What is the input signal that causes insulin to be released from a beta cell?

Glucose in the blood

Explanation:

When we eat and the level of glucose in the blood rises, it stimulates the release of insulin into the blood stream. It makes sense, because insulin is necessary for glucose to enter the cell across the membrane and be used as a source of energy. The problem arises when we eat too many carbohydrates for too long and the glucose level in the blood is constantly too high. After a number of years this causes insulin resistance. In other words, each time an ever greater concentration of insulin is needed to perform its function and eventually this leads to diabetes, when the pancreas just cannot produce any more insulin.