Few medications have changed the treatment of obesity as dramatically as the new generation of GLP-1 drugs. Semaglutide transformed what we thought medication-assisted weight loss could accomplish. Tirzepatide subsequently produced even greater weight loss. And now retatrutide, an investigational drug in late-stage clinical development, is pushing those numbers still further.
For most of medical history, our advice for weight loss could be summarized as “Eat less, move more, good luck”.
This was technically correct in approximately the same way that telling someone with insomnia to sleep more is technically correct.
The problem was never the arithmetic. The problem was biology.
Human beings come equipped with an extraordinarily sophisticated collection of hormones, neurons and metabolic feedback loops designed to prevent starvation. Unfortunately, this machinery was engineered during a “feast or famine” era when dinner occasionally escaped on four legs. It is considerably less helpful when dinner arrives by DoorDash.
The new weight-loss medications work because they finally begin speaking the language of that ancient machinery. And what makes their evolution particularly fascinating is that each successive drug adds another hormonal message.
The progression can be summarized remarkably simply:
- Semaglutide: GLP-1
- Tirzepatide: GLP-1 + GIP
- Retatrutide: GLP-1 + GIP + glucagon
One hormone. Then two. Then three.
And the results have become progressively more remarkable.
Semaglutide: Starting With GLP-1
Semaglutide, marketed for obesity as Wegovy, is a GLP-1 receptor agonist.
GLP-1, or glucagon-like peptide-1, is one of the body’s natural incretin hormones. It is released from the intestine after we eat and essentially announces, “Lunch has arrived”.
GLP-1 then begins coordinating the response. It stimulates insulin secretion when glucose is elevated, reduces glucagon when blood sugar is already elevated thus preventing the liver from unnecessarily releasing additional glucose, slows gastric emptying and communicates with appetite-regulating centers in the brain.
That last effect turns out to be enormously important.
People taking semaglutide frequently describe something that is difficult to appreciate until you have seen it clinically – food simply becomes less interesting.
Hunger decreases. Portions become smaller. The persistent internal monologue about what to eat next, the phenomenon now popularly called “food noise”, often becomes quieter. The refrigerator hasn’t changed. The conversation with the refrigerator has.
The FDA describes GLP-1 as a physiological regulator of appetite and caloric intake, with GLP-1 receptors located in several areas of the brain involved in appetite regulation. And the clinical effect is substantial.
In the landmark STEP-1 trial, overweight or obese adults receiving semaglutide 2.4 mg weekly lost an average of 14.9% of their body weight over 68 weeks, compared with 2.4% with placebo. Half of those receiving semaglutide lost at least 15% of their body weight.
For an obesity medication, those numbers were extraordinary. Then along came tirzepatide.
Tirzepatide: Add GIP
Tirzepatide, marketed for obesity as Zepbound, retains GLP-1 receptor activity but adds activation of a second incretin receptor, GIP, or glucose-dependent insulinotropic polypeptide.
We therefore move from GLP-1 to GLP-1 + GIP.
Like GLP-1, GIP is released from the intestine in response to nutrients. It enhances glucose-dependent insulin secretion and participates in metabolic signaling involving the pancreas, adipose tissue and central nervous system.
GIP has always been a somewhat more complicated character in the metabolic story. It has effects on adipose tissue and energy storage that initially made activating its receptor seem like an odd strategy for treating obesity.
Biology, however, has an annoying habit of refusing to read our textbooks.
When GIP receptor activation is combined with GLP-1 receptor activation, something quite powerful happens. Appetite falls further, caloric intake decreases, insulin sensitivity improves and metabolic regulation becomes more effective.
Tirzepatide activates receptors for both hormones in brain regions involved in appetite regulation. It also delays gastric emptying, particularly early in treatment.
And the weight-loss numbers climbed again.
In the SURMOUNT-1 trial, people receiving the highest dose of tirzepatide lost an average of 20.9% of their body weight over 72 weeks. More than half lost at least 20% of their starting weight, and more than one-third lost at least 25%.
Suddenly we had moved from roughly 15% weight loss with GLP-1 to 21% weight loss with GLP-1 + GIP.
These were numbers once associated primarily with bariatric surgery. The pharmaceutical industry, apparently unimpressed, decided to add another hormone.
Retatrutide: Now Add Glucagon
Retatrutide takes the same idea another step.
It activates three receptors: GLP-1 + GIP + glucagon.
The first two mechanisms are familiar from tirzepatide. The intriguing newcomer is the glucagon receptor.
Most of us learned glucagon in medical school as insulin’s counter-regulatory sibling, the hormone that shows up when glucose gets low and tells the liver to put more glucose into circulation. True enough. But glucagon has other talents.
It influences energy expenditure, lipid metabolism and hepatic fuel utilization. Glucagon-receptor activation can encourage the body to mobilize and oxidize stored energy. And suddenly the architecture of retatrutide becomes very interesting.
- GLP-1 primarily helps reduce energy coming in by decreasing appetite and caloric intake.
- GIP complements and amplifies the incretin and metabolic effects.
- Glucagon potentially works on the other side of the equation, influencing energy expenditure and fuel utilization.
In very simplified terms:
- Semaglutide says: Eat less.
- Tirzepatide says: Eat less, and let’s improve the metabolic conversation while we’re at it.
- Retatrutide says: Eat less, coordinate metabolism differently, and perhaps turn up the metabolic furnace a little.
That third mechanism may help explain the extraordinary weight reductions being observed.
In the Phase 2 obesity trial, participants receiving 12 mg of retatrutide lost an average of 24.2% of their body weight after only 48 weeks. Remarkably, the weight-loss curve had not clearly plateaued when the study ended.
The subsequent Phase 3 results have been even more striking. In the 2026 TRIUMPH-1 trial, participants receiving 12 mg lost an average of 28.3% of their starting body weight at 80 weeks. Nearly half lost at least 30%.
Thirty percent! For perspective, a 250-pound person losing 30% of body weight weighs 175 pounds afterward.
We are no longer discussing the five or ten pound weight reductions that characterized much of the old pharmacologic treatment of obesity. We are talking about changing the physical architecture of a human being.
Retatrutide remains investigational and should not yet be placed in the same clinical category as FDA-approved semaglutide and tirzepatide. But it offers a remarkable glimpse of where obesity pharmacology may be heading.
The Stomach Isn’t the Whole Story
One persistent misconception about GLP-1 medications is that they work primarily because they make food sit around in the stomach longer. Delayed gastric emptying certainly contributes, particularly early in treatment. But if that were the whole story, we could presumably treat obesity by swallowing a cork.
The more consequential effects occur in the brain. These medications alter biological signals governing hunger, satiety, reward and caloric intake. That distinction matters because obesity has too often been treated as a character flaw wearing stretch pants. It isn’t.
Body weight is biologically defended through an intricate conversation among the hypothalamus, gastrointestinal tract, pancreas, adipose tissue, liver and circulating hormones. Lose weight and the body frequently responds by increasing hunger and decreasing energy expenditure.
From an evolutionary perspective, this is brilliant. From the perspective of someone trying to lose forty pounds, it is infuriating. These medications work because they intervene directly in that physiology.
The Injection Is Only Part of the Treatment
As remarkable as these medications are, there is a danger in allowing their success to make obesity treatment look deceptively simple.
Take the injection. Lose the weight. Problem solved. If only human physiology were that cooperative.
In our clinic, we are very experienced using semaglutide, tirzepatide and the evolving generation of metabolic medications. But prescribing the medication is not where we begin. Before reaching for a prescription, we try to understand why the patient gained weight in the first place and what may be preventing that person from losing it.
We look carefully at diet, not merely calories, but food quality, protein intake, meal patterns, alcohol, refined carbohydrates and the patient’s actual relationship with food. We look at physical activity and, importantly, how much of that activity challenges and preserves muscle. We look at sleep, because chronically inadequate or disrupted sleep can profoundly alter appetite, insulin sensitivity and metabolic health. We look at stress and the patient’s ability to manage it, because human beings have an impressive ability to turn cortisol, exhaustion and a bag of potato chips into a single metabolic event.
We also look beyond lifestyle. When clinically appropriate, we evaluate thyroid, cortisol and sex hormones, glucose and insulin metabolism, and gastrointestinal health. The goal isn’t to blame every unwanted pound on a hormone or the microbiome. It is to identify meaningful contributors that might otherwise be missed and to treat the patient rather than simply treating the bathroom scale.
Then, if a GLP-1-based medication is appropriate, we use it as part of a larger strategy.
This becomes particularly important because these drugs are so effective at suppressing appetite. A patient who is suddenly eating dramatically less food can lose fat beautifully, but can also lose muscle, protein, vitamins, minerals and nutritional quality if nobody is paying attention. Studies of modern obesity medications suggest that lean mass can account for roughly one-quarter of total weight lost, which makes preservation of muscle an important part of treatment rather than an afterthought.
That is why our program includes our nutrition and wellness coordinator who is also a certified personal trainer, yoga instructor and highly experienced in nutrition. Medication may make weight loss possible; nutrition and movement help determine what kind of weight is lost and what kind of body remains afterward.
We emphasize adequate protein and nutrient-dense foods. We encourage resistance training to preserve, or ideally improve, strength and functional muscle. Aerobic activity remains important for cardiovascular and metabolic health, while yoga, mobility work and other forms of movement can help patients build something far more valuable than a temporary exercise prescription – a way of moving that they can actually continue for the rest of their lives.
This isn’t merely philosophical. Current obesity-treatment recommendations specifically emphasize adequate protein intake and muscle-strengthening exercise during pharmacologic weight loss, and evidence suggests resistance training can substantially reduce the loss of lean tissue that otherwise accompanies caloric restriction.
We Are Trying to Build the Life After the Drug
The most important question in medical weight loss isn’t, “How much weight can we help someone lose?” It’s, “What happens next?”
This sounds strange in an article about weight-loss drugs, but weight loss itself is not the ultimate objective. Health is.
As body weight decreases, we commonly see improvements in blood pressure, insulin resistance, glycemic control, triglycerides, fatty liver disease, sleep apnea, inflammation and mobility. But because obesity is a chronic biological condition, many patients may appropriately remain on medication long term. Others will eventually reduce the dose or discontinue treatment. Weight regain after stopping semaglutide or tirzepatide is common, which tells us something important: the medication may have controlled the biology, but it did not permanently rewrite it.
That makes the months spent losing weight an extraordinary opportunity.
While the medication quiets hunger, we can improve food choices. While body weight is falling, we can build muscle. We can improve sleep. We can develop an exercise routine. We can address metabolic and hormonal problems when they are present. We can work on stress, habits and the thousand small decisions that ultimately determine health.
In other words, we use the medication to create breathing room while we work on everything else.
And powerful appetite suppression comes with a responsibility. If someone spontaneously eats 30 or 40 percent fewer calories, they may also consume substantially less protein, vitamins, minerals and other essential nutrients.
The bathroom scale is an idiot. It cannot distinguish between twenty pounds of fat and twenty pounds consisting of fat, muscle and bone. We can.
That means nutrition and resistance exercise become more important, not less, when these medications are used. Adequate protein intake matters. Strength training matters. Physical activity matters. Micronutrient quality matters.
The injection can make someone less hungry. It cannot lift the weights. It cannot choose the food. It cannot go to bed on time. It cannot manage stress, repair a dysfunctional relationship with eating, or build a sustainable life around movement and health.
Those things still belong to the patient, and to the healthcare team helping that patient succeed.
The goal should never simply be to manufacture a smaller human being. The goal is to produce a healthier, leaner, stronger one.
That is why the best use of these remarkable medications isn’t to replace lifestyle medicine.
It’s to finally give lifestyle medicine a powerful metabolic ally.
A New Era in Obesity Medicine
For decades, physicians told people struggling with obesity to eat less and exercise more while those patients fought a biological system exquisitely designed to prevent weight loss.
We handed them a calorie sheet and sent them into battle against several million years of evolution. Evolution usually won.
These medications have changed the battlefield. Semaglutide demonstrated what could happen when we harnessed one powerful gut-hormone pathway. Tirzepatide showed us what could happen when we engaged two. Retatrutide is now showing us what may be possible when we engage three.
And somewhere, presumably, a pharmaceutical chemist is staring thoughtfully at number four.
The story of these medications therefore isn’t simply about increasingly powerful injections. It’s about learning to speak the body’s own metabolic language.
For the first time, rather than merely shouting “eat less” at the human organism, we are beginning to understand the biochemical conversation that determines whether it wants to eat in the first place. And that may prove to be the more important revolution.
Author
Scott Rollins, MD, is Board Certified with the American Board of Family Practice and the American Board of Anti-Aging and Regenerative Medicine. He specializes in bioidentical hormone replacement for men and women, thyroid and adrenal disorders, fibromyalgia and other complex medical conditions. He is founder and medical director of the Integrative Medicine Center of Western Colorado (www.imcwc.com) and Bellezza Laser Aesthetics (www.bellezzalaser.com). Call (970) 245-6911 for an appointment or more information.
