'Just one number': The problem with BMI

Key takeaways:

• BMI should only be considered alongside other health metrics for best care, experts said.
• Measuring waist circumference is an alternative to gauge patients’ body fat.

Although body mass index has long been used in clinical practice, many have recently questioned its effectiveness as a standalone measure for obesity.

Fatima Cody Stanford, MD, MPH, MPA, MBA, FAAP, FACP, FAHA, FAMWA, FTOS, an obesity medicine physician scientist at Massachusetts General Hospital and Harvard Medical School, told Healio that BMI has been used for categorizing patients into classifications like “underweight,” “overweight” and “obese” for decades.

However, previous research has indicated that, as a simple calculation of height and weight, BMI is not comprehensive and should be considered alongside other factors.

“The problem with BMI is that it doesn't tell me anything about what your weight really is like. Do you carry more fat mass? Do you carry more lean muscle mass? Is your weight distributed more around your midsection, which predisposes you to metabolic disease?” Stanford asked. “It doesn’t give me any of that. It just gives you this one number, and we’re supposed to just make all these determinations based on this one crude number, and that’s problematic in my thought process.”

An imperfect metric

Tochi Iroku-Malize, MD, MPH, MBA, FAAFP, president of the American Academy of Family Physicians, said BMI should not “be used as a one-size-fits-all measurement” and “is not always the best way to tell if patients are at a healthy weight.”

“There are some people whose BMIs may not be a good reflection of how much body fat they have,” Iroku-Malize said. “For example, some athletes may have a more muscular build. We know that muscle weighs more than fat, so their weight may not be a good indicator of how much fat they have in their body. Additionally, older adults tend to lose muscle mass as they age, so they could have higher BMIs.”

BMI also does not account for “the many socioeconomic and equity factors that determine someone’s diet, physical activity level, and their relationship with food and wellness,” she said, including income, where one lives, health literacy, transportation access and access to grocery stores carrying fresh foods.

“BMI should only be used as a screening tool and as a starting point for a conversation with a family doctor about weight,” Iroku-Malize said.

But many patients remain focused on their numbers, Stanford said, even when they have positive results from other important metrics, like cholesterol, blood pressure and liver function tests. Stanford said patients often request target numbers to reach, asking, “’What am I supposed to be?’” because “they’ve been told they have to fit into this number.”

“Is this really by itself a great measure of how healthy [patients] are? Or have they just been told they need to conform to this number because this number is very simple and that's what we've been using to define all humans, despite the fact that it's just one number? And that's where the equity issue comes in,” Stanford said.

Equity

As a health metric, BMI is also plagued by racial and ethnic disparities. For example, a recent study found that BMI particularly under-identifies obesity among Asian and Hispanic adults, and it has been suggested that primary care physicians should consider lower BMI cutoffs when screening patients of Asian descent for diabetes and CVD risk.

In June, the AMA House of Delegates adopted a policy encouraging physicians to use BMI alongside other valid measures when assessing patients for overweight or obesity, acknowledging that the use of BMI has led to “historical harm” and “racist exclusion,” partially because of the fact that its use is based on data collection from earlier generations of white people.

Stanford said BMI’s origin dates to 1800s Belgium, when a mathematician sought to determine what was considered an average white male soldier. That index, she said, “became the basis for eugenics and the basis for BMI.”

“Notice I said 1800s, white male, Belgium. I’m a Black woman in 2023, so we’re going to say that doesn’t include me,” she said.

Farzaneh Daghigh, PhD, a professor of biochemistry and director of a culinary medicine course at the Philadelphia College of Osteopathic Medicine, said racial and ethnic disparities “are evident in the relationship between BMI and body adiposity” and might “stem from differences in body fat distribution and metabolic factors.”

“Relying solely on BMI overlooks the complex socioeconomic, environmental and genetic factors contributing to obesity, hampering the identification and addressing of the underlying causes,” she said. “Such limitations can perpetuate health disparities and stigmatization of individuals who do not fit neatly into established BMI categories.”

Daghigh stressed the importance of PCPs having knowledge of BMI’s “limitations and equity issues,” and said that addressing them “is paramount to ensure accurate and equitable health assessments and interventions.”

“[PCPs] should consider individual patient circumstances, cultural variations and the potential for misclassification due to varying body compositions,” she said. “It's crucial to approach each patient with a comprehensive understanding of their unique health profile, using BMI as just one part of the assessment.”

Alternatives

For physicians seeking different ways to evaluate their patients’ body fat, alternatives exist.

“There are several other tools that may measure body fat, like using skin fold calipers or weighing while immersed in water or bone scans or even CT or MRI scans,” Iroku-Malize said. “However, these methods can be cost prohibitive and come with their own risks.”

One method that Daghigh, Iroku-Malize and Stanford all mentioned was waist circumference, especially considering that carrying weight around one’s midsection can be indicative of certain health issues. Stanford said the low-cost option also offers flexibility for patients using telehealth.

“A tape measure costs $3 to $5,” Stanford said. “So even for our patients with the lowest socioeconomic position, it's something that's affordable to have in a household, right? We don't have to go and spend exorbitant amounts of money to have a tape measure in our home.”

Considering drawbacks like the fact that waist circumference measurements cannot distinguish between fat and muscle mass, Iroku-Malize said it is “not a perfect tool for diagnosing obesity.” However, she said certain waist measurements could put people at higher risk for several health issues. For most women, a measurement of 35 inches or more “could put people at higher risk,” she said, and for most men, that number is 40 inches or more.

Daghigh said that other more comprehensive body composition measurements, like waist-to-hip ratio and body fat percentage “offer a more precise evaluation of an individual's health, especially concerning cardiovascular risk.”

“In primary care settings, it can be advantageous to incorporate these supplementary metrics alongside BMI to provide a more holistic appraisal of a patient's well-being,” she said.

Recently, studies have looked at alternatives to BMI that can differentiate between fat and muscle mass or more accurately gauge body composition and fat distribution — including the waist-calf circumference ratio and the weight-adjusted waist index.

Ultimately, Iroku-Malize said, obesity treatment “is about preventing health complications and improving the symptoms of obesity by reducing body fat” — not “getting to a specific number on the scale.”

“Family doctors and patients can work together to decide what individual treatment goals are and how to achieve them,” she said. “Obesity, like other chronic illnesses, requires working long term to achieve personal success.”