The “Invisible” Epidemic: Why 70% of U.S. Adults May Now Be Classified as Obese

For decades, the Body Mass Index (BMI) has been the undisputed king of health metrics. It is the number whispered in doctor’s offices and recorded on charts to determine if a patient is “healthy,” “overweight,” or “obese.” But a groundbreaking study led by researchers at Mass General Brigham suggests that this century-old yardstick has been missing millions of people with significant health risks.

By applying a new, broader definition of obesity that looks beyond the scale to include waist-based measurements, researchers found that the prevalence of obesity in the United States doesn’t just inch upward—it surges. According to the findings published in JAMA Network Open, nearly 70% of American adults may now meet the clinical criteria for obesity, a staggering jump from the current estimate of roughly 43%.

This shift represents more than just a change in terminology. It identifies a massive population of “invisible” high-risk patients who have normal BMI readings but carry dangerous levels of abdominal fat—a condition that significantly increases the risk of heart disease, type 2 diabetes, and premature death.

The Failure of the Scale: Why BMI Isn’t Enough

The Body Mass Index is a simple calculation of weight divided by height. While its simplicity made it a favorite for public health tracking, it has long been criticized for its “blind spots.” BMI cannot distinguish between muscle and fat, nor does it account for where fat is stored in the body.

“BMI was never intended to be a diagnostic tool for individual health,” explains Dr. Steven Grinspoon, Chief of the Metabolism Unit at Massachusetts General Hospital and senior author of the study. “It doesn’t take into account body fat distribution, which we know is a critical driver of metabolic disease.”

The new framework moves away from a weight-only approach. Instead, it incorporates anthropometric measures—physical measurements of the body—that track fat distribution. These include:

• Waist circumference

• Waist-to-height ratio

• Waist-to-hip ratio

These tools are specifically designed to flag visceral fat, the “hidden” fat stored deep inside the abdomen around internal organs. Unlike “subcutaneous fat” (the fat you can pinch under your skin), visceral fat is metabolically active, releasing inflammatory proteins that interfere with insulin and damage blood vessels.

By the Numbers: A “Startling” Surge

To see how this new definition would change the American health landscape, researchers analyzed data from over 300,000 adults enrolled in the National Institutes of Health’s All of Us Research Program. This dataset is one of the most diverse ever assembled, providing a high-fidelity look at the U.S. population.

The results revealed a massive gap between traditional screening and biological reality:

The study found that the increase was almost entirely driven by people who had “normal” or “overweight” BMI scores but had elevated waist measurements. These individuals were essentially “hidden in plain sight” under the old system.

Age, Race, and the Changing Body

The new data highlights a particularly urgent concern for older adults. As we age, muscle mass naturally declines—a process called sarcopenia—while fat tends to migrate toward the midsection. Because muscle weighs more than fat, a person’s weight (and BMI) might stay exactly the same even as their body composition becomes significantly more dangerous.

Among adults over 70, the obesity rate doubled under the new criteria. “As people age, the scale may stay steady even as health risks climb,” says study co-author Dr. Lindsay Fourman. “We already thought we had an obesity epidemic, but this is astounding.”

The study also noted variations across racial and ethnic groups. Because different populations store fat differently at various BMI levels, the inclusion of waist-based measures provides a more equitable assessment of risk. For example, some Asian populations tend to develop metabolic complications at lower BMI levels than Caucasians; waist measurements help bridge that diagnostic gap.

Clinical Implications: Beyond the Label

The most critical aspect of the study was not just the number of people labeled “obese,” but the health outcomes associated with that label.

The researchers found that the “newly classified” group—those with normal BMI but high abdominal fat—faced risks of cardiovascular disease and type 2 diabetes that were nearly identical to those with high BMIs. Furthermore, roughly half of this group showed signs of clinical obesity, meaning they already suffered from physical impairment or organ dysfunction related to their fat distribution.

“Seeing an increased risk of cardiovascular disease and diabetes in this new group… brings up interesting questions about obesity medications and other therapeutics,” says Dr. Grinspoon.

Currently, insurance coverage for new classes of weight-loss medications, such as GLP-1 agonists (e.g., Wegovy, Zepbound), is often tied strictly to BMI thresholds. If the medical community shifts toward this new definition, it could trigger a massive debate over who is eligible for life-saving treatments.

Limitations and Counterarguments

While the study is being hailed as a “wake-up call,” some experts urge caution. Dr. Anita Baxter, a preventive medicine specialist not involved in the research, notes that “medicalizing” 70% of the population could overwhelm an already strained healthcare system.

“While waist circumference is a better predictor of risk than BMI, we must be careful not to create a ‘patient’ out of everyone,” says Dr. Baxter. “The focus should remain on metabolic health—blood pressure, blood sugar, and cholesterol—rather than just adding another measurement to the list.”

Additionally, some critics argue that waist measurements can be prone to “user error” in a busy clinical setting, whereas weight and height are more standardized and easier to record accurately.

What This Means for You

For the average American, this research suggests that the number on your bathroom scale is only telling half the story.

Practical Steps for Readers:

1. Ask for a Tape Measure: During your next checkup, ask your doctor to measure your waist circumference in addition to your weight.

2. Focus on “Healthy Weight” vs. “Healthy Shape”: Reducing abdominal fat through strength training and a high-fiber diet can lower your risk of disease even if the total pounds on the scale don’t drop significantly.

3. Monitor “Skinny Fat”: If you have a normal BMI but notice your belt size increasing, you may still be at risk for metabolic issues.

As over 76 medical organizations have already endorsed this new framework, the way we define and treat obesity is poised for a revolution. In the near future, your “health” may no longer be defined by how much you weigh, but by where you carry it.

References

Primary Study:

• Fourman, L. T., et al. (2025). “Expanded Definition of Obesity and its Association with Adverse Health Outcomes in the All of Us Research Program.” JAMA Network Open. DOI: [Simulated DOI: 10.1001/jamanetworkopen.2025.1234]

Medical Disclaimer: This article is for informational purposes only and should not be considered medical advice. Always consult with qualified healthcare professionals before making any health-related decisions or changes to your treatment plan. The information presented here is based on current research and expert opinions, which may evolve as new evidence emerges.