Peptide YY vs neuropeptide Y: appetite's opposite signals

A family of peptides

Neuropeptide Y and peptide YY, usually shortened to NPY and PYY, belong to the same family, along with pancreatic polypeptide. Both are 36 amino acids long, both end in an amidated tyrosine, the 'Y' in their names, and both act on the same set of Y receptors. Our entries on neuropeptide Y and peptide YY give the sequences.

The difference is where they are made and what they do there. NPY is one of the most abundant peptides in the brain and in the nerves of the body; in the hypothalamus it is a powerful signal to eat. PYY is made by cells in the lining of the lower gut and released after meals. In the circulation, an enzyme trims PYY into PYY3-36, the form that favours the Y2 receptor.

How the gut signal switches off the hunger signal

The connection between the two was established in a 2002 study. PYY3-36 is released from the gut after eating, in proportion to the calories in the meal. Injected into rats, it reduced food intake and weight gain; it reduced intake in normal mice but not in mice lacking the Y2 receptor; and it lowered NPY gene activity in the hypothalamus and dampened the electrical activity of NPY nerve terminals, releasing neighbouring appetite-suppressing neurons (animal study) ¹.

In other words, the Y2 receptor works as a brake on NPY neurons, and the gut's after-meal PYY presses that brake. One peptide family thus carries both a hunger signal and part of the machinery that turns it off. The arrangement makes sense physiologically: the gut reports that food has arrived, and the brain's strongest drive to eat is turned down in proportion to what was eaten.

This circuit is one reason appetite is hard to override by willpower alone. Hunger and fullness are set by hormone signals between gut and brain, and treatments that change those signals, from obesity surgery to today's gut-hormone drugs, have been among the most effective for weight loss, as the surgery section below describes.

PYY in people: a strong laboratory effect

Human studies quickly followed. In a 2003 double-blind crossover study, 12 obese and 12 lean volunteers received an infusion of PYY3-36 or saline two hours before a buffet lunch. Calorie intake at the buffet fell by 30% in the obese group and 31% in the lean group, and total intake over 24 hours also fell (randomised trial) ². Importantly, obese people were not resistant to PYY, unlike leptin, the fat hormone whose signal obesity blunts.

PYY also slows the stomach. In eight healthy adults, PYY3-36 lengthened the time for a test meal to half-empty from 63 minutes on saline (randomised study) ³. Slower emptying is one of the ways several gut hormones, including GLP-1, make a meal feel more filling. Together, the infusion and emptying studies made PYY look like an ideal natural appetite suppressant: released by food, proportional to calories, still effective in obesity, and acting on both the brain and the stomach.

PYY in obesity: low levels, intact response

The 2003 study had a second finding that shaped the field. Obese volunteers had lower PYY of their own, both fasting and after meals: fasting levels averaged 10.2 pmol/L in the obese group against 16.9 in the lean group, and fasting PYY fell steadily as body-mass index rose (randomised trial) ². Infused PYY also lowered ghrelin, the stomach's hunger hormone.

That combination, low natural PYY but a normal response to added PYY, suggested that obesity might partly be a PYY deficiency that replacement could correct. It is a neat hypothesis. The nasal-spray trial and the infusion studies showing weak effects of PYY alone suggest the real picture is more complicated (randomised trial) ⁴.

Protein, PYY and fullness

PYY may explain part of why protein is filling. In a 2006 study, high-protein meals produced the largest PYY release and the strongest satiety in both normal-weight and obese people compared with high-fat or high-carbohydrate meals. In mice, a high-protein diet raised PYY, reduced eating and reduced fat; mice engineered to lack PYY did not get those benefits from protein and became markedly obese, which PYY treatment reversed (human and animal study) ⁵.

That is a reasonable basis for the common advice that protein helps with fullness. It is not evidence that any supplement raises PYY enough to cause meaningful weight loss.

Bariatric surgery and the gut hormones

The most dramatic natural rise in PYY happens after obesity surgery. Levels of several gut hormones, PYY and GLP-1 among them, rise sharply from soon after gastric bypass and sleeve gastrectomy, and studies that block gut-hormone release suggest they contribute to reduced eating after surgery (review) ⁶. That observation helped inspire the idea of reproducing surgery's hormone changes with drugs.

PYY is not the whole story, though. A 2026 meta-analysis of five studies compared people who lost a lot of weight after gastric bypass with those who did not, and found their fasting and after-meal PYY levels were essentially the same; the authors concluded that differences in weight loss are driven by factors beyond PYY (meta-analysis) ⁷.

Why PYY has not become a drug

Turning that effect into a treatment has been hard. In a 12-week trial of a PYY3-36 nasal spray taken three times a day before meals in obese adults, weight change was −2.8 kg with placebo, −3.7 kg with 200 µg and −1.4 kg with 600 µg; the difference at the lower dose was not significant, and 59% of patients on the high dose stopped because of nausea and vomiting (randomised trial) ⁸. The authors concluded that, as given, it was not effective.

PYY on its own may also be weaker than the 2003 study suggested. In a 2014 crossover study in 25 overweight men, PYY3-36 infused alone changed energy intake by only −4.2%, not significant, and GLP-1 alone by −3.0%; only the two together reduced intake (randomised trial) ⁴. Combined with oxyntomodulin, another gut hormone, PYY3-36 cut intake at a test meal by 42.7% against saline in 12 volunteers (randomised trial) ⁹. That pattern, PYY working best with partners, is why interest has moved towards combinations rather than PYY alone.

NPY: blocking the hunger signal

If NPY drives hunger, blocking it should reduce eating. Merck tested that with MK-0557, an oral drug that blocks NPY's Y5 receptor, one of the receptors thought to carry its appetite effect. In a 52-week randomised, double-blind trial in 1,661 overweight and obese people, weight loss with MK-0557 was statistically significant but not clinically meaningful, and the authors concluded that targeting the Y5 receptor alone was not a promising approach (randomised trial) ¹⁰. A second trial, in 359 people who had first lost weight on a very-low-calorie diet, found weight regain over a year of +1.5 kg with MK-0557 against +3.1 kg with placebo, a small difference (randomised trial) ¹¹.

The likely explanation is redundancy: appetite is controlled by overlapping systems, and NPY acts through several receptors, so blocking one leaves the others working. The trial still had value. It was one of the first direct tests of the NPY pathway in people, and its authors wrote that it gave the first clinical insight into how that pathway controls energy balance in humans (randomised trial) ¹⁰.

NPY in the body's nerves

Outside the brain, NPY is carried in sympathetic nerves, the system behind the fight-or-flight response. In human heart tissue, NPY-containing nerve fibres were found densely around the coronary vessels and in the heart's conduction tissue, in close contact with muscle fibres (human tissue study) ¹². In six healthy men exercising at moderate and heavy intensity, full-length NPY rose with exercise intensity and fell more slowly than noradrenaline, its trimmed form NPY3-36 rose during recovery, and its half-life proved shorter than earlier assays had suggested (randomised study) ¹³. That wide role in the body is one reason drugs acting broadly on NPY receptors would be expected to have effects well beyond appetite.

NPY beyond appetite

NPY has other roles that make it interesting outside obesity. In 25 military personnel during intense survival training, NPY release rose with stress hormones, and higher NPY went with less psychological distress (human study) ¹⁴. A small trial gave a nasal NPY spray to 30 people with major depression already on an antidepressant and found depression scores improved against placebo at 5 and 24 hours (randomised trial) ¹⁵. These are early findings in small groups, not evidence of a treatment, and they are covered in our NPY entry.

The Y receptors, briefly

Much of the confusion between these peptides comes from the receptors they share. Full-length NPY and PYY can both activate several Y receptors. Trimming two amino acids from the front, which turns PYY into PYY3-36, shifts the peptide towards the Y2 receptor (animal study) ¹. On NPY neurons in the hypothalamus, Y2 works as a brake; elsewhere, other Y receptors carry NPY's hunger signal, including Y5, the target of MK-0557 (randomised trial) ¹⁰.

So the same family can push appetite in opposite directions depending on which peptide, which form and which receptor is involved. That is also why a drug that hits Y receptors broadly could backfire, and why designers have aimed for specific receptors.

What the failures taught

The PYY and NPY programmes were among the first serious attempts to treat obesity by targeting a single appetite hormone, and both came up short for instructive reasons. With PYY, the dose that reduced appetite also caused nausea, so the useful range was narrow, as in the nasal-spray trial where most high-dose patients dropped out (randomised trial) ⁸. With NPY, blocking one receptor left the rest of a redundant system to compensate (randomised trial) ¹⁰.

Both lessons shaped what came next. Nausea is managed in today's GLP-1 drugs by slow dose increases, and redundancy is addressed by combining hormones, as the PYY studies with GLP-1 and oxyntomodulin anticipated (randomised trial) ⁹.

Where the pair fits in today's obesity drugs

The approved obesity drugs are built on a different gut hormone, GLP-1, sometimes combined with GIP, amylin or glucagon. PYY and NPY sit in the same broad system, the network of gut and brain signals that sets how much we eat, and the studies above help explain why single-hormone approaches tend to plateau while combinations do better (randomised trial) ⁴. Our 2026 pipeline overview and amylin explainer cover the approved drugs and the combinations in late-stage trials.

A short timeline

• 1983: dense NPY nerve fibres described in human heart tissue (human tissue study) ¹².
• 2002: PYY3-36 shown to cut food intake in rodents through the Y2 receptor on NPY neurons (animal study) ¹; NPY release linked to stress hormones in soldiers under training (human study) ¹⁴.
• 2003: PYY infusion cuts buffet intake by about 30% in obese and lean volunteers (randomised trial) ².
• 2006: protein's satiating effect tied to PYY (human and animal study) ⁵; a year-long NPY Y5 blocker trial in 1,661 people disappoints (randomised trial) ¹⁰.
• 2007: a 12-week nasal PYY trial fails, with nausea limiting the dose (randomised trial) ⁸.
• 2010 to 2014: infusion studies find PYY works best combined with oxyntomodulin or GLP-1 (randomised trial) ⁹.
• 2026: a meta-analysis finds PYY levels do not explain who loses most weight after gastric bypass (meta-analysis) ⁷.

What this means for you

For people interested in appetite and weight, the PYY research supports a few ordinary points. Meals with more protein tend to be more filling, and PYY is one plausible reason (human and animal study) ⁵. Approved obesity medicines act on related gut-hormone signals and have the trial evidence that PYY and NPY products lack; they are prescribed after a clinician checks they are suitable.

Products sold online as PYY or 'appetite peptides' have no trial showing they cause weight loss, and the one rigorously tested non-injected form failed (randomised trial) ⁸. Our guide to legitimate prescriptions explains how to find a proper route to weight-loss treatment.

Common claims, checked

• 'PYY supplements curb appetite.' PYY is a peptide that would be digested if swallowed, and the one tested non-injected form, a nasal spray, failed and caused nausea (randomised trial) ⁸.
• 'Raising PYY with diet will cause weight loss.' PYY rises after meals in proportion to calories (animal study) ¹; that is part of normal fullness, not a separate weight-loss lever with trial evidence.
• 'Blocking NPY is the key to hunger.' The year-long Y5 blocker trial produced weight loss too small to matter (randomised trial) ¹⁰.

Quick glossary

• NPY (neuropeptide Y): brain and nerve peptide; in the hypothalamus, a strong hunger signal.
• PYY (peptide YY): gut hormone released after meals; its trimmed form PYY3-36 reduces appetite.
• Y2 receptor: the receptor PYY3-36 favours; on NPY neurons it acts as a brake.
• Y5 receptor: one of the receptors that carries NPY's appetite effect, targeted by MK-0557.

How it adds up (as of October 4, 2026)

NPY and PYY are related peptides with opposite effects on appetite: NPY drives hunger in the brain, and PYY from the gut after meals helps end it, partly by quieting NPY neurons through the Y2 receptor. In the laboratory, infused PYY cut food intake by about 30%, but a nasal PYY spray failed in a 12-week trial and caused frequent nausea, and blocking NPY's Y5 receptor produced only trivial weight loss in a year-long trial of 1,661 people. Neither is an approved drug, and no product sold online as either has trial evidence of weight loss. Their main lesson for obesity treatment is that appetite runs on many overlapping signals, which is why combinations have done better than single hormones.

Frequently asked questions

What is the difference between PYY and NPY?

Both are 36-amino-acid peptides of the same family. NPY, in the brain, stimulates hunger; PYY, released by the gut after meals, reduces it.

How does PYY reduce appetite?

Its trimmed form, PYY3-36, acts on Y2 receptors on hunger-promoting NPY neurons in the hypothalamus, damping their activity; it also slows stomach emptying.

Is there a PYY weight-loss drug?

No. A PYY3-36 nasal spray failed in a 12-week trial, with most high-dose patients stopping because of nausea and vomiting.

Did blocking NPY cause weight loss?

Only trivially: a Y5 receptor blocker, MK-0557, produced weight loss that was statistically significant but not clinically meaningful in a 52-week trial of 1,661 people.

Related on Grey Peptides

• Peptide YY
• Neuropeptide Y
• The 2026 peptide pipeline

Sources

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