We wish to identify people with hypoglycemia (low blood sugar) and ultimately determine why the blood sugar levels are so low. To do so, we will contrast the responses of patients suspected to have hypoglycemia with those of healthy subjects during standardized tests. Those tests may include: 1) a controlled hypoglycemia produced by administration of insulin, a blood sugar-lowering hormone, and sugar, 2) a formula meal, and 3) a period of up to 72 hours without food.