Body mass index (BMI) is a calculation made by dividing a person’s weight in kilograms by the square of their height in meters. Don’t try to do the math—there are lots of BMI calculators online.
Doctors use BMI to determine whether a person is at a healthy weight. We know that obesity increases a person’s risk for diabetes, high blood pressure, coronary artery disease, stroke, osteoarthritis and some cancers.
BMI is an important piece of information but not the be-all and end-all of health, says UNC Health family medicine physician Sarah Ruff, MD.
“BMI is like a vital sign that doctors use,” Dr. Ruff says. “And while I think it is a useful tool, there are so many factors that it doesn’t take into account.”
1. BMI was developed without considering race or gender differences.
The weight-to-height ratio was developed originally in the 19th century by Lambert Adolphe Jacques Quetelet, a Belgian who was on a quest to find the “ideal man.” In 1972, American physiologist Ancel Keys developed the calculation we know as BMI. “Normal” is a BMI of 18.5 to 24.9, according to the Centers for Disease Control and Prevention (CDC).
“BMI was developed based on calculations of white men,” Dr. Ruff says.
It doesn’t consider gender, muscle mass, race, ethnicity, build or other factors that can have a significant impact on health.
For example, studies have shown that people from South Asia or of South Asian descent are at risk for developing metabolic disorders such as diabetes and high cholesterol at a lower BMI than white men typically do. And many Black women have a much higher BMI than the charts consider healthy before they exhibit any symptoms of a metabolic disorder.
One of Dr. Ruff’s patients, an athletic man with no signs of metabolic disease, had a BMI of 30 because he was very muscular. Metabolic disease is any of the diseases or disorders that disrupt your body’s ability to convert food to energy.
“BMI is imperfect,” Dr. Ruff says. “As doctors, we have to realize that it’s a useful tool, but it’s not the only tool.”
2. BMI is one indicator of health, but it doesn’t tell the whole story.
Other metrics are equally important in assessing health, Dr. Ruff says: cholesterol levels, blood pressure, blood sugar, diet, exercise habits and stress levels, to name a few.
“Losing weight may help relieve symptoms,” she says, “but it’s not the solution to every problem.”
For example, a patient with knee pain may be told only that he or she needs to lose weight, but physical therapy might also be helpful in solving the root cause of the knee pain.
A movement called Health at Every Size (HAES), which encourages health-promoting behaviors to improve symptoms and quality of life rather than focusing on dieting and weight loss, has gained traction in recent years. The HAES approach also tries to reduce people’s feelings of shame about living in a large body. So far, HAES has seemed to attract mostly white women, Dr. Ruff says.
HAES is a controversial philosophy because some people think it downplays the health risks of excess weight, Dr. Ruff says. But, “We’ve relied on BMI and told our patients to lose weight for a long time and we haven’t budged the obesity numbers. We need to think about what our goal is: for people to be healthier? Or for them to lose weight?”
Obviously, good health is the goal, she says, but weight loss still may be helpful.
“There are people who do need to lose weight because they have cardiovascular disease or high blood pressure,” she says. “But weight and health are an imperfect correlation.”
3. Factors other than weight contribute to metabolic diseases.
Chronic stress can lead to high blood pressure, cardiovascular disease and other conditions. Facing weight stigma can lead to stress. Stress can be caused by social, racial and economic inequality.
Racial, economic and ethnic factors, along with life experiences, often affect the choices people make about what, when and how much they want to eat.
“You can’t tell everyone to just drink more water and eat more fruits and vegetables,” Dr. Ruff says. “Some people are not going to stop eating certain foods, because it’s what they grew up with—it’s part of their culture.”
Dietitians can work with people to find ways to include their food preferences in a healthy diet.
4. BMI should be used with special caution when it comes to children.
BMI is a measure used with children as well as adults. With toddlers and young children, doctors talk about percentiles of children the same age who are larger or smaller than the patient—your 2-year-old might be in the 80th percentile for weight, for example, which means she is heavier than 80 percent of girls her age. This doesn’t mean she is overweight, however.
Even if a child is overweight, these labels aren’t helpful, Dr. Ruff says. Instead, doctors should ask parents and children questions about their daily habits: Do they play outside? How much time do they sit in front of a screen? What do they like to eat and drink?
When it comes to determining whether a child is healthy, “their weight is just a piece of the puzzle,” Dr. Ruff says. “It’s not the whole story.”
That goes the same for adults, she adds—consider your weight and BMI numbers not as diagnoses, but as information. Then talk to your doctor about any symptoms you experience and your goals for your health and life.
If you are concerned about your health, call your primary care doctor or find one near you.