Insulin is actually a storage hormone secreted in response to carb ingestion.It is important to keep it normalized and not let it cronically spike or it causes all sorts of problems.This is a reason it is important to eat protein and/or fat with carbs to normalize hormone(insulin)response.There probably is a hormone secreted to stimulte appetite(I think they are trying to discover how to turn it off for overweight people) but fit people generally burn more calories though exercise and elevated metabolism which leads to a need to consume more calories for fuel and thus appetite.