Yes, BMI Is An Outdated System—But Here’s Why We Still Use It

Published: July 8, 2021Updated: March 25, 2022
Does weight affect birth control pills? Why we still use BMI

As the Health At Every Size movement becomes more popular, body mass index (or BMI) has come under the microscope for not being an accurate reflection of a person’s health. 

So if that’s the case, why does the medical community—and SimpleHealth, in particular—still use it when recommending certain medications? And does weight affect birth control pills at all? We’ll explain it all below.

First, what is BMI?

BMI is a way of estimating body fat based on height and weight. Basically, it’s a person’s weight in kilograms divided by the square of height in meters. 

Here’s what most people get wrong about BMI: it’s not a way to precisely measure body fat (skinfold calipers, for example, are way more accurate). For many people, weight doesn’t necessarily correlate to body fat percentage—after all, consider an NFL linebacker who’s 6’2” and weighs 216 pounds. Their BMI would put them in the overweight range, not realizing that they’re probably all muscle and very little fat. 

But, BMI is *moderately* correlated with more direct measures of body fat. Plus, BMI appears to be as strongly correlated with co-morbidities (like heart disease) as are other, more direct measures of body fat. 

What are the advantages of measuring BMI?

Put bluntly, measuring BMI is cheap and easy. Anyone with a calculator can do it, and there’s no intense, expensive machinery required (like there would be for other ways to measure body fat, like hydrostatic weighing). 

It’s also been used for a really long time—since the 1830s, in fact. People understand it, and it’s been helpful as a screening tool. For example, someone whose BMI might place them in the “overweight” category could qualify for further testing for heart disease, diabetes, or other potential risks.

What are the disadvantages of measuring BMI?

Whew, there are a LOT. The most obvious one is that BMI measures excess weight, not excess fat—but most people interpret it as the latter. Plus, like we touched on before, muscle weighs more than fat. So, BMI isn’t accurate in people with lots of muscle. 

BMI also oversimplifies how we view a person’s health. Age, sex assigned at birth, race, and other variables should all be taken into consideration when screening a person for potential health risks. For example, BMI can underestimate the body fat of older individuals and others who have lost muscle mass. 

And as for reproductive health, BMI doesn’t account for whether a person is pregnant, post-partum, or breastfeeding—all of which affect weight.

Generally speaking, most people will not fall into these extreme categories. So despite BMI’s shortcomings, it is still a reliable tool for most people. 

So, why do we still use BMI when prescribing birth control?

While BMI clearly has its limitations, we still use it at SimpleHealth. It’s just one of the many indicators we take into account when finding the right birth control fit for you. Here’s more about how we use BMI when prescribing birth control.

To help our medical team make the best clinical decisions regarding your contraceptive medications, we collect your height and weight measurements to calculate your BMI. Bodies in various BMI ranges experience a number of physiologic differences that have the potential to affect how contraceptive medications are absorbed and metabolized. In that sense, we want to use the latest research about how weight affects birth control pills. Changes to these processes may affect the efficacy of the medication and simultaneously have a negative impact on your bodily health. 

No matter what your BMI is or what your complete medical history looks like, you can count on getting a totally personalized consultation with our doctors at SimpleHealth. We know how unique everyone’s reproductive health journey is, and our doctors make sure to take your *whole* life, medical history, and personal preferences into account when finding a birth control option that works for you. Birth control isn’t one-size-fits-all, and with our thorough, personalized consultations, you’ll be able to work with our care team to take control over your reproductive health.

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