Complex sentence: Incretins are a group of gastrointestinal hormones that cause an increase in the amount of insulin released from the beta cells of the islets of Langerhans after eating, before blood glucose levels become elevated.
Split into simpler sentences: Incretins are a group of gastrointestinal hormones that function to stimulate a decrease in plasma/blood glucose levels. Incretins cause an increase in the amount of insulin released from the beta cells of the islets of Langerhans after eating, before blood glucose levels become elevated.