Why BMI Is An Unhelpful (And Potentially Harmful) Measure Of Women's Health

I meet Susan at the first follow-up appointment after her mastectomy. She's a youthful 50, very blonde, and slender; she's traveled to see me from Connecticut, where she works in fashion retail at a high-end boutique where both customers and employees all look a bit like Gwyneth Paltrow.

Because Susan learned early on to equate good health with a smaller body, a lower number on the scale, she has never exercised outside of occasional walking; when it's cold outside, she sometimes does cardio on the elliptical machine. She has never lifted weights. She has always worried about being thinner. She avoids most physical activity, including cycling, because she's afraid it will make her thighs big. When I tell her she needs weight training to improve her bone density, she panics: She doesn't know how. She's afraid she'll bulk up. She read somewhere that lifting weights would increase her risk of lymphedema.

Susan is the living embodiment of all the ways in which the medical system fails to address women's muscular health. All of the information she should have known, information that is even more essential to her healing now, after a life-altering surgery, is information no doctor ever told her. And the result of having been deprived of this information is something worse than simply not knowing: Everything she believes about exercise is either needlessly terrifying, woefully inaccurate, or both.

During my first pregnancy in 2010, my doctor gave me a series of stern and frightening warnings about the dangers of physical activity. I was told never to elevate my heart rate above 140 beats per minute, the equivalent of a brisk walk. Lifting weights was completely out of the question. Raising my body temperature too high—in other words, working out hard enough to sweat—was also forbidden. All of this was in spite of the fact that I had always exercised vigorously and always found it beneficial not only to my physical health but my mental well-being.

My body, it was understood, was no longer mine; the energy I spent on exercising was energy I wasn't giving to the baby growing inside me. (Meanwhile, after I had given birth, I was praised for returning to my pre-pregnancy weight as fast as humanly possible, without a word from my doctor about preserving muscle mass or the injuries I sustained as a result of neglecting it.)

How different is this, really, from the Victorian edict that riding a bicycle would render a woman unmarriageable, infertile, insane? How different are the contemporary anxieties surrounding women exercising in pregnancy from those that gripped Dr. A.C. Simonton when he asked fearfully, "Will the female rider throw aside her wheel long enough to have a baby?"

Today, many of these misguided ideas—the Victorian plague of bicycle face, the disembodied uterus left behind on a ski slope—have long since been corrected in both medical and mainstream understanding. What lingers on is the myth that women and muscular development don't mix, which can manifest in everything from school athletics to medical research to the BMI chart hanging in your doctor's office.

But in recent years, doctors have at last begun to rethink using thinness as a stand-in for health and to reexamine the diagnostic tools they use to determine whether or not a patient has a healthy body composition. The body mass index (BMI) chart that compares height and weight to determine whether a patient is underweight, normal, overweight, or obese has been around for a long time, but its use in medical settings often overlooks as much as it reveals, particularly for women.

Neil Iyengar, M.D., a medical oncologist at Memorial Sloan Kettering Cancer Center, tells me that the BMI chart, which was developed in the 1830s, was never intended as a substitute for a more in-depth assessment of an individual patient's health: "BMI was originally developed as an epidemiologic tool," he says. "So at a population level, it is useful for understanding disease patterns—but we also have to remember the bias through which research has been conducted, which is essentially focused on men and male diseases."

Women, on the other hand, carry more fat but also carry it differently than men do1, which makes the BMI chart a markedly ineffective measure of what's happening inside their bodies. Consider two hypothetical patients: on one hand, a woman with healthy body fat levels and above-average muscle mass; on the other, a woman with elevated body fat levels but so little muscle that she can barely lift a 10-pound bag of groceries. It's the latter patient who is actually at elevated risk of multiple medical problems, but it's the former who will register on the BMI scale as overweight—and who will be wrongly instructed by physicians that she needs to be thinner.

Iyengar says, "To this day, almost every single weight loss intervention in women is geared toward body weight loss and achieving that skinny ideal. And we're learning that that is not always the best approach, especially with some of the new fad diets like fasting, for example. You lose fat, but you'll lose muscle as well."

For many reasons, this is a damaging paradigm and one that is fundamentally disempowering to women who have been systematically misled as to what a healthy body should look like. Doctors like Iyengar are working to help patients understand that maintaining a certain amount of muscle mass, even if it means sitting at a higher weight, is far more important to overall health than keeping the number on the scale low. Resistance training that builds muscle will make the body far more resilient than the calorie-burning cardio exercise to which most women gravitate.

This is especially true after menopause, when weight-bearing exercises become increasingly important for helping to maintain bone density and when maintaining a healthy body composition is essential to lowering the risk of certain diseases. In 2018, a study found that women with a normal BMI but elevated levels of body fat—women like the latter patient in the hypothetical above—were at nearly double the risk of developing breast cancer2. These are also the patients who would most benefit from increasing their level of exercise, as the dangerous combination of a thin physique but a high body fat percentage is almost invariably associated with a lack of physical activity.

This is where the medical establishment still needs to catch up: These women are the least likely to be advised to exercise and most likely to have their risk overlooked, simply because they don't "look" unhealthy.