This is the second in a three-part series on the obesity crisis. Part one tackles a complicated question – why does the obesity rate keep rising despite our efforts to stop it? – and can be found here. Part three shows how doctors and patients can make treatment better and can be found here.
In the mid-1980s, Louis J. Aronne, MD, strolled into a lab at Rockefeller University in New York where a colleague was breeding mice. “I will never forget what he showed me,” said Dr. Aronne, now the director of obesity research and treatment at Weill Cornell Medicine in New York. “He had a cage with 10 mice, one severely obese and the others normal weight. He took blood from one of the thin mice and gave it to the fat mouse.”
When Dr. Aronne returned 3 days later, that obese mouse had turned thin.
It was proof of something Dr. Aronne already suspected: Obesity had biological causes and wasn’t just a failure of willpower.
Years later, in 1994, that research led to the discovery of leptin, a hormone released from fat cells that’s involved in the regulation of body weight. It was a watershed moment in obesity research.
Since then, Dr. Aronne and others have worked to build the clinical field of obesity medicine, attempting to shift the public and medical view of obesity from a purely behavioral issue to a disease worthy of medical treatment.
All the while, the U.S. obesity rate soared.
Now, another watershed moment: We finally have highly effective obesity drugs. The hype is real, and so are the weight loss results.
“I’ve been saying for 30 years that when we find treatments that really work, people aren’t going to believe the results,” Dr. Aronne said. “It took longer than I expected, but it’s gratifying now to see.”
The big question
The emerging class of obesity medications known as GLP-1 agonists is indeed a game changer. The weight loss drug semaglutide (Ozempic, Wegovy) showed groundbreaking results, and studies suggest a parade of even more impressive drugs is on the way.
Yes, the drugs offer new hope to millions with obesity complications. But to truly turn the tide on our 42% obesity rate, much more work remains to be done, researchers said, including answering a big question:
How do these weight loss drugs work?
“We have new blockbuster drugs, and we don’t even know why they reduce body weight,” said Samuel Klein, MD, professor of medicine and nutritional science at Washington University in St. Louis. “It was by accident that this was discovered.”
Oops, we created a weight loss drug
Developed to treat diabetes, the GLP-1 drugs’ weight loss effects were a surprise. Now that those effects are confirmed, pharmaceutical companies and researchers are racing to figure out how these drugs work.
In the 1960s, scientists discovered the incretin effect – when you eat glucose (sugar), your body makes more insulin than it does if glucose is given intravenously. Glucose passes through the GI tract and the gut releases hormones that stimulate insulin secretion. It’s “essentially a feed-forward signal to your pancreas to tell it, ‘By the way, you need to be ready because there’s a bunch of glucose coming,’ ” said Randy Seeley, MD, director of the Michigan Nutrition Obesity Research Center, funded by the National Institutes of Health.
One of these hormones – or “incretins” – is GLP-1. In experiments, people with type 2 diabetes who were hooked up to GLP-1 saw their blood sugar go down.
“That led to the idea that if we could take this native hormone and make it last longer, we’d have a therapy for type 2 diabetes,” said Dr. Seeley. Thanks to a GLP-1-like compound in the saliva of the Gila monster, that idea became reality in the 2000s.
Along the way, a surprising side finding came to light: In early trials, diabetes patients on these drugs dropped weight.
Both Ozempic and Wegovy – brand names for semaglutide – are once-weekly injections (pill forms to treat obesity are on the way), but the latter is a higher dose.
“That dose results in about 40% of patients in the clinical trials achieving a 20% weight loss. We’ve just had nothing like that in terms of efficacy before,” said Dr. Seeley, who has worked with some of the drug companies (including Novo Nordisk, the maker of Ozempic and Wegovy, and Eli Lilly, maker of Mounjaro) that market the GLP-1s.
By contrast, semaglutide’s once-a-day predecessor liraglutide (Saxenda, made by Novo Nordisk) can lead to about 10% weight loss.
“And one of the ironies is, we don’t really know why,” Dr. Seeley said. “We don’t know why semaglutide is a better molecule for weight loss than liraglutide.”
Initially, scientists believed that the drugs, in addition to telling the pancreas to secrete more insulin, were also signaling the brain that you’re full. “Turns out that’s not really the way it works,” Dr. Seeley said. “GLP-1 made from your gut probably doesn’t get into your brain very much. But you make GLP-1 in your brain as well.”
For weight loss, it’s the brain’s GLP-1 system, not the gut’s, that the drugs are thought to hijack. But exactly which parts of the brain they affect and how is unknown. “That’s something lots of people are working on, including our own lab,” Dr. Seeley said. (Another surprise: The drugs may have potential as an anti-addiction treatment.)
The diabetes medication tirzepatide (Mounjaro), expected to be approved for weight loss as early as this year, is also a weekly injection, but it has a unique feature: It starts a response not just for GLP-1 but also for another incretin called GIP. Turns out, two is better than one: Trial participants on tirzepatide lost up to 22.5% of their body weight.
More of these hybrid drugs are on the way, Dr. Seeley said. In mid-stage clinical trials, the drug retatrutide, which targets three hormones, led to 24% weight loss. “The idea is the more bullets we can load into the gun, the more we can push the biology into a place where it’s easier to lose weight.”