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Here’s a weighty question for you: What is a healthy BMI?

For years, BMI—body mass index—has been the go-to tool for plotting our weight into categories: underweight, normal, overweight, or obese. BMI is a number that comes from a formula based on your weight and height. If your number is 25 or higher, you fall into the overweight or obese category.

So, does that mean that a BMI over 25 is unhealthy?

Holly Russell, MD

It’s complicated, says UR Medicine family medicine physician Holly Russell, MD, in her commentary, “”

Many of us cling to the belief that a thin body is a healthy body, but recent evidence suggests that’s not necessarily true. For example, new research has shown that some people in the “overweight” BMI category have a lower risk of death from heart-related causes than those with a “normal” BMI.

Studies have also shown that BMI can mislabel a person’s health when compared to objective measures, like results from tests for cholesterol and blood-sugar levels.

“Despite assumptions we’ve come to accept as facts, the evidence linking weight and health is inconsistent,” Russell says. “And using a weight-based tool like BMI to draw conclusions about a person’s health adds to the confusion.”

When you consider the odd, Victorian-era beginnings of the modern-day BMI calculator, you’ll understand why.

The Myth of the Math

Here’s a fact that may surprise you: When it was developed in the 19th century, the BMI formula was never intended as a measurement of health—especially the health of a diverse population. It wasn’t even created by medical experts.

Developed by a 19th century astronomer as a population census tool, the BMI formula was never intended as a measurement of health.

BMI came from the work of a 19th century Belgian astronomer who was designing a population census in the Netherlands. His sample group of high-income, mostly white men aimed to estimate typical sizes of the total population for the purpose of distributing resources.

In the early 1900s, studies were done—primarily based on a white, male population—to try to determine the “ideal body weight.” Mixed results from tests of basic tools to measure body fat—like water displacement and skin calipers—led to the conclusion that the simple math of BMI should set the standard. And it’s been that way ever since.

“When you consider all the things that have changed in the last 100+ years, it’s puzzling to think that we’ve clung so tightly to BMI and, beyond that, the notion that there actually is a universal ideal body weight,” says Russell, who also serves as director of clinical and community-based programs at the Center for Community Health & Prevention.

Why Weight?

“The fact is, there is not a precise link between weight and health outcomes, nor is there evidence to support a fixed belief that higher weight always equals worsening health,” Russell says. “A person’s health is influenced by a complex mix of health behaviors, genetic factors, lean mass, fitness, and environmental risks.”

The bottom line: no simple math formula or number on the scale can measure a person’s health. And using one for that purpose may actually cause harm.

A person’s weight and BMI—and the stigma when those numbers aren’t “normal”—can stand in the way of people getting the care they need. Anyone who has ever postponed a doctor’s appointment due to their weight, or purposely gone on a diet before a doctor’s appointment, is evidence of that.

Instead of asking, “What is a healthy BMI,” spend your time and energy on science-based tactics that are known to be good for your health.

“It’s time to shift our focus away from BMI and ‘ideal weight’—and the judgment that goes along with higher weight—and to focus on things like that are consistently proven to help us live longer, healthier lives, like spending time with those you love, moving your body, and smoking or vaping less,” Russell says.

“Placing attention on health rather than body size can help us focus our priorities to what is truly important to us in the new year,” she adds.