Today’s Dietitian
Vol. 28 No. 1 P. 24
Does weight affect health? If so, to what degree? Most research shows that as BMI increases, cardiometabolic health decreases. But population-based trends can’t account for individual health, and the assumption that “fat” means unhealthy and “thin” means healthy can lead to inadequate health care for patients across the weight spectrum, including misdiagnoses and interventions that miss the mark.
Accordingly, the concepts of “metabolically healthy obesity” (MHO) and “metabolically unhealthy normal weight” (MUNW) began picking up steam in the research literature 15 years ago, but how do they fit into the larger conversation about weight inclusivity and evidence-based care, including reducing long-term risk for cardiometabolic diseases such as diabetes and CVD?
What Are MHO and MUNW?
The origin of the term “metabolically healthy obesity” came from clinical observations 75 years ago that not every person with a BMI in the “obese” range has the same predisposition to type 2 diabetes and atherosclerosis. Since then, further clinical observations and a rapidly growing body of research has established that not all higher weight people have cardiometabolic abnormalities—such as insulin resistance, impaired glucose tolerance, hypertension, and dyslipidemia—and some people with a BMI in the “normal” range do, thus the term “metabolically unhealthy normal weight.”1
Characteristics of MHO include higher subcutaneous body fat in the lower body, but lower liver and visceral fat, as well as greater cardiorespiratory fitness and insulin sensitivity, normal adipose tissue function, and lower levels of inflammatory markers.1 Characteristics of MUNW include excess abdominal subcutaneous and visceral fat, fat deposits in organs and muscle tissue, reduced skeletal muscle mass, low cardiorespiratory fitness, adipose tissue inflammation, and higher levels of inflammatory markers. While exact numbers vary based on the definition of metabolic health used, it’s estimated that around 10% to 30% of people with a BMI in the “obese” range are metabolically healthy. Similarly, an estimated 5% to 45% of people with a “normal” BMI are metabolically unhealthy.1
Evaluating the Controversy
Some researchers argue that MHO and MUNW can be applied to individual people, but don’t represent a distinct phenotype that can be applied to a larger subgroup of people within a weight category.1 However, others debate that MHO can be a distinct and stable phenotype.2 One issue is the heterogeneity of research on this topic. Studies on MHO and MUNW use different criteria and cutoff values to define metabolic health. Some researchers define it as the absence of any metabolic disorder or CVD. Others allow some degree of cardiometabolic abnormalities or risk factors, or use a single biomarker, such as insulin resistance or HDL cholesterol.
Although total fat mass has been thought to be a more accurate predictor of a metabolic phenotype than BMI, on an individual level, amount of body fat does not predict metabolic health—it appears that what matters more is the function and location of that fat tissue.3 For example, while most MUNW studies have used individuals on the higher end of the “normal” range, other BMI-matched studies have found that metabolic dysfunction can exist in the absence of higher-than-usual body fat for BMI, which means that having more body fat isn’t the cause. Also, waist circumference is not significantly different between these BMI-matched subjects and controls. 4 A 2024 review in Nature Reviews Endocrinology stated that liver fat content is one of the strongest predictors ofthe transition from MHO to metabolically unhealthy obesity (MUO) status.5
Individuals classified as MHO have about half the risk of developing type 2 diabetes compared with MUO, but a greater risk (50% to 300%) than the metabolically healthy normal weight group.6 Results from studies looking at the long-term association between MHO and CVD risk are conflicting.7-9 Many medical organizations define obesity as a chronic, relapsing progressive disease. Under this view, MHO is a transitory state that may change over time based on weight loss and gain, and patients in this category may still benefit from weight loss interventions. 1 However, any individual has the potential to transition between metabolically healthy and unhealthy states, regardless of BMI.9 Data from the Nurses’ Health study published in 2018 found that a large proportion of metabolically healthy women converted to an unhealthy phenotype over time across all BMI categories, and that metabolically unhealthy phenotypes were much more strongly associated with CVD risk than BMI.10
“In the absence of a standard definition, it’s very hard to have the kind of evidence that lets us group a reasonably homogenous group of people together in order to assess the links to health outcomes, then design specific interventions for those populations,” says Ellen Schur, MD, MS, a professor of medicine at the University of Washington School of Medicine in Seattle and director of the clinical research unit at the UW Medicine Diabetes Research Institute. She points out that in research on metabolic syndrome, each individual risk factor has an independent effect on disease risk over time. “So, if we create an umbrella of, say, metabolically unhealthy normal weight or metabolically healthy vs unhealthy obesity, we’re in a situation where some of those people are going to be very different from each other. I think those are the challenges for adopting the actual names and concepts clinically as a physician.”
A 2022 study in the Mayo Clinic Proceedings found that, based on NHANES data, the prevalence of MHO has increased significantly in the past two decades, and that clinical risk assessments of all adult patients with an “obese” BMI should include detailed and repeated metabolic phenotyping.11 “I think that a lot of larger bodied people experience medical care as being surveilled rather than being supported in health,” says Lisa Erlanger, MD, a clinical professor of family medicine at the University of Washington School of Medicine and director of medical services for The Emily Program in Washington state. “I worry about those people designated as ‘metabolically healthy obese’ being subjected to this kind of increased scrutiny, like there must be something wrong, and if there’s not, then there will be soon.”
Looking Beyond BMI
It’s well known that BMI can’t identify the amount or distribution of body fat, but the MHO/MUNW research also illustrates why BMI can’t accurately predict cardiometabolic risk.1 Recently, groups such as the American Medical Association seem to agree. “The new recommendations from the American Medical Association are actually going to put more folks in the obesity category because there’s preclinical obesity and then clinical obesity,” says Kellene Isom, PhD, MS, RD, an associate professor of nutrition at California State Polytechnic University-Pomona and an obesity researcher. “It truly is going to delve into the biomarkers and the organ health and the overall health aspects of that adiposity or the body composition overall. So that can be explored instead of just saying, ‘Oh yeah, your BMI is 31.’”
But are MHO and MUNW classifications any better than BMI-based assessments? “Those terms are defined by the BMI typically, which has its own complicated and rather unscientific history,” says Janice Dada, MPH, RDN, CDCES, CEDRD, owner of SoCal Nutrition & Wellness in Newport Beach and author of Intuitive Eating for Diabetes. “Why wouldn’t we just say ‘metabolically healthy’ or ‘needs metabolic improvement’? If they’re aiming for something to further classify or to be clearer than the BMI, then how are they defining obesity? Is that still based on the BMI classification, or is it something else?”
Schur says the MHO and MUNW concepts “hit within a framework of holistically assessing someone’s health and their weight and how they may or may not be related to each other.” Schur says, “Utilizing measures of metabolic health in conjunction with measures of adiposity that are available to us in the clinic is an appropriate way to give people the best advice about how to move forward.”
Schur points out that everyone, regardless of weight, is on a bell curve of risk for cardiometabolic diseases. “It’s not that it’s fated. We all still have some control, whether it’s the way we treat any risk factors that have developed or what we do lifestyle wise to modify that risk,” she says. “It’s just that when people are in the overweight or obesity categories, that bell curve of risk is shifted higher.”
Is MHO Always a Precursor to MUO?
Some research argues that people with BMIs in the “obese” range who are metabolically healthy will not stay that way over time, so weight loss interventions are warranted. Erlanger says this is a problem, pointing to large population-based studies, that, when carefully analyzed, showed that weight cycling—repeated episodes of weight loss and regain—is more metabolically harmful at any BMI than staying stable at that BMI. A 2025 study that used 23 years of health records from Vanderbilt University Medical Center found that weight cycling was associated with increased cardiometabolic risk independent of having a high baseline BMI.12 “If we suggest that weight loss might be an intervention for higher weight people who are metabolically unhealthy, or who are currently metabolically healthy to try to prevent metabolic unhealthy ‘obesity,’ we are probably down the line triggering worse metabolic health,” she says.
Patient-Centered Care
Isom says the MHO and MUNW concepts could be helpful for patient-centered care. “I think a big criticism in medicine in general is that we’re taught the general or the averages, so we tend to ignore the small percentage that don’t fit these guidelines of what is high risk,” she says. However, she says these concepts aren’t being discussed or explored outside of obesity medicine. “We are seeing, I hope, in dietetics education, a focus shifting away from BMI and talking more about the rest of the patient’s story. But these two terms are not employed in clinical practice and they’re not being employed in education of dietitians. I haven’t seen them employed in the education of medical providers either.”
Dada says these terms imply that we must qualify the fact that a patient is in a larger body, but they happen to be metabolically healthy. “Why is it that we have to refer to the weight at all? Can we just focus on the metabolic condition at hand if it exists? Like somebody with diabetes or somebody without diabetes. And when we describe that this is somebody who has a metabolic condition, plus what their weight classification is, it sort of moves away from the purpose of how we would really treat the individual.”
Erlanger says that while we don’t want to miss the opportunity to diagnose metabolic risk factors early and treat them before they develop into CVD or diabetes or life-threatening complications, MHO is very weight-centric and possibly reductive formulation of health. “They frame the finding of metabolic well-being as an anomaly in a larger bodied person,” she says. She hears stories from her larger-bodied patients about doctors who keep looking until they find something wrong. “We also know that simply the act of continuing to look for something increases the chance that you come across a false positive, but in itself is also stigmatizing and can both keep patients from the office and directly lead to higher blood pressure, higher inflammation, and insulin resistance.”
Erlanger also says the idea of MUNW isn’t much better. “It suggests that only higher weight people should be sick and therefore there’s something unusual about this group of people that are smaller rather than the two facts that body size exists on a continuum and people of all sizes can be metabolically healthy or unhealthy. And if we take the weight piece out of it, then we give each person a chance to be evaluated and diagnosed without bias.”
Impacts on Weight Stigma
In some respects, the terms MHO and MUNW manage to confirm the stereotype that there’s an inverse correlation between weight and health, while at the same time poking holes in it. So, can these terms help reduce weight bias and stigma in health care?
“If these words were used with a patient, that would be pretty stigmatizing,” Dada says. “But if it was going to help a clinician who is otherwise very entrenched in weight normative care practices see that there are differences in metabolic health that span the weight spectrum, then I could maybe see it being helpful in that instance. When we assume that people who are thin are not going to have metabolic conditions and people who are in larger bodies are, we miss people on both ends. But certainly this language should be kept behind the scenes and not used with patients.”
Isom says these terms have been used to try to reduce weight bias and weight stigma. “That being said, there could be a subset of folks who view it as a negative because you’re now identifying someone as unhealthy or healthy. Going forward, if we get more research on these specific populations, it could be an opportunity to look at all health parameters around the individuals in each category. If we can create categories or distinguish metabolic health, there can be a reduction of weight stigma or obesity stigma.”
Schur says she typically uses the word “weight” with her patients, because not everyone hears the word “obesity” the same. “There are some people who really embrace a medical model or chronic disease model of obesity, and they feel that that describes their experience. And there are other people for whom that’s a very difficult term. And understandably so.”
She says there are particular body phenotypes that are at high risk and she hopes clinicians take that into account. For example, weight gain around the waist can be a sign of increased intra-abdominal fat, which is a strong predictor of metabolic syndrome. “This is where, in some ways, our cultural obsession with body weight and shape gets in the way of good care. Because it’s a little hard to say to somebody, ‘you know, I noticed you put a lot of your weight in your tummy.’ It’s a medical statement that some people will find very hard to hear.”
Dada says MHO and MUNW could be a steppingstone. “Maybe it’s a way to move from weight-based assumptions about health to, ‘Okay, here’s this in-between,’” Dada says. “But of course, I’d want to see that the research is also controlling for some of the really important variables like access to care and dieting history and health care avoidance because of past poor experience.”
Erlanger says framing the concepts we work with through the lens of weight isn’t helpful. “We need an approach to screening, prevention and treatment that doesn’t leave people out at any end of the weight spectrum. And that directly addresses modifiable risks and behaviors. And weight is not a behavior.”
Putting the Focus on Behaviors
Diet and lifestyle behaviors may better explain differences in metabolic health across the weight spectrum. A 2012 study found that adopting four healthy habits—eating five or more servings of fruits and vegetables daily, participating in 30 minutes of exercise daily, not smoking, and moderate alcohol intake—leveled mortality risk across the BMI spectrum.13 And a 2016 study found that, regardless of BMI, metabolically healthy groups were less sedentary and more physically active than metabolically unhealthy groups.14 Multiple studies—including those in a 2025 systematic review and meta-analysis15—that controlled for physical activity or cardiorespiratory fitness found no significant difference in CVD or all-cause mortality rates among different BMI categories, calling into question the assertion made in other studies that there is no “benign” obesity.16,17
Schur says improving metabolic health doesn’t require losing enough weight to move BMI into the “normal” range. “The metabolic benefits of weight loss happen very early. We start seeing reductions in blood sugar, triglycerides, and blood pressure with body weight loss of 5%. If you want a strategy to improve your metabolic health, then modest weight loss is a strategy for that.” She says she has patients who are frustrated because they’re eating healthy and exercising but not losing weight. “I look at all their numbers, and they look great. So, I tell them they are clearly having health benefits from what they’ve been doing.” As for patients who don’t want to pursue weight loss, she says they choose to address their individual risk paths with medication.
Dada says she would like to see more consistency in screening and intervention. “I see some clinicians who are so eager to run labs and give a diagnosis and to give a medication. And then I also see clinicians who are more like, ‘Oh, your A1c is 9, see if you can do some diet and exercise changes and bring it down.’ We can’t leave an A1c at 9,” she says. “I think some really want to check A1c constantly on their larger bodied individuals and then watch it less closely in someone who’s thinner because they assume that their habits might be better.”
Final Thoughts
“It’s a challenging field and we always want everything simplified. But we can’t simplify it,” Isom says. “Maybe the take-home is that there’s heterogeneity in response to treatments for overall health, and heterogeneity in response to treatments for obesity. Therefore, there’s also heterogeneity in responses to how one’s body handles and presents the overall anthropometrics and biomarkers that we tend to look at.”
Erlinger says she thinks everyone would be better cared for if we just removed weight from the equation. “I do see how, from a research perspective, it might actually help a weight-inclusive approach to care if we understood better which people do get metabolically ill and don’t. If we accept that BMI particularly, but weight and size in general, were very poor measures of that. And we were able to clearly define who a metabolically healthy person is and who a metabolically unhealthy person is and then really ask: who are each of these people? How do we identify them clinically? And then what risks are modifiable and what is the best way to treat them? That would be so helpful. But unfortunately, it seems like researchers only get 90% of the way there.”
— Carrie Dennett, MPH, RDN, is the nutrition columnist for The Seattle Times, owner of Nutrition By Carrie, and author of Healthy for Your Life: A Non-Diet Approach to Optimal Well-Being.
References
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