Over the years, body mass index (BMI) has been both hailed and excoriated as a measure of health. Because research ties obesity to negative health outcomes, including heart disease and cancer, BMI has been considered a more nuanced look at weight than just the numbers on a scale. While certain charts offer suggested weight ranges, BMI takes that information one step further by including your height in the equation to determine whether you are overweight, obese, or in the so-called “normal” range.

That BMI equation: Divide your weight in kilograms by your height in metres squared, (or kilograms/height-squared = BMI). According to the Centers for Disease Control and Prevention, a BMI under 18.5 is underweight; between 18.5 and 24.99 is considered normal; 25 to 29.9 is overweight; and 30 and above is considered obese.

“Having a BMI of 30 percent or above, which puts you in the obesity category, is associated with an increased risk of obesity-related health issues and with co-morbidities, such as cardiovascular disease and diabetes,” Dr Deborah Riebe, from the College of Health Sciences at the University of Rhode Island tells Runner’s World. “Even the overweight range has been linked with problems such as diabetes and certain cancers.”

On the surface, all of this information seems clear and useful, but in fact, there are a number of complications. The truth is, the equation may not be a useful tool for everyone. Moreover, categorizing people in ways that appear to divide them into good and bad can be detrimental both to personal body image and receiving healthcare.

The History of BMI and Its Shortfalls
Developed in the early 1800s by a Belgian sociologist, mathematician, and astronomer who wanted to find “the perfect man,” according to Endocrine Web, BMI is inherently flawed from the get-go as it was never meant to be used for women and people of colour. Its equation began with measurements from white males and has “little validity for other racial and ethnic groups,” according to experts at Harvard Medical School.

Similarly, the numbers appear to be hard and fast boundaries between “good” and “bad,” even though that’s not how health outcomes work.

“There are limitations to its performance because what is the difference in health between a BMI of 24.9 and 25.1?” Dr Tracy Richmond, director of the eating disorder program at Boston Children’s Hospital and associate professor of paediatrics at Harvard Medical School tells Runner’s World. In other words, is an individual really at that much more risk for disease if their BMI is .2 percent higher at the time of measurement?

The dividing line between healthy weight and overweight and obese is arbitrary and not based on health research, Richmond adds. In fact, she says, there are no measures of health that work in a linear correlation with weight. For example, no one can say that after 80 kilograms, a person, male or female, tall or short, is more likely to develop heart disease or any other health problem. In fact, in its first health iteration, BMI insurance tables found that tall men lived longer than short men, and that information had nothing at all to do with weight.

Also, BMI does not measure body composition, that is the amount of fat versus muscle (as well as bone and everything else that is in the body) which contributes to total weight. “Body fat composition is different from BMI,” says Riebe. “BMI is like a proxy for that information, but it is imperfect.” In fact, a bodybuilder or muscular person may read at a higher number on the scale, but they don’t carry a high amount of fat for their height. Even still, that might put them in the overweight or obesity category.

Why Weight Doesn’t Always Reflect Health
On the flip side, those who fit in the normal or underweight BMI categories are also not immune to disease and potentially negative health outcomes, just like those who are overweight or obese. Genetics, as well as other habits such as smoking or a poor diet, are all important in the health equation.

While the recommended annual exams, such as fasting blood tests, are the same for people of all weights (though not all ages), when someone is thin and goes to a doctor with a complaint, a doctor may assume an illness is not metabolic in nature. However, this person could still very well have metabolic issues, including insulin resistance.

In other words, as Richmond says, metabolic health does not correspond directly to a specific weight nor to a specific amount of fat in relation to muscle. In fact, in a 2018 study of more than 8,000 Americans, researchers found that less than one-third of that group were “metabolically healthy,” and that included those who were at normal weight and normal BMI.

Having too little fat can also be detrimental to health, especially when it comes to athletes. “Yes, people do tend to perform a little bit better if they have reduced adiposity, but there’s very much a limit to that,” Richmond explains. “If you push too far, you lose your competitive edge.”

Riebe says that “essential fat levels are 3 to 5 percent for men and 12 to 13 percent for women.” You do see people get too lean in sports, she adds, and that is dangerous for both physical and mental health.

Is It Time to Ignore BMI?
This is a timely question because Richmond and other experts have made this exact suggestion, citing that BMI information is often not well-used by physicians. Moreover, “people hate getting on a scale,” Riebe adds. “Health professionals have to read people before asking them to get on a scale.”

On the other hand, Riebe considers BMI to be another data point that can be helpful for both patients and physicians. All medical information, she says, is pointing toward the odds of disease. “It’s like tobacco,” she explains. “The odds are that the person who uses tobacco is more likely to have issues. Likewise, the risk of obesity-related diseases is higher in someone who is within a specific weight range. You’re playing the odds.”

To that end, both Richmond and Riebe think that the research showing that the use of BMI in conjunction with waist-to-hip ratio may correlate with health outcomes. Fat distributed around the waist may increase disease risk more, compared to fat distribution in other places, but as Richmond notes, “BMI does not take that into account.”

How to Reshape Thoughts on BMI in Healthcare—and at Home
Today, many in the athletic community work toward a body-positive mindset. That is, embracing the idea that people of any size can be athletic and healthy. However, not everyone has adopted this point of view. Physicians, like many in the general public, demonstrate bias against overweight and obese people. In fact, according to commentary published in The Lancet, studies have shown that patients frequently encounter a weight bias in medical settings.

That stigma stands in the way of successful healthcare. Studies suggest that the stigma around obesity, particularly when brought into the medical setting, can create mistrust of doctors, poor adherence to medical advice, and that it can reduce the quality of care.

Richmond hopes physicians will start learning more about functionality and other markers to identify poor health. For example, they should ask questions that deliver more lifestyle information, including: Do you take a walk every day? Can you go up the stairs without breathing heavily? Do you eat whole foods rather than processed foods? How do you feel about your weight?

If doctors couple that information with some tests, such as those looking at cholesterol levels and blood pressure, they can determine if a patient needs further tests for disease, and any other follow-up.

Finally, instead of encouraging their patients to lose (or gain) weight, Richmond says, physicians should “focus on behaviour versus numbers.”

The best health-promoting programmes are interventions that give people tools to make behaviour changes, Riebe adds. Adding fruits and vegetables or adding movement to your day are more likely to bring about positive changes.

“Just always check that you’re doing things that bring joy to your life and seem sustainable,” Richmond says. If you like to run, run when you can. If you like to cook, fix meals that are healthy. Focus on enjoyment, not diets.

Also, decouple movement from gains and losses on the scale, Richmond adds. Try to dial down the self-critique. All scientists agree that excess weight is not a moral or personal failure, and that fat shaming is likely to lead to weight gain.

One more thing: Remember that people, like dogs, come in all different shapes and sizes. “Some people are English Bulldogs and some people are whippets. A starved English bulldog will never be a whippet,” Richmond says.

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