I'd really like to hear you elaborate on this a little bit.
First of all on your own situation, if you are as you describe yourself you are either very muscular or you are obese.
Secondly as to BMI in general. I'm not sure of a population to whom BMI cannot be applied other than the very muscular. Obviously we as clinicians and people with common sense can look at the NFL running back stacked with muscle and say, "that person is neither out of shape or obese." But even among active, athletic adults very few people are going to have enough muscle mass to meaningful skew BMI measurements.
To put it another way, I think if you took a cross section of the population with BMI>30 you would find less than 5% and probably less than 1% for whom an obese level BMI was a false positive.
ok, so yes i am very muscular and that throws off my bmi, big time.
what i am getting at is that, while not in the majority, people like me, and even people that workout enough to have dropped bf% and increased lean tissue (muscle and bone), will have a skewed bmi. so if you walk into a serious gym with serious members, whether that be a weight gym, basketball gym, martial arts gym, gymnastics, etc, and took bmis on everybody, they would be artificially high.
what i meant by dillution factor, is that i don't know what to name the phenomenon that occurs when you measure LARGE populations of people, and you get a bmi average that seems to work fairly well. it seems that the muscular people that measure high, and the anorexic or "skinny fat" people that measure low, may simultaneously be corrected by both cancelling each other out and by the rest of the population.
whether this "dillution factor" is true or not, i dont know.
what i do know is that, for a year, i was one of the researchers on a project dealing with body composition. this project was funded by GE and we used the newest generation DEXA scanning machines plus, hydrostatic weighing, plethysmography, bioelectrical impedance (and not those stupid hand-held or step on scales either), anthropometric measurements including caliper skinfold measurements, as well as waist/hip ratio and body mass index. i participated in measuring close to n=1000 participants from a huge spectrum of the population: old people, young, middle aged, male, female, fat and fit, plus a sub-population of competetive athletes.
besides the fact that plethysmography sucks really badly (those machines are so tempermental), body mass index was the winner of being the measurement that was the least consistent. and i'm sure that i don't have to say it was the most prominent among the athletes and the fit people. it was also off in some of the old people with low bone mineral density and low muscle mass (skinny fat), but of course they measured low bmis as compared to their body fat percentage.
the project should be totally finished in the next year or so and published as a series of posters before the journal articles. seing as how the main objective of the research was to set bodyfat norms for DEXA, the fact that bmi kind of sucks may not be a huge part of the discussion - although, i'm sure that it will be mentioned that bmi is an outdated method of measuring body composition and that it can be very inaccurate in quite a few circumstances.
furthermore, i was a professional trainer for many years before med school, and i performed lots and lots of anthropometric bodyfat measurements on many of my clients. also, my undergraduate major was kinesiology and we had a really nice human physiology lab, where i was one of the leaders of the fitness assessment program where we did fitness tests (including bmi and hydrostatic weighing) on fit and unfit students, faculty, alumni, and even community residents.
so, i also saw, first hand, the inaccuracy of bmi in tons of people that i either personally measured, or supervised the measurement of, in these situations. bottom line is that although bmi can be right, it can also miss the mark in many people. the problem is that, unless you have the person in front of you in their underwear so you can really tell their body morphology (and even then you can't tell for sure about their bone density), you can't tell for
which people the bmi will be wrong. sometimes way wrong. and if you DO happen to have a person in front of you in their underwear, so you can see their morphology, why the heck do you need bmi to tell you what you can already tell by looking at them???
this is why i just usually shrug my shoulders and say, 'whatever' when people talk about bmi as a way of measuring body comp. i think it pretty much sucks, since it can be off in more people than you give it credit for.
of course if there is a usmle question talking about a 45 year old smoker with a bmi of 40, i know that they're not talking about a bodybuilder, so i'll assume she's obesity class III. eventually (probably many years), we'll get rid of this crappy measurement and use body fat%. i hope.
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