In brief: in the TRIUMPH-1 trial, retatrutide reduced knee osteoarthritis pain in people with obesity over 80 weeks. That result does not establish cartilage regeneration, and the drug is investigational, not MHRA-authorised in the UK. [1] [2]

Osteoarthritis of the knee has long been framed as a problem confined to the joint. The cartilage deteriorates, the knee starts to hurt, and a proportion of patients head towards a replacement.

I believe that picture is now too limited.

Recent trial data on retatrutide show clear falls in knee pain together with marked weight loss. What strikes me is not merely that a new obesity drug may ease sore knees. It is that the condition is increasingly understood through a broader frame: metabolic state, body composition, inflammation, how well the muscles work and the mechanical forces on the joint, in addition to the cartilage.

That opens up a field worth watching: metabolic treatment for knee osteoarthritis.

Much of my working life has gone into one question: how do we protect a joint instead of waiting until it has to be replaced? Metabolic health may turn out to be one more piece of the answer.

What the TRIUMPH-1 knee osteoarthritis data tell us

The headline is plain: retatrutide lowered knee pain.

TRIUMPH-1 enrolled people with obesity. Within its knee osteoarthritis subgroup, those given retatrutide reported significantly bigger improvements than those given placebo on the WOMAC knee pain score across 80 weeks. WOMAC is a questionnaire that scores osteoarthritis symptoms, pain among them. [1]

The findings apply to the people who were studied. They cannot be assumed to carry over to someone at a healthy weight who has a focal cartilage injury or a torn meniscus, and an average across a group never guarantees what will happen to you personally.

Nor should anyone read this as evidence that retatrutide regrows cartilage, or that every person with knee arthritis ought to take it. It remains an investigational medicine, without MHRA authorisation for use in the UK. [2]

Even so, the signal is meaningful. We usually ask, “What can be injected into this knee?” Perhaps we should also ask a second question:

What could we change in the whole person to give the knee a better setting in which to cope?

Why “wear and tear” undersells the condition

The phrase “wear and tear” has never sat comfortably with me. A knee is living tissue within a living body.

Your symptoms depend on cartilage and bone, yes, but also on the synovium (the lining of the joint), the surrounding muscle, the loads you place through the knee, inflammation, your body composition and your general metabolic state.

Obesity shows this well. Extra body mass raises the forces passing through the knee. But the link with osteoarthritis is not only mechanical: fat tissue is hormonally and metabolically busy, and inflammatory and metabolic pathways now figure more and more in our understanding of the disease.

For that reason I see metabolic orthopaedics as a potentially valuable strand of joint preservation.

The aim is bigger than shedding kilograms so the knee carries less. A fuller aim would combine several things:

Better metabolic health, less excess fat, muscle that is kept or improved, better movement, and treatment of the joint itself when it is needed.

That is a different way of thinking about the problem.

What makes retatrutide scientifically interesting

Retatrutide is a triple agonist. It stimulates receptors for GLP-1, GIP and glucagon, so a single medicine engages three hormone pathways that influence metabolism. [3]

The three hormones in plain terms

GLP-1 plays a role in appetite and, when blood glucose is high, in prompting insulin release. GIP likewise supports insulin release that depends on glucose. Retatrutide pairs those effects with stimulation of the glucagon receptor. [3]

Precision matters here, because glucagon and insulin do different jobs in controlling blood glucose.

Insulin helps the body take up and store glucose. It brings blood glucose down by driving glucose out of the bloodstream and into tissues such as muscle, and by dampening the liver’s own glucose output.

Glucagon keeps blood glucose steady between meals. It prompts the liver to release glucose, and it also affects how fat and amino acids are handled.

So retatrutide does not imitate insulin. It switches on GLP-1, GIP and glucagon receptors at once, which produces a combined metabolic effect.

The glucagon question

Glucagon draws interest because it may add to energy expenditure (the energy the body burns) and alter how fuel is used. In early retatrutide work in mice, a glucagon-related rise in energy expenditure was seen. That explains the thinking behind the design. It does not show how far this mechanism drives lasting weight loss in people. [3]

I would also avoid saying that glucagon “activates muscle”. At present there is no human evidence that retatrutide directly switches on or strengthens skeletal muscle.

Muscle during weight loss

Muscle still deserves a central place in this discussion.

When a patient loses a large amount of weight, my goal is not a smaller version of the same person. I want to understand what is happening to their muscle, power and mobility.

For a painful knee that is crucial. The quadriceps absorb load, steady the joint and let you stand, walk and manage stairs. Shedding surplus fat could help the knee, but if working muscle is lost alongside it, that is an outcome I would not welcome.

In a human body-composition substudy of retatrutide in people with type 2 diabetes, lean tissue was lost along with fat, in a proportion the authors describe as similar to other obesity treatments. Lean tissue is not the same thing as skeletal muscle, and a scan of body composition cannot tell you whether strength changed. These data must not be taken as proof that muscle is automatically protected. [4]

It is why I think any metabolic approach to osteoarthritis should eventually be paired with objective measurement of muscle and movement. Sound nutrition and suitable exercise stay important whenever weight loss is medically supervised. [2]

Does this make retatrutide a regenerative treatment?

No: the knee pain findings do not establish regeneration.

Pain can fall without any cartilage being rebuilt. Nothing in the knee osteoarthritis results examined in this article demonstrates cartilage regrowth, a reversal of arthritis, or protection from knee replacement. [1]

Because my own work is in regenerative medicine, I feel we must be strict with our wording. Easing pain is valuable in itself.

Whether improving metabolic health could lead to better results after cartilage repair is a distinct question for future research. These pain results have not answered it.

Looking beyond weight: metabolic joint preservation

This is the point where I find the future of knee care most promising.

Picture a clinic that tracks far more than a pain score and a number on the scales. In principle we could follow:

• your body composition and metabolic markers;
• how strong and functional your muscles are;
• how you walk, squat and rise from a chair;
• knee symptoms;
• scans and joint structure, where they are appropriate; and
• whether the patient is growing more capable, instead of merely lighter.

That sits much closer to how I think about preserving a joint. The measure of success stops being how many kilograms were lost and becomes a different question:

Has your knee ended up in a healthier biological and mechanical setting?

I use the phrase metabolic joint preservation as a description of a clinical intention: weighing metabolic health together with symptoms, strength and the joint. It is not a statement that retatrutide protects cartilage structure.

Might metabolic care join mainstream knee osteoarthritis treatment?

My view is that it might.

That is my reading of the direction of travel, not a proven fact.

Retatrutide is not an approved treatment for knee osteoarthritis, and the trials so far do not demonstrate cartilage regeneration or disease modification.

They do, though, push us to think afresh. For selected patients with knee osteoarthritis who also have metabolic disease or excess weight, care could in future address the joint and its metabolic surroundings together.

Such care might take in nutrition, medically supervised weight management where suitable, protection of muscle, progressive strengthening, better movement and targeted treatment for the knee itself.

I find that far more compelling than regarding retatrutide as one more injection for weight loss.

If you are living with knee pain now, a painful knee deserves assessment with the options available now. At London Cartilage Clinic we assess knee osteoarthritis on that basis, rather than asking you to wait for an investigational medicine.

The bigger question is this:

Could we change the biology of the person in order to change the outlook for the joint?

References

The TRIUMPH-1 source is a conference presentation from the manufacturer. The other two papers address separate research questions, on mechanism and on body composition.

1. Lilly: TRIUMPH-1 Phase 3 results, ADA Scientific Sessions, June 2026 (conference presentation).
2. MHRA: No summer shortcut for safe weight loss, 24 July 2026.
3. Coskun et al. LY3437943: from discovery to clinical proof of concept. Cell Metabolism, 2022.
4. Coskun et al. Effects of retatrutide on body composition in people with type 2 diabetes: a substudy of a phase 2 randomised trial. Lancet Diabetes & Endocrinology, 2025.