When BMI Doesn’t Quite Measure Up
Last month, we did a “Friday Mythbusters” edition of 10almonds, tackling many of the misconceptions surrounding obesity. Amongst them, we took a brief look at the usefulness (or lack thereof) of the Body Mass Index (BMI) scale of weight-related health for individuals. By popular subscriber request, we’re now going to dive a little deeper into that today!
The wrong tool for the job
BMI was developed as a tool to look at large-scale demographic trends, stemming from a population study of white European men, who were for the purpose of the study (the widescale health of the working class in that geographic area in that era), considered a reasonable default demographic.
In other words: as a system, it’s now being used in a way it was never made for, and the results of that misappropriation of an epidemiological tool for individual health are predictably unhelpful.
If you want to know yours…
Here’s the magic formula for calculating your BMI:
- Metric: divide your weight in kilograms by your height in square meters
- Imperial: divide your weight in pounds by your height in square inches and then multiply by 703
“What if my height doesn’t come in square meters or square inches, because it’s a height, not an area?”
We know. Take your height and square it anyway. If this seems convoluted and arbitrary, yes, it is.
But!
While on the one hand it’s convoluted and arbitrary… On the other hand, it’s also a gross oversimplification. So, yay for the worst of both worlds?
If you don’t want to grab a calculator, here’s a quick online tool to calculate it for you.
So, how did you score?
According to the CDC, a BMI score…
- Under 18.5 is underweight
- 18.5 to 24.9 is normal
- 25 to 29.9 is overweight
- 30 and over is obese
And, if we’re looking at a representative sample of the population, where the representation is average white European men of working age, that’s not a bad general rule of thumb.
For the rest of us, not so representative
BMI is a great and accurate tool as a rule of thumb, except for…
Women
An easily forgotten demographic, due to being a mere 51% of the world’s population, women generally have a higher percentage of body fat than men, and this throws out BMI’s usefulness.
If pregnant or nursing
A much higher body weight and body fat percentage—note that these are two things, not one. Some of the extra weight will be fat to nourish the baby; some will be water weight, and if pregnant, some will be the baby (or babies!). BMI neither knows nor cares about any of these things. And, this is a big deal, because BMI gets used by healthcare providers to judge health risks and guide medical advice.
People under the age of 16 or over the age of 65
Not only do people below and above those ages (respectively) tend to be shorter—which throws out the calculations and mean health risks may increase before the BMI qualifies as overweight—but also:
- BMI under 23 in people over the age of 65 is associated with a higher health risk
- A meta-analysis showed that a BMI of 27 was the best in terms of decreased mortality risk for the over-65 age group
This obviously flies in the face of conventional standards regards BMI—as you’ll recall from the BMI brackets we listed above.
Read the science: BMI and all-cause mortality in older adults: a meta-analysis
Athletic people
A demographic often described in scientific literature as “athletes”, but that can be misleading. When we say “athletes”, what comes to mind? Probably Olympians, or other professional sportspeople.
But also athletic, when it comes to body composition, are such people as fitness enthusiasts and manual laborers. Which makes for a lot more people affected by this!
Athletic people tend to have more lean muscle mass (muscle weighs more than fat), and heavier bones (can’t build strong muscles on weak bones, so the bones get stronger too, which means denser)… But that lean muscle mass can actually increase metabolism and help ward off many of the very same things that BMI is used as a risk indicator for (e.g. heart disease, and diabetes). So people in this category will actually be at lower risk, while (by BMI) getting told they are at higher risk.
If not white
Physical characteristics of race can vary by more than skin color, relevant considerations in this case include, for example:
- Black people, on average, not only have more lean muscle mass and less fat than white people, but also, have completely different risk factors for diseases such as diabetes.
- Asian people, on average, are shorter than white people, and as such may see increased health risks before BMI qualifies as overweight.
- Hispanic people, on average, again have different physical characteristics that throw out the results, in a manner that would need lower cutoffs to be even as “useful” as it is for white people.
Further reading on this: BMI and the BIPOC Community
In summary:
If you’re an average white European working-age man, BMI can sometimes be a useful general guide. If however you fall into one or more of the above categories, it is likely to be inaccurate at best, if not outright telling the opposite of the truth.
What’s more useful, then?
For heart disease risk and diabetes risk both, waist circumference is a much more universally reliable indicator. And since those two things tend to affect a lot of other health risks, it becomes an excellent starting point for being aware of many aspects of health.
Pregnancy will still throw off waist circumference a little (measure below the bump, not around it!), but it will nevertheless be more helpful than BMI even then, as it becomes necessary to just increase the numbers a little, according to gestational month and any confounding factors e.g. twins, triplets, etc. Ask your obstetrician about this, as it’s beyond the scope of today’s newsletter!
As to what’s considered a risk:
- Waist circumference of more than 35 inches for women
- Waist circumference of more than 40 inches for men
These numbers are considered applicable across demographics of age, sex, ethnicity, and lifestyle.
