In an effort to address America’s obesity problem, a
government panel recently issued guidelines calling for
routine obesity screening, including
calculating their patient’s Body Mass Index (BMI). The
BMI is a simple formula: (Weight in Pounds / [Height in inches
x Height in inches]) x 703. A score
of 18.4 or lower indicates underweight; 18.5 to 24.9 indicates
normal weight; 25 to 29.9 is overweight; and a BMI of 30 or
higher suggests obesity.
According to a
CBS News story, “A government
panel renewed a call Monday for every adult to be screened for
obesity during checkups, suggesting more physicians should be
routinely calculating their
patients’ BMIs. And when someone crosses the line into
obesity, the doctor needs to do more than mention a diet.
It's time to refer those patients for
intensive nutrition-and-fitness help, say the guidelines
issued by the U.S. Preventive Services Task Force.”
Bashing the BMI
Yet a lot of people are not impressed; in fact the BMI had
been attacked and ridiculed for years. Just how reviled is the
BMI among many the public? Do a
Google search for “BMI is…” and the top
three returns are “bullshit,” “wrong”
and “a joke.”
Why such hate for an otherwise boring, uncontroversial medical formula?
Part of the reason is that the BMI has come under
attack—not so much by medical professionals (who use it
on a fairly regular basis) but by fat acceptance
activists who believe that the BMI unfairly discriminates
against overweight people. Author and Salon.com blogger Kate
Harding, for example, created a
slideshow explaining why
BMI is badly flawed. Many other bloggers and writers have echoed her
complaints, including at Jezebel, Huffington Post, and
elsewhere.
By far the chief complaint is that BMI doesn’t give
accurate readings for everyone, and overemphasizes the role of
weight in health. An
article
provided by Men’s Health magazine for the
Huffington Post notes “The next time you happen
to catch a Minnesota Vikings game, take a look at Adrian
Peterson, the team's 6'1, 217-pound
running back. Now ask yourself: what kind of physical
characteristics would you attribute to him? Athletic? Lean?
Fit? All of these certainly sound like
valid answers to us—but his clinical classification
might surprise you. By any normal standards, Peterson is one
of the fittest men on the planet. But by
our country's system of measuring body fat, he’s
overweight.”
For many social activists and feminists the issue is not
really about a height-weight ratio formula but instead what
they believe the BMI represents:
judgment against overweight women (and men), and an attempt to
impose high beauty standards on women. As a
blogger
for Change.org asks in a piece typical
of anti-BMI pieces, “If the BMI doesn’t…take
into account any distinctions of age, race, and sex, than
perhaps its at best an outmoded, ineffective, and
ultimately demoralizing standard by which to judge a
woman’s health?”
There are several problems with this characterization. The
questions of whether the BMI is outmoded and ineffective will
be addressed shortly, but
suggesting that the measure is a “demoralizing standard
by which to judge a woman’s health” is bizarre:
The BMI is not a “standard [of] health” (for men
or
women); it is a measure of adiposity (fatness). Though there
is a strong positive correlation between excess weight and
poor health, it is quite possible
to be overweight (or even obese) and healthy. Thus the
suggestion that the BMI is a measure of health is patently
false. And any medical measure can be
“demoralizing” in some way if it does not give its
user the desired results, from a blood sugar strip to a
bathroom scale. The purpose of any weight scale
or measure is to be accurate, not to provide comforting
assurances (like the magical mirror in “Snow
White,” proclaiming that its user is the
fairest—or
thinnest—of them all).
BMI Limitations
It is absolutely true that the BMI overestimates the amount of
body fat (mistaking muscle for fat) in people like football
running backs. However the
critics gloss over the fact that most of us are not
professional football running backs; the vast majority of
people are not in categories where BMI’s
validity is skewed. The limitations of the BMI are well-known
to doctors; the measurement was developed as a general
guideline. No medical professional
would classify a patient as underweight, normal, or obese
based only on a BMI score; it’s a starting point, a
general guideline, not a strict rule that
correctly predicts overweight in everyone.
To use another medical example, doctors issue guidelines to
the public about warning signs of a stroke (including arm
weakness, speech difficulty, and face
drooping). These guidelines are not perfect (people experience
these symptoms without having a stroke), and are not a
substitute for a doctor’s medical
diagnosis. Like the BMI, the guidelines were not designed to
be—and never claimed to be—the best diagnostic
tool out there, but instead a general rule of
thumb to give the layperson a guide to their health status.
Nobody would suggest getting rid of the simple stroke
checklist because it does not correctly
diagnose stroke in everyone (and has the potential for false
positives), yet many suggest getting ride of the BMI for
exactly the same reason.
Dr.
Steven Novella, a Yale physician, has examined the BMI
“controversy”
in-depth. Writing on his
Science-Based Medicine blog, Novella
notes that “It is widely recognized and admitted that
BMI is problematic as applied to individuals. Muscular and
athletic people may have a high BMI and
not have excess adiposity, for example. Also at the extremes
of height the BMI becomes harder to interpret. But this does
not mean the BMI is useless. In
fact, for most people BMI correlates quite well with
adiposity.
In one study
researchers compared BMI to a more direct measure of body fat
percentage using skin-fold thickness. They found that when
subjects met the criterion for
obesity based upon BMI, they were truly obese by skin-fold
thickness 50-80% of the time (depending on gender and
ethnicity). When they were not obese by
BMI they were not obese by skin-fold 85-99% of the time. So
BMI is a rough but useful estimate, good for large
epidemiological studies where more elaborate
fat percentage measurements are not practical.”
Flawed Logic
For those who reject the BMI because it is imperfect, using
that logic nobody should take any medical drugs or undergo any
medical procedures or surgeries,
since they are imperfect and have varying success rates. The
usefulness of any drug or medical procedure does not depend on
whether it is completely
appropriate or useful for everyone. For example a drug
prescribed to treat high blood pressure is not completely
effective for every single patient; for
most patients it will significantly improve their
symptoms—but individuals vary, and it works better for
some people than others. This is not a logical
reason not to use them.
Another criticism is that the BMI was developed over 150 years
ago, and its longevity somehow discredits it. What would
become the BMI was developed around
1850 by a Belgian statistician named Adolphe Quatelet, and has
been used more or less since then. This is, of course, flawed
logic: if anything the fact
that the BMI has been widely used for so long is actually
evidence that it works—not that it doesn’t work.
To use only one example of many, germ theory has
been around since at least 1815 (when Agostino Bassi did
experiments showing that the etiology of disease could be
traced to germs), and no one suggests
that germ theory is “outmoded” or incorrect merely
because it’s been around for nearly 200 years.
Ironically, efforts by critics (including many feminists) to
challenge the BMI’s validity often undermine the very
research they promote. By challenging
the validity of the BMI, they are also indirectly (but
significantly) challenging the instrument validity of research
that uses BMI in its
methodology—often research they themselves cite in
support of their claims. Hundreds of studies related to
women’s body image, self-esteem, and media
exposure dating back decades have used the BMI as a measure of
weight in the study’s subjects. Some of those studies
are widely-cited and referenced,
offered as evidence, for example, that exposure to images of
thin women on television encourages eating disorders
(“Eating behaviours and attitudes
following prolonged exposure to television among ethnic Fijian
adolescent girls.” 2002. Becker A.E., Burwell R.A.,
Gilman S.E., Herzog D.B., and Hamburg P.
British Journal of Psychiatry. June; 180:509-14.).
If the BMI really is an invalid measure as often claimed, the
validity of countless studies that using that measure must be
questioned. This problem has
received little if any attention, likely because most people
promoting social agendas don’t actually read the
original studies they cite in support of
their arguments and claims. They don’t examine the
research for strengths, flaws, and important caveats but
instead skim the abstract, or rely on news
stories to tell them what the research means.
One reason that BMI is useful is that
many people do not notice weight gain. Contrary to the popular
idea that most people (especially women) weigh themselves
constantly in fear of gaining an ounce, studies show that that
a significant number of women
evaluated at six-month intervals did not recognize recent
gains in weight. Nearly one-third of women did not notice a
weight gain of approximately 4.5
pounds over a six-month time, and one-quarter of women did not
notice a weight gain of nearly 9 pounds over the same period.
Such findings concern
researchers because if people don’t realize they are
overweight they won’t make effort to lose weight.
The BMI is also useful in that it is a calculation that anyone
can make, and does not require medical knowledge nor a medical
office visit. Not everyone
has affordable access to medical care and private physicians,
and the BMI helps poor and underprivileged people determine
whether or not they are at a
healthy weight.
Many of the BMI critics’ complaints are straw man
arguments stemming from a fundamental misunderstanding of what
the BMI is and what it claims to do. It is
not a tool used by the patriarchic medical establishment for
oppressing women, nor pressuring them to conform to impossible
standards of beauty. It is
instead a generally useful, accurate guide to helping average
men and women determine their weight status.