Peptide YY
Peptide YY (PYY) also known as peptide tyrosine tyrosine is a peptide that in humans is encoded by the PYY gene.  Peptide YY is a short (36-amino acid) peptide released from cells in the ileum and colon in response to feeding. In the blood, gut, and other elements of periphery, PYY acts to reduce appetite; similarly, when injected directly into the central nervous system, PYY is also anorexigenic, i.e., it reduces appetite.
Dietary fibers from fruits, vegetables, and whole grains, consumed, increase the speed of transit of intestinal chyme into the ileum, to raise PYY3-36, and induce satiety. Peptide YY can be produced as the result of enzymatic breakdown of crude fish proteins and ingested as a food product.

# Structure
Peptide YY  is related to the pancreatic peptide family by having 18 of its 36 amino acids located in the same positions as pancreatic peptide. The two major forms of peptide YY are PYY1-36 and PYY3-36, which have PP fold structural motifs. However, the most common form of circulating PYY immunoreactivity is PYY3-36, which binds to the Y2 receptor (Y2R) of the Y family of receptors. Peptide YY3-36 (PYY) is a linear polypeptide consisting of 34 amino acids with structural homology to NPY and pancreatic polypeptide.

# Release
PYY is found in L cells in the mucosa of gastrointestinal tract, especially in ileum and colon. Also, a small amount of PYY, about 1-10%, is found in the esophagus, stomach, duodenum and jejunum. PYY concentration in the circulation increases postprandially (after food ingestion) and decreases by fasting. In addition, PYY is produced by a discrete population of neurons in the brainstem, specifically localized to the gigantocellular reticular nucleus of the medulla oblongata. C. R. Gustavsen et al. had found PYY-producing cells located in the islets of Langerhans in rats. They were observed either alone or co-localized with glucagon or PP.

# Function
PYY exerts its action through NPY receptors; it inhibits gastric motility and increases water and electrolyte absorption in the colon. PYY may also suppress pancreatic secretion.  It is secreted by the neuroendocrine cells in the ileum and colon in response to a meal, and has been shown to reduce appetite. PYY works by slowing the gastric emptying; hence, it increases efficiency of digestion and nutrient absorption after a meal. Research has also indicated PYY may be useful in removing aluminium accumulated in the brain.

# Animal studies
Several studies have shown acute peripheral administration of PYY3-36 inhibits feeding of rodents and primates. Other studies on Y2R-knockout mice have shown no anorectic effect on them. These findings indicate PYY3-36 has an anorectic (losing appetite) effect, which is suggested to be mediated by Y2R. PYY-knockout female mice increase in body weight and fat mass. PYY-knockout mice, on the other hand, are resistant to obesity, but have higher fat mass and lower glucose tolerance when fed a high-fat diet, compared to control mice. Thus, PYY also plays a very important role in energy homeostasis by balancing food intake. PYY oral spray was found to promote fullness.  Viral gene therapy of the salivary glands resulted in long-term intake reduction.

# Relevance to obesity
Leptin also reduces appetite in response to feeding, but obese people develop a resistance to leptin.  Obese people secrete less PYY than non-obese people, and attempts to use PYY directly as a weight-loss drug have met with some success.  Researchers noted the caloric intake during a buffet lunch offered two hours after the infusion of PYY was decreased by 30% in obese subjects (P<0.001) and 31% in lean subjects (P<0.001).
While some studies have shown obese persons have lower circulating level of PYY postprandially, other studies have reported they have normal sensitivity to the anorectic effect of PYY3-36. Thus, reduction in PYY sensitivity may not be one of the causes of obesity, in contrast to the reduction of leptin sensitivity. The anorectic effect of PYY could possibly be a future obesity drug.
The consumption of protein boosts PYY levels, so some benefit was observed in experimental subjects in reducing hunger and promoting weight loss.  This could partially explain the weight-loss experienced with high-protein diets, but the high thermic effect of protein appears to be the leading cause.
Obese patients undergoing gastric bypass showed marked metabolic adaptations, resulting in frequent diabetes remission 1 year later. When the confounding of calorie restriction is factored out, -cell function improves rapidly, very possibly under the influence of enhanced GLP-1 responsiveness. Insulin sensitivity improves in proportion to weight loss, with a possible involvement of PYY.