Hypoglycemia refers to a low blood sugar level and is a common complication of diabetes mellitus. Hypoglycemia is usually defined as a blood sugar level that has dropped to below 4 mmol/L. Healthy people usually maintain a blood glucose level within the range of 3.9 to 5.6 mmol/L over a 24-hour period.
Blood glucose fluctuates throughout the day. For example, after a meal the carbohydrates (eg, bread, rice, pasta, vegetables) present in food are broken down by enzymes to give simple sugar molecules. The main sugar molecule that provides the body's energy is glucose, which is absorbed into the bloodstream.
As the blood glucose level rises, the hormone insulin is secreted which aids the uptake of glucose from the blood to fuel various cell functions. This, in turn, causes a fall in blood sugar. Periods of fasting and starvation lead to a fall in blood sugar and can lead to hypoglycemia. Among diabetics, hypoglycemia is more likely to occur in those who take insulin, if more exercise than usual is carried out or if a meal is missed or not eaten on time.
Blood sugar levels are expressed in milligrams per deciliter (mg/dL or mg/100 mL) in the United States, Japan, Spain and France, with a healthy blood glucose level defined as within the range of 72 to 144 mg/dL over 24 hours. Values in mg/dL can be converted to mmol/L by dividing by 18.0 g/dmol (that is the molar mass of glucose).
A blood sugar level of between 3.3 or 3.9 mmol/L (60 to 70 mg/dL) is generally considered the lower limit for blood glucose. However, symptoms of hypoglycemia do not usually manifest until levels have reached a low of between 2.8 to 3.0 mmol/L (50 to 54 mg/dL).
Although the normal range of blood glucose has been established and is generally greed upon, there is still debate over what extent of hypoglycemia is enough to be damaging to a person's health and require treatment. In order to accurately determine exactly how low a blood sugar level needs to be for hypoglycemia to be diagnosed depends on several factors including:
The blood glucose measurement method used
The patient's age
Absence or presence of hypoglycemia symptoms/effects
The reason for defining low blood sugar
Reviewed by Sally Robertson, BSc