More than 50 years ago, I began my career believing I’d spend my life studying addiction. I never imagined the journey would lead to obesity medicine and compulsive eating—from Gatorade to Ozempic.

Looking back, what surprises me most is not where the science ended up, but how often progress began with unexpected observations made caring for patients. One of my earliest mentors at the University of Florida was the late Robert Cade, M.D., the brilliant nephrologist who invented Gatorade to prevent dehydration in athletes.

Dr. Cade taught me that a scientifically perfect oral rehydration solution accomplishes nothing if people refuse to drink it. After listening to his wife’s advice, he added lemon flavoring and later carbohydrates to his electrolyte mixture, making Gatorade useful and palatable. This always reminded me that physiology and behavior are inseparable.

I certainly never imagined that, decades later, gut physiology would become central to my own work through GLP-1 biology. But Cade’s lesson stayed with me.

In 1969, Woodstock proved an unexpected natural experiment in competing rewards. Food, music, sex, social connections, and psychoactive drugs all competed for attention. I found myself wondering whether the brain prioritized drugs over natural rewards. I had no way to test that idea, but the question never left me.

During my Yale fellowship in the 1970s, I became immersed in the neurobiology of addiction, studying opioid actions, withdrawal, craving, and compulsive drug use. One of my mentors, Eugene Redmond, M.D., conducted research on the island of St. Kitts. He described watching free-ranging vervet monkeys actively seeking fermented sugar cane and even soft drinks provided by local residents. Because fermented sugar cane naturally contains alcohol, their attraction to both suggested that sugar and alcohol might engage related brain reward processes.

Meanwhile, my patients at Yale taught me equally important lessons. At Herb Kleber’s New Haven storefront addiction clinics, cigarette smokers gained weight after quitting. Conversely, cocaine users often forget to eat. Patients withdrawing from opioids developed intense cravings for sweets. During a New Haven blizzard interrupting daily methadone treatment, some patients described eating large amounts of sugar, drinking vodka, or even recreating the ritual of heroin use by heating sugar cubes in spoons to ease withdrawal. Again and again, food and drugs appeared to substitute for one another in remarkably predictable ways.

The opioid, cocaine, and speedball epidemics created another unexpected observation. Residential programs originally designed for alcoholism found many patients developed severe overeating and substantial weight gain after detoxification. Recovery programs soon added nutrition classes, healthier food choices, and treatment for overeating. Clinical experience repeatedly suggested addiction and eating behavior were closely intertwined.

My friend and colleague, Bart Hoebel, at Princeton performed pioneering experiments demonstrating that sugar activates many of the same dopamine circuits engaged by addictive drugs. His laboratory became an ideal place to explore ideas originating in addiction neurobiology and treatment. Those studies grew into my collaborations with Bart and his mentee, Nicole Avena. Elegant animal models showed that intermittent access to sugar could produce behaviors resembling addiction. Highly palatable foods were far more than calories; they engaged many of the same reward systems recruited by cocaine and other drugs of abuse. Later, as Nicole’s mentor on her NIDA K01 Career Development Award examining pharmacotherapies for food addiction, we explored the possibility that some forms of binge eating share important behavioral and neurobiological features with addiction.

Kelly Brownell moved these ideas into mainstream obesity research. We brought together leading addiction scientists and obesity researchers at the Yale Historic Conference on Food and Addiction, challenging us to think differently about overeating. Those discussions ultimately led to our Oxford textbook, Food and Addiction: A Comprehensive Handbook, with 66 chapters written by 100+ experts, helping establish food addiction as a legitimate scientific field.

Another important advance came through Kelly’s mentee, Ashley Gearhardt, who helped organize the historic conference and later developed the Yale Food Addiction Scale. For the first time, researchers had a standardized way to study addictive eating behaviors in humans. Our collaborations additionally reinforced the close relationship between smoking, appetite, body weight, and food reward. Smoking suppresses appetite; smoking cessation commonly leads to weight gain, and both nicotine and highly palatable foods engage overlapping motivational circuitry.

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My interests gradually expanded into obesity treatment itself. Collaborating with U.F. bariatric surgeons Juan Cendan, M.D., and Robert Rout, M.D., we studied surgical and behavioral interventions, medications including metformin and fenfluramine-phentermine, and gastric stimulation devices. At the time, many surgeons and endocrinologists believed the key to obesity lay primarily in the gut. I believed the answer resided mainly in the brain. I hoped medications developed for addiction—particularly combinations such as bupropion and naltrexone—might also help treat hedonic overeating and binge eating since binge eating shares important behavioral and neurobiological features with addiction, including craving, loss of control, compulsive consumption despite adverse consequences, and relapse.

The eventual FDA approval of the bupropion-naltrexone combination (Contrave) provided one of the first clinical demonstrations that anti-addiction medications acting on reward pathways could also reduce overeating.

But where I was really wrong—and where my obesity colleagues were right—was the importance of the gut-brain pathway.

The Gut Changed the Story

I expected addiction neuroscience to transform obesity medicine. Instead, obesity research transformed addiction neuroscience.

Like many addiction researchers, I focused on dopamine, reward, reinforcement, and cortical control. We understood motivation reasonably well but knew comparatively little about the remarkable communication between the gut and the brain. The neurobiology of GLP-1 signaling, hormonal regulation of reward, and dopamine modulation had yet to emerge.

GLP-1 receptor agonists like Ozempic and Mounjaro completed the picture. They revealed that metabolic signals originating in the gut influence neural circuits governing reward, craving, decision-making, and reinforcement. The brain’s reward circuitry had always been only one part of a much larger integrated system. Early studies suggest these medications also reduce alcohol consumption and alcohol-related harms, providing further evidence that gut-brain signaling influences reward processes across both overeating and substance use disorders.

Looking back, I no longer think addiction researchers and obesity researchers were studying different diseases. We were studying the same remarkably complex biological system from different directions. Addiction researchers emphasized craving and reinforcement. Eating-disorder researchers focused on binge eating and loss of control. Obesity researchers emphasized appetite and metabolism. Behavioral scientists studied learning. Neuroscientists mapped dopamine circuits. Endocrinologists unraveled gut hormones. Only recently have these perspectives begun to converge.

One lesson has remained constant throughout my career. Scientific progress rarely follows a straight line from laboratory discovery to patient care. More often than not, patients reveal something unexpected. Those observations inspire laboratory experiments. New discoveries lead to better treatments, which generate new clinical questions and send us back to the laboratory. That cycle—from bedside to bench and back again—has defined my career.

I now realize many observations that first appeared unrelated were actually pieces of the same puzzle. Together they revealed a common biology of reward that links addiction, food, obesity, B.E.D., and now GLP-1 therapeutics.

What once seemed like separate fields increasingly look like different windows onto the same biology.