If you really want to know whether you are obese, just undress and look at yourself in the mirror. Don’t worry about our fancy laboratory measurements; you’ll know!
—Dr. Ancel Keys, 20th century American physiologist/biologist
What is BMI?
Body Mass Index (BMI), simply put, is a weight-to-height ratio.
The formula is weight in kilograms divided by the square height in meters.
(As such, BMI does NOT directly measure body fat.)
In and of itself, BMI is a completely objective and quantifiable measurement.
It tells one the proportion of one’s weight to height. Boom. That’s it.
So why all the hullabaloo surrounding BMI?
Once we assign certain numbers to predetermined categories–once we interpret the data–that’s where those sneaky averages start to come into play.
[Want to know where I stand on comparing ourselves to the “average”? Check out last week’s post here.]
Okay, so BMI does not directly measure body fat; however, it correlates moderately well as a predictor of risks associated with body fat.
The premise is the higher the BMI, the higher the risk of higher body fat.
And from there, the higher the body fat, the higher the risk for __________.
You can fill in that blank with a very, very large number of diseases and illnesses and reasons for shortened life expectancy and lowered quality of life.
But let’s back up….
What are all of the sources of “weight” on our bodies?
These fall into two categories: body fat and lean body mass.
Body fat = body fat
Lean body mass = everything else (bones, organs, muscles)
So if we’ve got a high BMI, it’s because our ratio of weight-to-height strongly favors weight. Our weight per centimeter is hefty. We are dense.
It could be due to body fat or it could be due to lean body mass OR possibly both.
(Assuming we don’t have abnormally heavy, burly organs, we can make the statement a little tidier and just say, that a high BMI could be due to body fat or due to muscle mass.)
In looking at ourselves in a mirror, we could probably figure it out.
If we look around at the general population (eek, now looking at typical/average/standard), we can see that for most people, it’s due to body fat.
In fact, research shows that same “norm.”
All of this to say…
BMI is utilized because it is a super-simple and efficient, time- and money-wise, method of screening in order to assess risk associated with fatness.
The assumption is that an excess of weight (the reason for one’s weight to height ratio to be high) will usually be caused by an excess of body fat. Generally, I think we can agree that it is the most likely scenario–as compared to an excess of weight being caused by significant amounts of muscle mass.
So, we take it for what it is. From this perspective, we need to be conscious to not rely on the BMI too much–or to consider it anything but merely one tool–in attempting to draw any sort of conclusion about our health.
If we look at the BMI chart critically, it doesn’t take a physiologist to raise some concerns. [In fact, as evidenced by the quote above, one of the first physiologists to begin applying the BMI in this way specifically states that laboratory science is likely unnecessarily to determine your own state of leanness.]
This chart is intended to apply to all adults, regardless of gender, aged 20 and above. Imagine a 215-lb. college athlete and compare him to a 215-lb. great-grandfather. As long as they are similar in height, they will have the same “weight status.” They may even have the same BMI.
(Pause for laughter.)
There are undeniably problems with using BMI as the primary tool for diagnosis of conditions that are linked to the scientist-created categories of overweight or obese. But it really never is used as the primary tool. It’s a screening tool.
The qualifications for overweight and obese, by the way, are somewhat arbitrary (though obviously research-based) in that scientists have assigned those titles to certain sets of characteristics. Even if we look at the objective numbers for body fat percentage, clearly there is a continuum of unhealthy to healthy to unhealthy as we move from body fat percentages of zero to 100. You’re not perfectly healthy and at low risk of premature death at 30% and, after a large meal, suddenly unhealthy and at high risk.
The main drawback of using BMI as an indicator of risk factors is that it has been standardized to apply to too wide of a population. A simple ratio cannot accurately apply to males, females, athletes, moms, grandmas, Africans, Asians, Latinos, those with debilitating illness, those with disordered eating, those wasting away due to old age, etc.
Does BMI matter?
If you use BMI to monitor your ratio and you compare it to your ratio over time, etc., then it’s a fine tool.
If you use your BMI to estimate your body fatness, you should pair it with another measure from time to time to ensure that the reading is accurate enough for your usage.
If you use your BMI to identify your category of “weight status,” then I caution you to take the values only as what they are. Also use the mirror. And how you feel. And how healthy your lifestyle is.
What is BMI NOT?
I think an equally important question to ask is, “what is BMI not?”
BMI is NOT…
- the end-all, be-all of one’s health outlook
- the only measure that should be considered to determine risk associated with fatness
- anything more than a quick tool to estimate your general leanness
- necessarily an indicator of body fat
BMI is also NOT as accurate of an leanness measure for certain populations. There are pretty notable limitations of application depending on age (as in the example above), as well as ethnicity. (For instance, many of the risks associated with body fatness “kick-in” at a lower BMI for Asians than would be expected based solely on the standard interpretation of the BMI.)
Ya know DOES appear to be a very strong indicator of health (longevity and quality of life)? Actual amount of muscle mass! I’m thinking I might keep train rolling into next week…
Want to burn fat and work towards a debatably healthier BMI?
40s HARD, 20s REST for 5 minutes:
Sprawl outs (like a burpee with no push-up),
Squat jacks (like a jumping jack but landing in & jumping from a squat).
P.S. To pick up where I left off last week: Do not aim to have an average BMI. Aim to have health. An average BMI, if using the norm of American adults, is unhealthy; so if you compare yourself to average and give yourself a pat on the back, you are doing yourself a disservice. Instead, determine healthy for you, and aim for that relentlessly.
P.P.S. If you want to be sure your high BMI is only due to muscle mass and not to body fatness, you’ll probably want to check out Pat’s Inner Circle. I’ve heard it’s good for that.