Stigma to Science: Yale’s Role in Changing Our Understanding of Obesity and Its Treatment

For much of modern medical history, obesity was treated as a failure of willpower. Physicians commonly advised patients to “eat less and move more,” reflecting a calorie-balance model that underestimated the complexity of human metabolism.

Today, obesity is recognized as a chronic, relapsing neurometabolic disease— a condition rooted in how the brain and metabolism regulate hunger, energy use, and fat storage. That shift reflects decades of research into metabolism, endocrine signaling, and brain–gut communication—much of it advanced by investigators at Yale.

Long before the era of GLP-1 medications, Yale scientists were helping redefine how metabolism works.

In the 1980s and 1990s, Gerald I. Shulman, MD, PhD, George R. Cowgill Professor of Medicine (Endocrinology) and professor of cellular and molecular physiology, and colleagues developed and refined methods to measure insulin resistance, a condition in which the body’s cells do not respond properly to insulin, the hormone that regulates blood sugar. Their work demonstrated how excess adipose tissue—fat—disrupts cellular energy signaling and contributes to type 2 diabetes and cardiovascular disease. Obesity was not merely excess weight; it was a state of altered metabolic regulation.

At the same time, Robert S. Sherwin, MD, C.N.H. Long Professor of Medicine, Emeritus, and collaborators were clarifying how the brain and peripheral organs communicate to regulate glucose production and energy balance. Their research helped show that metabolism is centrally regulated through hormonal and neural feedback loops, not simply determined by calorie intake.

Together, these discoveries helped establish the biological framework that underpins modern obesity treatment: body weight is actively regulated by powerful physiological systems.

As metabolic science advanced, obesity prevalence steadily rose.

According to the World Health Organization, in 2022 approximately one in eight adults worldwide were living with obesity. Researchers had been documenting the steady rise for decades, well before it reached today’s global scale. Long-term epidemiologic studies helped confirm that obesity is associated with increased risk for cardiovascular disease, stroke, type 2 diabetes, certain cancers, liver disease, and joint disorders, as well as depression and anxiety.

Even modest weight loss—5% to 10% of body weight—can significantly improve metabolic health. Yet sustained weight reduction proved difficult, not because of weak resolve, but because the body resists it.

As researchers deepened their understanding of endocrine regulation, a more complex picture emerged.

The body maintains what scientists call a “defended fat mass set point”—the body’s tendency to protect a certain level of stored fat. When calorie intake is reduced, compensatory mechanisms activate: hunger hormones increase, metabolism slows, and neural circuits amplify the drive to eat. These changes can persist long after weight loss begins.

Historically, weight loss meant starvation. The same survival mechanisms that once protected humans from famine now make long-term weight reduction difficult in calorie-dense environments.

This evolving science reframed obesity from a lifestyle issue to a disorder of metabolic signaling.

Insights from bariatric surgery further strengthened the biological model.

Patients undergoing weight-loss surgery often experienced improvements in blood glucose control within days—before significant weight loss occurred. This suggested that hormonal signals from the gut, not just calorie restriction, were altering metabolism.

One of those hormones, glucagon-like peptide-1 (GLP-1), became a central focus.

Initially approved by the U.S. Food and Drug Administration in 2005 for type 2 diabetes, GLP-1 receptor agonists mimic a naturally occurring hormone that regulates blood sugar, slows gastric emptying, and reduces appetite. Clinicians soon observed significant weight loss in patients receiving these medications.

In 2017, semaglutide (Ozempic) was approved for diabetes. A higher-dose formulation, Wegovy, was approved for obesity treatment in 2021 after clinical trials demonstrated average weight reductions of approximately 15%—results previously achievable primarily through bariatric surgery.

The next advance came with tirzepatide, which targets both GLP-1 and glucose-dependent insulinotropic polypeptide (GIP), another hormone involved in blood sugar and appetite regulation. In NIH-funded trials led by Ania Jastreboff, MD, PhD, associate professor of medicine (endocrinology) and of pediatrics (pediatrics endocrinology) at Yale School of Medicine, patients achieved weight reductions exceeding 20% of body weight—comparable to surgical outcomes. Tirzepatide is now approved under the brand names Mounjaro and Zepbound for type 2 diabetes and obesity.

These results represented the clinical culmination of decades of metabolic science. Medications were no longer simply suppressing appetite; they were targeting the biological systems that regulate energy balance.

Obesity arises from a convergence of biological and environmental forces.

Modern environments are increasingly “obesogenic.” Sedentary lifestyles, chronic stress, sleep disruption, medications that promote weight gain, and the widespread availability of ultraprocessed foods all contribute. These foods digest rapidly, spike blood glucose, stimulate insulin release, and promote energy storage.

Environmental exposures may also play a role. A 2018 Yale study led by John Morton, MD, professor of surgery (bariatric, minimally invasive), found that individuals with higher blood levels of bisphenol A (BPA)—a chemical used in plastics—lost less weight following bariatric surgery, suggesting that endocrine-disrupting compounds may influence metabolic outcomes.

In 2023, Yale launched the Obesity Research Center (Y-Weight) under the direction of Jastreboff, formalizing decades of Yale research into obesity and metabolic disease.

The center integrates discovery science, human physiology studies, clinical trials, and outcomes research. Investigators are studying how anti-obesity medications affect brain signaling, examining long-term safety and cardiovascular outcomes, and developing next-generation dual- and triple-hormone receptor agonists.

The goal is not simply to reach a target number on a scale. Yale clinicians emphasize improving metabolic health, reducing complications, and addressing stigma—recognizing that successful treatment differs from patient to patient.

Semaglutide and tirzepatide mark the beginning of a transformation in obesity care. More than a dozen hormone-based therapies are now in clinical development, including agents designed to preserve lean muscle mass while promoting fat loss.

At Yale School of Medicine, decades of research—from insulin resistance and brain-glucose signaling to gut hormone biology and clinical trials—have reshaped how obesity is understood and treated.

What was once framed as a failure of discipline is now recognized as a complex biological disease. At Yale, the science—and the treatment—continue to evolve.