Infused PYY3-36 cut food intake in trials; oral PYY3-36 alone did not

What is PYY3-36?

PYY3-36 is a fragment of PYY, a hormone secreted by intestinal L cells after a meal alongside GLP-1 and then cleaved to the PYY3-36 form.

Pharmacokinetic work shows both peptides reach their highest concentrations in splanchnic blood near the gut rather than systemically. PYY3-36 is thought to be substantially degraded before it reaches general circulation, which is one reason infusion studies, which bypass gut and liver clearance, and oral studies have produced different results ^[1].

The proposed downstream mechanism is well-established physiology: PYY3-36 preferentially engages a receptor subtype on hypothalamic appetite-regulating neurons and dampens hunger signaling. That pathway is the standard explanation for how a gut peptide can act as a satiety brake.

Does infused PYY3-36 reduce food intake in humans?

Yes. Intravenous PYY3-36 infusion reduced caloric intake at a test meal by roughly 30% in obese adults and 31% in lean adults, and it also reduced cumulative 24-hour intake ^[3].

A separate trial found a smaller but still significant reduction over 24 hours: 17% in obese subjects and 24% in lean fasting subjects ^[4].

The direction of effect across independent human infusion studies is more consistent than the rodent data turned out to be. That asymmetry is the core of the PYY3-36 story: the translational claim that matters for a drug, the human satiety response, held up better than the preclinical claim that launched the excitement.

Did PYY3-36's rodent results replicate?

Reportedly not. The 2002 demonstration that peripheral PYY3-36 injections cut food intake in mice and rats is widely reported to have failed replication in several independent labs by the mid-2000s. That original finding made the molecule famous, and a contemporaneous drug-development commentary cites it in passing as an established starting point for human testing ^[4].

The rodent effect became one of the more publicly discussed replication disputes in metabolic research. Multiple independent groups reportedly could not reproduce the original magnitude of feeding suppression, and stress or illness from the injection procedure itself was raised as a possible confound in reading reduced feeding as "satiety" rather than "sickness."

Treat the rodent effect size as unsettled rather than as the field's bedrock.

Does PYY3-36 cause nausea?

Yes. High-dose PYY3-36 infusion caused enough nausea that 8 of 12 subjects could not finish the protocol ^[2]. Among those who did complete it, PYY3-36 lowered self-reported well-being and raised heart rate alongside reducing intake ^[2].

That pattern matches the same illness confound raised in the rodent dispute, documented directly in humans. Reduced eating that co-occurs with nausea is not the same finding as reduced eating that co-occurs with normal well-being, and the human literature does not fully separate the two.

Does oral PYY3-36 work?

No. An oral formulation designed to mimic natural post-meal PYY3-36 release, using an absorption-enhancing delivery agent, produced high plasma concentrations and did not significantly reduce food intake on its own in a randomized crossover trial ^[1].

Total energy intake dropped meaningfully, by about 21.5%, only when PYY3-36 was combined with oral GLP-1 ^[1]. Why peptides can't be swallowed covers the general problem oral peptides face, and why oral semaglutide needs large doses covers a drug that solved it.

Why did PYY3-36 monotherapy get abandoned?

PYY3-36 monotherapy disappeared from clinical development because it did not work on its own in the oral trial, and it now reappears as a component of combination drugs ^[1]⁠^[7]⁠^[8]⁠^[9].

Current obesity-drug reviews describe PYY as one of several gut peptides, alongside GLP-1, GIP, oxyntomodulin, and glucagon, being tested in combination ^[7]⁠^[8]⁠^[9]. The rationale is that single-hormone approaches, including GLP-1 alone, have shown limits on efficacy or tolerability that combinations aim to offset.

That is a pharmacological rationale, not a resurrection of the original monotherapy claim. Why oxyntomodulin never became a weight-loss drug covers another gut peptide on the same list.

What is still unknown about PYY3-36?

Long-term effects, whether obesity blunts the response, and whether a hormone change produces a behavior change all remain open for PYY3-36:

• Long-term data. Human data on PYY3-36, alone or oral, are limited. The 2004-era commentary calling for longer studies in freely-fed subjects still describes an evidence gap that combination-focused trials have only partly addressed ^[4].
• Response in obesity. Whether obesity blunts the response to exogenous PYY3-36, the way it blunts leptin signaling, is unsettled. Obese subjects do show altered intestinal nutrient-sensing and gut hormone responses compared with lean subjects in other paradigms ^[6]. Leptin therapy in common obesity covers that parallel.
• Hormone change versus behavior change. Lowering ghrelin pharmacologically in adults with a hyperphagic genetic condition did not reduce food intake, even though the hormone dropped substantially, and it lowered PYY as a side effect ^[5]. That study does not test PYY3-36 directly. It shows that a hormone moving in the "right" direction on a blood test does not guarantee a behavioral effect, which is the gap the rodent replication dispute exposed and the gap combination trials are still working to close.