During the early 2000s, researchers at the Centers for Disease Control and Prevention (CDC) looked into the association between body size and early death. They wanted to determine how much being underweight or overweight increased a person’s risk for mortality.

Their findings, which appeared in JAMA, revealed some surprises.

“Being overweight was associated with lower mortality than being underweight, which we didn’t expect to find,” says Katherine Flegal, PhD, one of the authors of the CDC study and a consulting professor at Stanford University.

In fact, her study found that people who were overweight according to their body mass index (meaning a BMI between 25 and 30) had no increased risk of death compared with people whose BMIs fell in the “normal” range (18.5 to 25).

“The fixation on BMI and weight loss is overemphasized,” Dr. Flegal says. “I think it contributes to fat shaming and stigma and misperceptions.”

Her work helped usher in new thinking within the medical community — and the wider public — about body weight and its relationship to health. It has also contributed to the recognition that some people with obesity may be metabolically healthy — a concept that more widely became referred to as “fat but fit” — and that considering BMI alone can be misleading.

That said, there is nonetheless a lot of evidence linking obesity — and particularly severe obesity — with poor metabolic health. Beyond metabolism, research has associated obesity with increased rates of cancer (in EBioMedicine), osteoarthiritis (in Maturitas), and other health issues.

Experts say all these factors — and many more — need to be taken into account when assessing the health effects of body size and the benefits of weight loss.

What Does the Current Evidence Say About Metabolically Healthy Obesity?

The word metabolism refers to all the processes that occur inside the body to sustain its health and functioning, per StatPearls. Metabolic disease or dysfunction is associated with an increased risk for type 2 diabetes, heart disease, stroke, liver disease, and some cancers, much research shows.

When assessing a person’s metabolic health, medical experts usually take into account the following five things, which are components of what’s known as the metabolic syndrome, according to Johns Hopkins Medicine:

Blood pressure
Triglycerides (blood fats)
Blood HDL cholesterol (aka “good” cholesterol)
Blood glucose
Waist circumference

Experts who have studied the relationship between body weight and metabolic health say there are some consistent patterns, but also lots of person-to-person variation.

“Many people who are obese have most or even all of these metabolic abnormalities, but some have few or none of them,” says Samuel Klein, MD, a professor and director of the Nutrition Obesity Research Center at Washington University in St. Louis.

Some of Dr. Klein’s work, including a study published in 2019 in the Journal of Clinical Investigation, has examined the science of “metabolically healthy obesity,” which he says is an ill-defined term.

For example, he says some research efforts have defined “medically healthy obesity” as individuals who have two or fewer of the above five abnormalities that comprise metabolic syndrome. Meanwhile, other researchers have set the bar at one or zero of these abnormalities. In all, he has found more than 30 different definitions in the medical literature.

He says these inconsistencies make some of the work on metabolically healthy obesity difficult to interpret. “In some of these papers, people reported as having metabolically healthy obesity were not metabolically healthy, they just had fewer metabolic abnormalities,” he says.

An important takeaway, however, remains that a small subset of people with obesity have no metabolic abnormalities at all. “These people appear to be resistant to the adverse metabolic effects of excess body fat, and if you follow them over time, they don’t develop diabetes or heart disease,” he says.

How Common Is Metabolically Healthy Obesity?

But, Klein adds: “But this group represents a small percentage of the population who are obese.”

Furthermore, a precise number is hard to estimate, and it depends in part on a person’s age, sex, and ethnic background, he says. But if you’re using the most sensitive and rigorous measurement tools, about 7 percent of all people who are obese may qualify as metabolically healthy obese, his work has found.

Also according to that work, metabolically healthy obesity is more common in women than in men; more common in younger people than in older adults; and more common in people with BMIs less than 35 than in people with BMIs of 35 and higher.

“We usually see it in women with lower body fat distribution, meaning they have increased fat in the thighs, legs, and buttocks, but not in the abdominal area,” he says.

Increased waist circumference and abdominal fat are almost always indications of metabolic abnormalities, he adds, and very few men are metabolically healthy obese.

That’s because abdominal fat tends to be visceral fat, the more problematic type of fat that’s linked to increased risk of several chronic diseases linked to chronic inflammation, as well as high blood pressure, according to Harvard Medical School. And this is why abdominal fat tends to be indicative of more health risks than fat accumulation around the thighs, legs, and butt areas.

If You’re Overweight or Obese, What Determines if You’re Metabolically Healthy?

Genetics certainly play a role when it comes to whether or not someone with a high weight is metabolically healthy or not. “We don’t understand the relationship fully, but clearly there has to be a genetic cause for why some people are more resistant than others to the adverse effects of obesity,” Klein says.

Lifestyle and behavioral factors likely also play a part. Klein says people who eat a healthy diet low in processed foods, who exercise regularly, and who get plenty of sleep may be better protected from the metabolic harms of obesity.

“But a lot of this is theoretical and not borne out by epidemiological studies,” he adds.

Researchers’ ability to measure those lifestyle factors is “not very good,” he explains, and so that makes it difficult to say for certain whether lifestyle factors can promote metabolically healthy obesity. He adds: “It makes sense that eating a healthy diet and exercising would improve metabolic health, but whether it leads to metabolically healthy obesity is something we can’t say for certain.”

What Role Does Fitness Play in Protecting Against the Risks of High Weight?

A research review published in 2014 found that people who were obese but fit — as determined by their maximal oxygen uptake, or VO2 max, a common measurement of overall fitness — experienced similarly low rates of mortality when compared with people who were normal weight and fit. Both groups had lower mortality than people who were unfit, the study found.

In line with these findings, more research, including a review published in 2021 in the journal iScience, argued that fitness — rather than body weight — is a more helpful health measurement. The authors of that review made the case that improving fitness among people with obesity would offer more health benefits than weight loss.

“There are research data showing that being fat and fit — not being an athlete, but having an ability to consume more oxygen during exercise — is clearly associated with a lower risk of developing heart disease or diabetes, or dying from heart disease, than if you’re lean and unfit,” says Klein.

He says that becoming fit will lead to health benefits among many people who are overweight or obese. Regular aerobic and endurance training can improve metabolic health and may also be protective against metabolic diseases, he says.

“But whether becoming more fit will make you metabolically healthy obese will vary from person to person,” he says. “It may make you metabolically healthier, but not necessarily healthy.” He also says that, in most cases, someone who is fit and lean is going to be at lower risk for metabolic or cardiovascular diseases than someone who is fit but obese.

There are surely people out there who meet the “fat but fit” label. But it’s unclear whether people who are overweight or obese can avoid all weight-related health problems by focusing solely on improving their fitness, Klein says. “Weight loss may still provide additional risk reduction benefits,” he adds.

So, Should Weight Loss Always Be the Goal if You’re Overweight or Obese?

For those who are obese, the answer to this question is pretty straightforward.

“If you’re obese, even if you’re metabolically healthy, it’s reasonable to try to lose weight,” Klein says.

Looking beyond metabolic health, he says people who are obese are at increased risk for age-related mobility impairments, joint problems such as arthritis, dementia (per a study in Alzheimer's and Dementia), and some cancers (such as liver and uterine cancers, per The New England Journal of Medicine) compared with people who are not obese. “I think it’s very rare to find a person who is obese who would not benefit from losing weight,” he says.

Other experts echo his sentiments.

“Eventually, high weight takes its toll,” says Steven Heymsfield, MD, a professor of metabolism and body composition at Louisiana State University’s Pennington Biomedical Research Center. “People who are obese their whole lives often need new hips and knees, or develop other functional problems, and healthy weight loss can prevent that to some extent.”

Everyone, at every size, benefits from eating a healthy diet with more whole foods and few ultra-processed foods. Likewise, getting regular exercise, a good night’s sleep, and generally moving more (and avoiding sedentary behaviors) are healthy behaviors. If adopting these healthy habits leads to weight loss in someone who is obese, that’s almost always a good thing, he says.

But when it comes to people who are overweight, as opposed to obese, the benefits of weight loss are less certain. Especially if a person who is overweight is living a generally healthy lifestyle, trying to drop weight may not always lead to health benefits, says Flegal. (And in older age, carrying some excess weight can be beneficial even, research shows.)

Regardless of your body size, if your healthcare provider recommends that you lose weight, Klein says it’s helpful to ask a few questions.

“Ask what is the benefit of me losing weight, or why should I do this,” he says. “If they say your triglycerides are high, your blood pressure is on the edge of being abnormal, you have a family history of diabetes or you have prediabetes — all of those are major indications for weight loss benefits,” he says.

On the other hand, if your parents were both obese and “lived to 100 without major diseases,” and your body resembles their bodies — and you’re metabolically healthy — Klein says losing weight may not improve your metabolic health.

Should People Who Are 'Fat but Fit' Take Weight Loss Medications?

There’s been much attention paid recently to prescription weight-loss drugs thanks to the impressive results people have experienced taking Wegovy and Zepbound (and the related diabetes drugs Ozempic and Mounjaro). Are they appropriate for someone who is obese but metabolically healthy?

Klein notes that these drugs have not been studied specifically among people who are known to be both obese and metabolically healthy. So for now, judging their usefulness is going to require some guesswork.

The current FDA guidelines state that Wegovy is only appropriate for someone whose BMI qualifies them as obese (meaning a BMI of 30 or higher), or for someone with a BMI of 27 or greater who also has weight-related health problems such as high blood pressure or type 2 diabetes.

Klein says these criteria “are very reasonable,” but there are situations where even someone who meets these criteria may not benefit from a weight loss drug.

First of all, he emphasizes that BMI can be a flawed measurement tool. A person with little body fat but a lot of muscle mass (such as a bodybuilder) could, based solely on BMI, meet the definition of obesity. This sort of person doesn’t need weight loss drugs, he says.

He also says that the FDA-approved weight loss drugs can be very expensive — “around $1,200 or $1,300 per month” — and insurance doesn’t always cover the cost. If a person is obese but doesn’t have obesity-related diseases or risk factors, paying that much money may not make sense.

On the other hand, he says these drugs appear to be “very safe.” Also, if someone is obese, he says that taking a weight-loss drug might “improve their ability to walk and to play with their children, and maybe also lower their risks for non-metabolic diseases such as cancer.”

The relationship between body weight and health is complicated. The right answers will depend on many individual factors.

“The decision to lose weight and the best approach to losing weight is really for the patient and their physician to discuss,” Klein says.