This is a condition where the amount of sugar in the blood stream declines to a low level, which produces symptoms. Relative hypoglycemia occurs in situations where the blood sugar declines from an elevated level to a lower, but not too low, a level at a rate such that the brain reacts to this change. Generally, the blood sugar must fall below 50 for a patient to experience symptoms of hypoglycemia. The brain's two most critical sources of energy are sugar and oxygen. When the sources of these are too low, the brain reacts to try to restore them.