. It wasn't a natural balance for me.

These days (turned 49 today), unless I watch what I eat and work out regularly, I have a tendency to gain fat. I've flirted with letting diet and exercise lapse for extended periods a couple different times since I turned 40, and it hardly takes anything to pack on some pounds. But when I'm watching my diet and working out, and my metabolism is humming on all cylinders (for my age, of course), 200-205 is right where I land, and that's where I look and feel my best. BMI be damned.

[Pandora]

I have known that the BMI is bullspit from the moment I was measured by it. According to the BMI my ideal weight is 175. But I am comfortable, and look and feel healthy when I weigh 200-205. I could not achieve 175 without losing all my fat and allowing muscle mass to atrophy. No possible way. Several years ago I worked out and watched my diet like a madman for several months and basically burned away all my fat. I got down to 190. My wife thinks I am unattractively skinny at 190, and my body did NOT want to stay at that weight. It wasn't a natural balance for me.

These days (turned 49 today), unless I watch what I eat and work out regularly, I have a tendency to gain fat. I've flirted with letting diet and exercise lapse for extended periods a couple different times since I turned 40, and it hardly takes anything to pack on some pounds. But when I'm watching my diet and working out, and my metabolism is humming on all cylinders (for my age, of course), 200-205 is right where I land, and that's where I look and feel my best. BMI be damned.

49?!  Baby-face! 

Seriously, IDP, Happy Birthday and many more.

A few local forum posters live and die by whatever the government shovels at 'em and the bullspit BMI was just another thing they're determined to swallow whole.  So, I went out of my way to find the debunk and thought y'all might like to see it as well.

When I was weight-lifting seriously -- 10/12 hours a week -- I weighed 98 lbs, but it was mostly muscle.  I still weigh 98 lbs, but the muscle-fat ratio has switched.  I really need to get on that.

[Predator Don]

I'm 5'9". If I go by what some machine tells me, to have the proper BMI I need to weigh 155 lbs. I look like a ghost at 155. I feel better and much stronger at 175.

[Libertas]

I'm not dropping 40 pounds or gaining a foot of height so to heck with 'em!

[LadyVirginia]

Happy Birthday, IDP!
 

[Libertas]

"turned 49 today"

Happy Birthday...youngster!

 

[LadyVirginia]

I forgot to ask, IDP, how many times have you turned 49?

[Pandora]

I forgot to ask, IDP, how many times have you turned 49?

 

*ahem*

[charlesoakwood]

"turned 49 today"

Happy Birthday...youngster!

 

!

[IronDioPriest]

Thanks guys! Had a nice evening with the family. Wife and I were supposed to go to the caucus together, but my daughter and her family showed up late for dinner, so I had to choose, and chose to make it a family evening.

*sigh* I guess I have one good year of life left worth living, then hit the big five-oh. Anyone have any tips on how to deal with being a senior citizen, and facing the imminent end of life? I've got a year to figger it out.

 

[charlesoakwood]

The sun still comes up in the east and you are still you and fifty is just a number.
Save your money, buy land, love and enjoy your family.

[Libertas]

I thought 40 was more significant than 50, hell I know I'm not making it to 100, so if I get into the 80's I'll call that a good run!

Mile markers, they are what they are.

And like CO says, what we choose to do with the time we have is more important than the actual amount we have allotted!

 

[ToddF]

Happy Birthday...youngster old fart!

Fixed/ 

[Pandora]

Thanks guys! Had a nice evening with the family. Wife and I were supposed to go to the caucus together, but my daughter and her family showed up late for dinner, so I had to choose, and chose to make it a family evening.

*sigh* I guess I have one good year of life left worth living, then hit the big five-oh. Anyone have any tips on how to deal with being a senior citizen, and facing the imminent end of life? I've got a year to figger it out.

 

O rly.      

Tell ya what, just off yerself in about six months and avoid the next-birthday rush.   

Glad to hear you had a nice family dinner this year.  Too bad the next birthday will render you so far into antiquity you won't be able to enjoy it.

[Sectionhand]

At the tender age of 60 I'm 6' 1 1/2" and weigh in at 175 lbs. I'm okay with where I am . I was an athlete in high school and college but .... exercise now ? I never stand when I can sit and never sit when I can lie down . 

Happy Birthday , IDP