A hormone secreted by L-cells in the lining of the small intestine that acts as an appetite suppressant and counteracts the effects of the hormone ghrelin. Active peptide YY consists of 36 amino acids and its release is stimulated by the presence of food in the digestive tract. It circulates in the bloodstream and binds to receptors in the brain to reduce appetite and promote satiety. It also acts on the stomach and intestine to slow the movement of digesta through the digestive tract. Levels in blood gradually fall following a meal and are at their lowest when fasting, e.g. overnight. It may have potential as an anti-obesity treatment.