Obesity has long been a word wrapped in stigma, controversy, and complexity. For decades, its definition seemed straightforward: a simple calculation of weight relative to height, known as the Body Mass Index (BMI). If your BMI tipped over a certain threshold, you were labeled obese—a term heavy with clinical implications and social connotations.
But what if the way we’ve been defining obesity is fundamentally flawed?
In a groundbreaking international study published in PLOS Global Public Health, researchers from six institutions across four continents—including Emory University, Johns Hopkins University, and universities in China, Australia, and Peru—have revealed the potential consequences of redefining what it means to be obese. The results are startling and, for some, unsettling. Under the proposed new criteria, which would require the presence of at least one obesity-related health condition such as diabetes, hypertension, or high cholesterol, the global prevalence of obesity would drop dramatically—by more than 50% in some countries.
On the surface, that might sound like progress. But for public health experts, it raises red flags.
Beyond the Scale: Why BMI Might Not Be Enough
The Body Mass Index has been both a tool and a trap. Developed nearly 200 years ago by Belgian mathematician Adolphe Quetelet, BMI was never intended to be a diagnostic measure. It was a population-level statistical tool. Yet over time, it morphed into a clinical litmus test—used by doctors, insurers, and policymakers to determine not just physical health, but eligibility for treatment, insurance coverage, even surgery.
Critics have long warned that BMI is a blunt instrument. It doesn’t differentiate between muscle and fat, doesn’t account for fat distribution, age, ethnicity, or metabolic health. A muscular athlete might be classified as obese, while a person with high visceral fat but normal BMI might be overlooked. Still, in the absence of a better metric, BMI has persisted.
That’s where the new proposed definition comes in. It attempts to shift the focus from body size alone to metabolic health. The logic is intuitive: not everyone with a high BMI has poor health outcomes, and not everyone with a lower BMI is metabolically healthy. So why not reserve the “obese” label for people already showing signs of health complications?
But as the multinational research team discovered, changing the definition comes at a cost.
An Artificial Drop: The Risks of Redefining Health
The study analyzed data from 142,250 adults across 56 countries, comparing current obesity prevalence based on BMI with what the numbers would look like if the new, stricter criteria were adopted. The outcome was a seismic shift.
In countries like Malawi, obesity prevalence among men dropped by 68%, and among women by 53%. Other nations saw similarly dramatic declines. At first glance, these numbers might look like a public health victory. But Dr. Rodrigo M. Carrillo-Larco, the study’s lead author and a global health expert at Emory University, warns that the decrease is an illusion.
“At the population level we would expect the prevalence of obesity to drop, but we should be mindful that it’s an artificial drop,” Carrillo-Larco said. “It’s not real. The definition is just more strict.”
And therein lies the danger. By waiting until someone already has a condition like diabetes or hypertension to classify them as obese, the new definition could delay intervention—missing the window where lifestyle changes can be most effective. Prevention, after all, is only powerful when it happens before disease takes hold.
False Reassurance in a Dangerous World
For individuals who are currently considered obese based solely on their BMI, the new definition could create what researchers call a “false sense of security.” If their classification changes, they might believe they’re suddenly healthy—even if their metabolic risk remains high.
Carrillo-Larco puts it plainly: “Just because people who have high BMI but no additional comorbidities would not be considered clinically obese, it does not mean that by some miracle their risk has reduced.”
This nuance matters deeply in real-world care. A patient with a BMI of 32—technically obese—may not yet have high blood pressure or type 2 diabetes, but their trajectory points clearly in that direction. Under the current guidelines, that patient can access weight management support, preventive care, and insurance-covered interventions. Under the proposed definition, they might be told to wait—until symptoms emerge, and the disease is harder to manage.
The Equity Equation: Who Gets Left Behind?
The consequences of redefining obesity don’t fall evenly across the globe—or even within nations. The study found gender disparities in how the new criteria would apply. In some countries, women saw smaller reductions in obesity prevalence than men. That might seem like good news for women’s access to care, but it also reflects underlying structural inequalities in how health conditions manifest, are reported, and are treated across genders.
Furthermore, low- and middle-income countries face unique challenges. In places where healthcare infrastructure is under-resourced, requiring clinical tests to determine obesity status—such as blood work for cholesterol or A1C for diabetes—could mean millions of people are left undiagnosed and untreated. Measuring height and weight is relatively easy, but diagnosing comorbidities? That requires labs, equipment, trained personnel—resources that are often scarce.
Shifting the definition could therefore widen health disparities, disproportionately impacting vulnerable populations.
What Defines a Disease? The Philosophical Weight of Obesity
At the heart of this debate lies a deeper question: What defines a disease?
If we wait for symptoms to appear, have we already failed the patient? Can we truly promote wellness if we don’t count risk factors as part of the diagnosis?
Obesity, unlike many other conditions, is visible. But it is also incredibly misunderstood. It’s influenced by genetics, environment, socioeconomic status, mental health, diet, and physical activity. It’s not simply a matter of willpower. And it doesn’t always follow a straight path from excess weight to illness. Two people with the same BMI may have radically different health profiles.
By redefining obesity based on comorbidities, the new standard may help align terminology with disease severity. But it may also subtly reinforce the myth that weight is only a problem if it leads to illness—as if the biological changes occurring silently beneath the surface don’t matter until they reach crisis level.
A Middle Path Forward?
Public health, like science, is not static. It evolves. It must.
The researchers behind this study are not advocating for blind adherence to the status quo. They acknowledge the limitations of BMI and the value of better, more holistic definitions. But they urge caution—deep, evidence-based caution—before rewriting the world’s obesity guidelines.
Carrillo-Larco emphasizes the importance of balance: “If we’re going to use the definition at some point, we need to remember that doesn’t mean preventing obesity and the associated comorbidities are not important.”
Prevention must remain a pillar. Whether someone meets the strict new criteria or not, healthy eating, physical activity, and routine monitoring should remain cornerstones of care.
The Road Ahead: Science in the Service of Humanity
Ultimately, the goal of medicine is not just to name diseases but to alleviate suffering, prevent illness, and empower individuals to live healthier lives.
This global study is not just about numbers—it’s about lives. It’s about the mother in rural Peru who needs early intervention to avoid diabetes. It’s about the office worker in China quietly developing high cholesterol while still falling under the BMI radar. It’s about the millions of people who don’t yet show symptoms but are walking a path that leads inevitably to them.
The definition of obesity may change in the coming years. It will undoubtedly continue to evolve as science deepens our understanding. But one thing must not change: our commitment to early care, equitable access, and compassion-driven policy.
Because beneath the debate over definitions is a simple truth: health cannot wait.
Reference: Rodrigo M. Carrillo-Larco et al, Prevalence of pre-clinical and clinical obesity in adults: Pooled analysis of 56 population-based national health surveys, PLOS Global Public Health (2025). DOI: 10.1371/journal.pgph.0004838