Starch is one of the major chemicals involved in the digestive system of creatures. It is generally broken down into glucose and not created in the body. Some mutations, such as the Vampire Norns, create starch by performing certain actions.
In Creatures 1, its description was:
Emitted by food objects - converts to Glucose for energy. Does NOT decrease hunger in pre-Life Kit norns - this is done directly by Hunger-- (saccharin), so that junk food can contain high saccharin levels but low starch, etc.