Millions of adults in the United States and the United Kingdom may be living with undiagnosed obesity, according to a wave of groundbreaking studies published in October 2025. These new findings, reported by JAMA Network Open, Communications Medicine, and other major outlets, suggest that traditional ways of measuring obesity—mostly relying on the timeworn body-mass index (BMI)—are missing a huge swath of people at risk. The culprit? Hidden fat lurking around the waist and deep inside organs, invisible to the bathroom scale but dangerous all the same.
For decades, doctors and health officials have used BMI—a simple ratio of weight to height—to determine whether someone is underweight, healthy, overweight, or obese. Under the current system, a BMI of 30 or above marks the threshold for obesity. But as scientists dig deeper into the nuances of body composition, it’s becoming clear that BMI alone doesn’t tell the full story. In fact, it may be missing millions who are at risk for diabetes, heart disease, and even early death.
According to a major study led by Dr. Lindsay Fourman at Massachusetts General Hospital and published in JAMA Network Open on October 15, 2025, incorporating waist measurements and fat distribution into the definition of obesity would catapult U.S. obesity rates from 42.9% to a staggering 68.6%. That’s nearly seven in ten adults—up from less than half. The study analyzed health data from over 300,000 Americans enrolled in the All of Us research program between 2017 and 2023, tracking them for four years to see how weight and fat patterns translated into real health outcomes.
“The prevalence of obesity increased by 60 percent when using the new definition compared with the traditional BMI-based one,” the study authors wrote, as reported by The Daily Mail. The new framework, endorsed by 76 professional medical organizations and supported by a 2025 consensus from global experts, goes beyond BMI. It considers not just how much you weigh, but where you carry your fat—especially around the belly and organs.
Why does this matter? Because fat that settles deep in the abdomen (visceral fat) and liver (hepatic fat) is far more dangerous than the kind that sits just beneath the skin. As explained by researchers from McMaster University in Ontario, Canada, in a study published in Communications Medicine, this so-called “skinny-fat” phenomenon means even those who appear slim can have unhealthy fat deposits putting them at risk. “This study shows that even after accounting for traditional cardiovascular risk factors like cholesterol and blood pressure, visceral and liver fat still contribute to artery damage,” co-lead author Russell de Souza told The Independent. “The findings are a wake-up call for clinicians and the public alike.”
Using MRI scans and data from more than 33,000 adults in the UK and Canada, the McMaster team found that visceral and liver fat are strongly linked to the thickening and clogging of carotid arteries—the main blood vessels to the brain. This narrowing is a key predictor of stroke and heart attack. And crucially, these associations held up even after adjusting for lifestyle factors and metabolic risks. In other words, you can’t just look at someone’s waistline or BMI and know if they’re safe.
Dr. Sonia Anand, a vascular medicine specialist and corresponding author of the McMaster study, emphasized, “You can’t always tell by looking at someone whether they have visceral or liver fat. This kind of fat is metabolically active and dangerous; it’s linked to inflammation and artery damage even in people who aren’t visibly overweight.”
The American study led by Dr. Fourman underscores this point. Researchers identified a group they called “anthropometric-only obesity”—people with normal BMI scores but elevated waist measurements. Among 78,047 such participants, more than one in five had BMI scores labeled as normal or underweight. But their health outcomes told a different story: compared to people without obesity, those with this hidden form of belly fat were 76% more likely to develop organ problems, more than twice as likely to get diabetes, and had a 55% higher risk of heart disease. High blood pressure, sleep apnea, and physical limitations were also more common in this group.
Age and ethnicity play a role as well. The JAMA Network Open study revealed that older adults saw the biggest jump in obesity rates under the new definition—78.3% of Americans over 70 would now be classified as obese, with more than half showing signs of organ dysfunction or physical limitations. Asian participants experienced a 90.3% increase in obesity classification, from 27.0% to 51.4%, reflecting how traditional BMI thresholds may not account for ethnic differences in fat distribution. Men were also more likely than women to have hidden (anthropometric-only) obesity: 32.5% versus 21.7%.
Across the Atlantic, British researchers echoed these concerns. As reported by The Daily Mail, millions of Britons who have been told they are perfectly healthy could in fact be obese under the new definition. The proposed overhaul would expand the number of people classed as obese by nearly 60%, potentially raising the UK’s obesity tally from 13 million to almost 21 million adults. The NHS currently defines an unhealthy waist circumference as 37 inches or more for men and 31.5 inches or more for women. But the new guidelines also factor in waist-to-height ratio, so even those with average BMIs could be flagged as obese if their waistlines are disproportionately large.
Experts argue that BMI charts alone are not nuanced enough to capture health risks related to fat distribution. “Visceral adiposity can cause coronary artery disease even in people with a normal or near-normal BMI. This is more common in specific ethnic groups like South Asians,” said Alex Miras, professor of endocrinology at Ulster University, speaking to The Independent. He added, “Any form of weight loss, through behavioural interventions or obesity pharmacotherapy, is likely to have a positive effect in reducing cardiovascular disease. They also need to have their cholesterol and blood pressure aggressively treated.”
The implications for healthcare policy and clinical practice are enormous. Under the new framework, eligibility for obesity treatments—including medications and surgery—would shift. Some people currently eligible for weight-loss drugs might be downgraded to lower priority, while others with serious hidden obesity could be excluded under current guidelines. The new approach introduces two categories: “clinical obesity,” where health problems or physical limitations are already present, and “preclinical obesity,” where concerning fat patterns exist but serious health issues have not yet developed. Both carry heightened risks for future diabetes, heart disease, and death, but the distinction could help tailor treatment strategies more precisely.
For governments, insurers, and public health officials, these findings are a wake-up call. In England, a raft of anti-obesity measures has already come into force, banning “buy one, get one free” deals on unhealthy snacks and imposing stricter advertising rules. But as obesity rates climb and the definition of risk expands, more comprehensive strategies may be needed—ranging from improved screening to better access to weight management programs and medications.
Ultimately, these studies challenge us all to rethink what it means to be healthy. The scale may not always tell the truth, and hidden fat can quietly undermine health for years before problems arise. For millions, the message is clear: pay attention to your waistline, not just your weight, and don’t assume that a “normal” BMI means you’re in the clear.
