What is peptide yy (PYY)? Satiety, honestly

What is peptide yy? Peptide YY (PYY) is a gut-made satiety hormone released after food reaches the intestine. Its PYY3-36 form can reduce appetite in humans, and exaggerated PYY after gastric bypass probably helps the surgery work. Yet efforts to turn that biology into a tolerable weight-loss drug keep underdelivering.

What does peptide YY do?

Peptide YY tells the brain and digestive tract that food has arrived, helping reduce appetite and slow the movement of food through the gut. The intestine releases PYY after a meal, roughly in proportion to the calories eaten. Calling PYY a satiety hormone peptide is fair, although no single hormone owns the feeling of fullness.

The body makes a 36-amino-acid form called PYY1-36. An enzyme named dipeptidyl peptidase-4 (DPP-4) clips off its first two amino acids, producing the 34-amino-acid PYY3-36, often written pyy 3-36. The NCBI record for the human PYY gene confirms that gut endocrine cells produce two processed peptides with different receptor preferences.

PYY3-36 prefers the Y2 receptor. In plain English, that receptor helps turn down appetite-driving neurons in the hypothalamus, the brain region that tracks energy needs.

Does PYY3-36 actually reduce food intake in people?

PYY3-36 has reduced short-term food intake in controlled human infusion studies. The famous 2002 experiment reported that an intravenous infusion producing normal post-meal concentrations cut 24-hour food intake by 33%. That is strong evidence for an acute biological effect, but an infusion-day result is not the same as durable peptide yy weight loss.

The Batterham Nature study established the signal. A 2003 follow-up studied 12 adults with obesity and 12 lean adults. PYY3-36 reduced 24-hour intake by 16.5% and 23.5%, respectively, without reported nausea at the tested infusion level. Peak circulating PYY reached about 57 and 54 pmol/L in the two groups.

Those are human controlled studies, the highest rung on this page for the claim that PYY can suppress eating for hours. They do not show that repeated PYY injections produce large, sustained fat loss. Acute appetite, months of body-weight change, and tolerability are three different endpoints. Drug development has been learning that distinction the expensive way.

Is PYY3-36 one reason gastric bypass works?

PYY3-36 is probably one meaningful part of the gastric bypass effect, alongside GLP-1 and other changes in anatomy, food transit, bile acids, and eating behavior. Roux-en-Y gastric bypass sends nutrients rapidly into lower sections of the intestine, where L cells release an unusually large wave of PYY and GLP-1 after meals.

The size of that wave matters. In one randomized gastric-bypass study, median peak total PYY reached about 115–125 pmol/L after surgery, more than twice the roughly 54–57 pmol/L peaks produced by the classic infusion. That comparison is total PYY, not isolated PYY3-36; assays and meals also differed, so it is context rather than a clean head-to-head result.

The better evidence is functional. In 12 people after bypass, researchers blocked GLP-1 signaling while also reducing formation of PYY3-36. Food intake rose by about 20%; blocking either pathway alone did not do so. The gastric bypass gut hormones study supports teamwork, not a solo act.

A newer assay that separates the two PYY forms found that meal-triggered PYY1-36 and PYY3-36 secretion was amplified one year after Roux-en-Y bypass, but not after sleeve gastrectomy. That makes PYY one of the more credible explanations for post-bypass satiety without pretending a complex operation is merely a hormone drip with stitches.

Why has peptide YY weight loss failed as a drug?

Peptide YY weight loss drugs keep running into a narrow window: too little PYY may do little, while higher or sustained exposure brings nausea, vomiting, and other gastrointestinal problems. Native PYY3-36 also disappears quickly, so drug makers must extend its action without turning normal post-meal satiety into all-day digestive misery.

Early intranasal PYY3-36 illustrates the problem. In a 12-week study summarized in the later development paper, 200 micrograms produced 3.7 kg of weight loss versus 2.8 kg with placebo. At 600 micrograms, most participants discontinued because of nausea and vomiting. The efficacy gap was small; the tolerability gap was not.

This is the part basic explainers usually skip. A hormone can clearly work in human physiology and still make an awkward medicine. Gastric bypass changes several signals in timed bursts after eating. A drug must reproduce enough of that pattern, week after week, without making the patient feel poisoned. Biology does not owe pharmaceutical engineering an easy translation.

What happened when PYY1875 was added to semaglutide?

PYY1875 added only modest extra weight loss to semaglutide and caused enough gastrointestinal trouble to limit its clinical usefulness. PYY1875 is a long-acting, selective Y2-receptor agonist designed for weekly dosing; the 2025 phase 2 result was a serious test of the PYY idea, not a failed nasal spray from decades ago.

Participants first received semaglutide for 32 weeks and lost an average 14.3% of body weight. During the next 16 weeks, adding 1 mg PYY1875 produced 5.6% further loss versus 3.1% with placebo plus semaglutide: a treatment difference of 2.2 percentage points, or 2.2 kg. The investigators called that modest and not clinically meaningful.

Tolerability supplied the harder verdict. Gastrointestinal adverse events occurred in 68% of the 1 mg PYY1875 group versus 29% with placebo; 21.3% versus 10.7% discontinued early. Only 20 of 47 participants were still on the 1 mg target dose at the end. The 2 mg group was assessed only for safety, and just three participants reached and stayed on that target dose.

The PYY1875 trial was sponsored by Novo Nordisk and is listed as completed phase 2 on ClinicalTrials.gov. As of July 2026, PYY1875 remains investigational, not an approved obesity treatment.

How does PYY compare with semaglutide and amylin?

PYY, semaglutide, and amylin analogues all influence fullness, but they reach that outcome through different receptors and time patterns. PYY3-36 mainly targets Y2 signaling; semaglutide copies GLP-1 activity; amylin analogues act through amylin receptors and also slow gastric emptying. Combining appetite pathways can help, but side effects can add too.

Semaglutide has large, completed weight-loss programs and FDA-approved products. PYY1875 has one modest phase 2 add-on signal with poor tolerability. Amylin analogues explained covers another combination strategy, while the broader peptides for weight loss guide separates approved medicines from early candidates.

The honest answer to what is peptide yy is therefore more useful than “the fullness hormone.” PYY is established human appetite biology, credible gastric-bypass biology, and unfinished drug technology. The signal is real. The medicine has not caught up.