PYY

PYY is a gut hormone that says “you have eaten enough.” Released after meals from the lower intestine, it dials down hunger over the hour or two following a meal, acting as a natural brake on how much you eat.

Peptide YY is secreted by L-cells in the ileum and colon — the same cells that make GLP-1 — in proportion to the calories and especially fat and protein you have consumed. Its active form acts on appetite-regulating circuits in the hypothalamus and brainstem to reduce food intake, and it also slows gut motility (the “ileal brake”), giving the intestine time to absorb nutrients. Together with GLP-1, it is a key satiety signal from the lower gut.

PYY helps explain why some treatments curb appetite. After certain weight-loss surgeries, PYY and GLP-1 rise sharply, which is thought to contribute to reduced hunger and lasting weight loss. As a stand-alone appetite drug, however, PYY has been difficult to use because high doses tend to cause nausea.