Weight is a funny thing. It seems that experts
can never agree on what constitutes the "right" weight. Should we eat a
low calorie diet and be a little underweight? Should we be a little overweight?
Who decides what is overweight or underweight anyway? One of the sticking
points in these conversations is the
Body Mass Index, more commonly known as BMI.
I recently told a friend that I was trying to
reach a healthy BMI. He immediately went off on how BMI is a scheme concocted
by capitalism and physicians to scare the American populace into losing weight,
and how I should not even try to reach a healthy BMI because I might kill
myself in the process. Now, I don't put much stock in such an extreme reaction,
especially since this same person told me I couldn't be "serious"
about losing weight without restricting my exercise to only cardio and cutting
out certain "bad" foods. I'm certainly trying to exercise and eat
healthier for better reasons than to reach a number on a scale. But it did get
me wondering: how good of a goal is a healthy BMI?
BMI is a measurement calculated by taking a
weight (in kilograms) and dividing it by a height (in meters) squared. It was
originally designed in the mid-1800s, and first started to be used as a proxy
for body fat percentage in the 1970s. Despite warning that BMI should only be
used on a population level and not to determine any one individual's health, it
is currently used by insurance companies and physicians as a diagnostic tool
for personal obesity. In the average person, BMI can aid physicians in determining who may be at increased risk. Anything below 18.5 is considered
underweight, while 18.5 to 24.9 is considered normal. 25 to 29.9 makes you
overweight, and anything over 30 enters you into the "obese"
categories.
The problem with BMI is that it cannot
distinguish between fat and lean body mass. This means that it fails to mark
some people as obese who have excess body fat, while it marks some people (like
athletes) as obese who clearly do not have excess body fat. It also fails to
adjust for the extremes of height, which gives taller people a deceptively high
BMI and shorter people a deceptively low BMI. At least one physician has
altered the formula to correct for this, and you can view the comparison
between the two here.
So if BMI is so flawed, how can we use it?
A study conducted in 2007 looked at the relationship between BMI and body fat
percentage in college students: males and females, athletes and nonathletes.
They suggested altering the cutoffs for what should be "overweight"
and "obese" based on the population being examined. For example, if
you're a female athlete, your cutoff for overweight should be 27.7, but if you're
a female nonathlete your cutoff should be 24.0. Linebackers shouldn't be
considered overweight until they reach above 34.1. Clearly, there's a lot of
wiggle room.
Why not just measure body fat? Measuring body
fat percentage directly is a real chore, requiring an individual to wear
special clothing and be submerged in a water or gas chamber with trained
personnel at special sites. Slightly more intensive than hopping on a scale and
backing up against a wall. Even the body fat percentage calculated by electrical
currents or measurements with skin calipers can be way off if performed by
inexperienced people or on someone with significant obesity.
But there are other measures that can add good
information to BMI. For example, waist-to-hip ratio is a good indicator of how
much fat a person stores in the belly. Where you store your fat is just as
important as how much of it you have. More fat around the abdomen (i.e., a
higher waist-to-hip ratio) is associated with poorer health outcomes than more
fat around the hips or thighs. Your level of cardiovascular fitness is also
important, like the fastest possible speed you can run on a treadmill.
So my original question: how good of a goal is
a healthy BMI? I think no one would argue that I am an athlete, and I'm smack
dab in the middle of the curve as far as height. So none of the pitfalls of
using BMI apply to me, and I can trust that my BMI correlates with my body fat
percentage fairly well. If anything I should aim a little lower than the upper
end of "normal" given the 2007 study. For me, at least, BMI is a good
measure of progress in my attempt to get healthier, in addition to waist-to-hip
ratio and cardiovascular fitness.
What do you think about using BMI? Is it fair for physicians to
calculate BMI and potentially base treatment decisions on it? Should we be
required to obtain other complementary information to get a more complete
picture of a patient's obesity status?
I think most physicians are well aware of the pitfalls of the BMI. No doctor is telling a mostly healthy looking tall female athlete to lose weight to reach a normal BMI. Nor are they telling linebackers to lose weight.
ReplyDeleteBMI is a pretty good initial tool to judge a person's body fat content. And of course we judge a person's BMI by looking at a person as well. Trying to do actually exacting body fat measures would just be a waste of time and resources. Often the losing weight part comes with healthier habits, such as exercise and eating more fruits and vegetables. Those two can never really hurt anyone. Even if you don't need to lose weight, those two activities are great for both physical and mental well being.
And more often than not, when the treatments go further than just telling patients to lose weight, most physicians search for evidence of CV disease, diabetes, HTN, joint pain, etc.
I agree that physicians aren't telling athletes to lose weight to get a healthy BMI. But I also think my friend is a good example of the opposite being true. Many of us have skewed ideas of what obese actually looks like, and we're not great at judging a person's BMI by how they look. So I've seen physicians not mention to people (like me) when their BMIs or overweight or obese because they don't feel that person "looks" overweight, and well, there are problems with BMI, etc. Understanding what the problems actually are I think would help physicians use it more appropriately, instead of just throwing it out as a flawed measurement when they want to avoid a conversation about weight. You don't want to wait until a person's BMI is 40 and they "look" obese to start having the weight conversation.
DeleteAnd I agree exercise is important regardless of whether or not you need to lose weight. I'm actually going to talk about that next week :)
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