What is considered 'obese' — and does it mean you aren't healthy? Experts say it's complicated.

Are good health and obesity mutually exclusive? Not necessarily — but it’s complicated.

For years, body size was considered a barrier to most sports, but today that’s starting to change — with plus-size influencers proving that physical activity is for everyone.

Dancers like Lizzy Howell — who can whip through fouetté turns and command a stage while breaking the skinny mold — are also social media darlings inspiring others to lace up their pointe shoes. “Recreational ballet as an adult has radically healed my relationship with my body,” one newbie ballerina says on TikTok, along with the hashtag #DoItFat. “Instead of seeing my size, I can focus on my strength and technique.”

Another TikToker who goes by EmilyJeanAro has built a following posting videos of her impeccable lines and sleek poses — and how she got started “as a 200-lb yoga practitioner.”

And Mirna Valerio, a celebrity in the running world and founder of the "Fat Girl Running" blog, is teaching a new generation of plus-size athletes that runners don’t need to “look a certain way.”

More than two in five U.S. adults (or 42%) have obesity, yet many in the medical field say that the way we currently diagnose that condition isn’t reliable — and maybe even misleading about patients’ health.

And while physical activity is important, experts say there’s more to consider.

What exactly is obesity?

The World Health Organization defines obesity as “abnormal or excessive fat accumulation that presents a risk to health.”

While fat does serve a purpose as fuel that’s stored up for future use by the body, having too much can cause or exacerbate chronic issues.

“Some conditions such as joint pain, arthritis, exercise intolerance and asthma may be due to the effects of simply carrying extra weight,” Dr. Andrea Coviello, director of the Weight Management Program at UNC Health, tells Yahoo Life.

Body mass index (BMI) is the commonly used measure to define obesity, and is calculated by taking an individual’s body weight (in kilograms) and dividing it by the square of their height (in meters).

Here are the Centers for Disease Control and Prevention’s numbers of what constitutes obesity based on BMI for adults age 20 and older:

• Underweight: BMI less than 18.5

• Healthy weight: BMI 18.5 to less than 25

• Overweight: BMI 25 to less than 30

• Class 1 Obesity: BMI 30 to less than 35

• Class 2 Obesity: BMI 35 to less than 40

• Class 3 Obesity (Severe Obesity): BMI 40 or greater

Can you have obesity and still be healthy?

A recent Lancet Commission report on clinical obesity said that “obesity is generally considered a harbinger of other diseases, not a disease in itself,” and that “the idea of obesity as a disease remains therefore highly controversial.” So, while it is possible to have obesity without having any other health issues, an obesity diagnosis usually means it’s likely that health issues will arise at some point unless measures are taken to get it under control.

Dr. Sun Kim, an endocrinologist at Stanford University who specializes in the treatment of type 2 diabetes and obesity, tells Yahoo Life it’s possible to be classified as obese yet still be healthy — but even the term “healthy” is a subjective one.

“We do not have a clear definition of ‘healthy,’ but technically, when we say someone has obesity, we are implying that they have excess body fat,” Kim says.

Part of the problem, Kim explains, is that BMI is used as a “surrogate” for measuring body fat, but BMI isn’t actually a direct measurement — meaning that BMI can falsely over-diagnose or under-diagnose people with obesity. For example, Coviello says one group that may be misclassified as obese is younger, muscular men who might have a BMI in the “overweight” range (BMI 25-29) but actually have normal amounts of fat with high muscle mass. Conversely, older people who have lost a lot of bone and muscle mass might not technically meet the BMI threshold for obesity but have higher body fat.

“You can't presume risk exclusively from a BMI. That's the bottom line,” Dr. William Dietz, director of Stop Obesity Alliance and a professor at George Washington University’s Department of Exercise and Nutritional Sciences, tells Yahoo Life. “Just because you have an increased BMI doesn't mean that you have body fat that is likely to cause disease.”

But even if an individual has higher body fat, is that enough to say definitively whether they are “healthy” or “unhealthy”? Can someone with obesity who’s also physically active still be considered healthy? Dietz says it depends.

“Physical activity is important, but it doesn't necessarily counter all of the potential adverse impacts of body fat,” Dietz says. That means that just because someone is capable of physical feats like a 5K doesn’t necessarily mean they don’t have underlying conditions that could raise problems later.

“You can have an increased BMI and increased lipids, or increased blood pressure or prediabetes.” Dietz says. “Those are not necessarily considered ‘good health,’ but they don't have a functional impairment — meaning people can still be physically active. And that [physical activity] will reduce your risk, but it won't completely eliminate it.”

Coviello adds that it’s possible for people to be in the obese BMI category and not have any obesity-related disorders — “however, the likelihood that people with obesity develop related conditions over time is very high.”

Should we be rethinking how we define obesity?

Current measures for obesity lean heavily on BMI, but experts are increasingly sounding the alarm that that method needs an overhaul.

The recent Lancet report sought to differentiate obesity from clinical obesity. Both conditions, the commission proposed, should be defined not only by an increased BMI, but also by whether the person has an increased waist circumference. And to qualify as having “clinical obesity,” the individual needs to also have one of 18 “obesity-related diseases,” such as a cardiovascular disease or a metabolic disease like type 2 diabetes. In other words, simply having excess body fat wasn’t enough to be categorized as “clinically obese.”

When measuring health and disease risk, these are some of the characteristics besides BMI that experts say we should be taking into account.

• Waist circumference. Waist circumference considered in tandem with BMI is key. In general, a waist circumference greater than 35 inches for women or greater than 40 inches for men puts you at elevated risk for obesity-related diseases. But one of the lingering questions, Dietz says, is how or where exactly to measure waist circumference — which hasn’t been standardized yet. For example, Dietz says that many people with more severe obesity tend to have abdominal fat that overlaps the waist. “So do you measure waist circumference around that overhang of fat, or do you measure it underneath that overhang? That's part of the debate about the complexity of adding that measure,” Dietz says.

• Visceral fat vs. subcutaneous fat. Not all fat is created equal. Subcutaneous fat is easy to see and feel under the skin. Visceral fat, though harder to detect, can be hazardous; it’s stored deep in the abdomen and surrounding organs, and is made up of biologically active cells that can cause inflammation and other harmful effects on the body. “You can have two people of the same BMI who have a completely different fat distribution,” Dietz says. “Some people have a lot of subcutaneous fat and no visceral fat, and at the same BMI other people have visceral fat but very little subcutaneous fat.”

• Fat location. The location of excess fat can also tell you a lot about your health. “For example, in primarily women, gluteal fat (fat in the buttocks) is not a risk factor, whereas abdominal circumference is a risk factor that says something about the likelihood that not only do you have increased body fat, but you also have an increased clinical risk,” Dietz says. In other words, weight in the midsection may be more cause for concern than weight in the thighs or butt area.

• Chronic disorders. Increasingly, experts are also saying that clinical obesity should be defined by whether a person has comorbidities — and not just whether they have a high BMI and waist circumference. Coviello says her team at UNC takes chronic disorders into consideration when gauging a patient’s health. “Hypertension, diabetes, cholesterol problems, fatty liver disease, obstructive sleep apnea, cardiovascular disease are common conditions that are related to obesity,” Coviello says.

Since some of these risk factors are impossible to see, it’s another reason, experts say, why we shouldn’t make snap judgments about other people’s health based on their weight. While an obesity diagnosis can flag to patients and their health care providers that steps need to be taken to prevent adverse health outcomes later on, it doesn’t necessarily mean that obesity-related diseases are impeding life at that moment. And that's especially important to consider given that even the way we diagnose obesity is increasingly being called into question.