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Glucose, a form of sugar, is the body's main fuel. Hypoglycemia, or low blood sugar, occurs when blood levels of glucose drop too low to fuel the body's activity.

Carbohydrates (sugars and starches) are the body's main dietary sources of glucose. During digestion, the glucose is absorbed into the blood stream (hence the term "blood sugar"), which carries it to every cell in the body. Unused glucose is stored in the liver as glycogen.

Hypoglycemia can occur as a complication of diabetes, as a condition in itself, or in association with other disorders.

Blood Sugar Range
The normal range for blood sugar is about 60 mg/dl (milligrams of glucose per deciliter of blood) to 120 mg/dl, depending on when a person last ate. In the fasting state, blood sugar can occasionally fall below 60 mg/dl and even to below 50 mg/dl and not indicate a serious abnormality or disease. This can be seen in healthy women, particularly after prolonged fasting. Blood sugar levels below 45 mg/dl are almost always associated with a serious abnormality.

How Does the Body Control Glucose?
The amount of glucose in the blood is controlled mainly by the hormones insulin and glucagon. Too much or too little of these hormones can cause blood sugar levels to fall too low (hypoglycemia) or rise too high (hyperglycemia). Other hormones that influence blood sugar levels are cortisol, growth hormone, and catecholamines (epinephrine and norepinephrine).

The pancreas, a gland in the upper abdomen, produces insulin and glucagon. The pancreas is dotted with hormone-producing tissue called the islets of Langerhans, which contain alpha and beta cells. When blood sugar rises after a meal, the beta cells release insulin. The insulin helps glucose enter body cells, lowering blood levels of glucose to the normal range. When blood sugar drops too low, the alpha cells secrete glucagon. This signals the liver to release stored glycogen and change it back to glucose, raising blood sugar levels to the normal range. Muscles also store glycogen that can be converted to glucose.

What Are the Symptoms of Hypoglycemia?
A person with hypoglycemia may feel weak, drowsy, confused, hungry, and dizzy. Paleness, headache, irritability, trembling, sweating, rapid heart beat, and a cold, clammy feeling are also signs of low blood sugar. In severe cases, a person can lose consciousness and even lapse into a coma.

The symptoms associated with hypoglycemia are sometimes mistaken for symptoms caused by conditions not related to blood sugar. For example, unusual stress and anxiety can cause excess production of catecholamines, resulting in symptoms similar to those caused by hypoglycemia but having no relation to blood sugar levels.

Hypoglycemia in Diabetes
The most common cause of hypoglycemia is as a complication of diabetes. Diabetes occurs when the body cannot use glucose for fuel because either the pancreas is not able to make enough insulin or the insulin that is available is not effective. As a result, glucose builds up in the blood instead of getting into body cells.

The aim of treatment in diabetes is to lower high blood sugar levels. To do this, people with diabetes may use insulin or oral drugs, depending on the type of diabetes they have or the severity of their condition. Hypoglycemia occurs most often in people who use insulin to lower their blood sugar. All people with insulin-dependent diabetes mellitus (IDDM, or type 1) and some people with noninsulin-dependent diabetes mellitus (NIDDM, or type 2) use insulin. People with type 2 diabetes who take oral drugs called sulfonylureas are also vulnerable to low blood sugar episodes.

Conditions that can lead to hypoglycemia in people with diabetes include taking too much medication, missing or delaying a meal, eating too little food for the amount of insulin taken, exercising too strenuously, drinking too much alcohol, or any combination of these factors. People who have diabetes often refer to hypoglycemia as an "insulin reaction."
Managing Hypoglycemia in Diabetes
People with diabetes should consult their health care providers for individual guidelines on target blood sugar ranges that are best for them. The lowest safe blood sugar level for an individual varies, depending on the person's age, medical condition, and ability to sense hypoglycemic symptoms. A target range that is safe for a young adult with no diabetes complications, for example, may be too low for a young child or an older person who may have other medical problems.

Because they are attuned to the symptoms, people with diabetes can usually recognize when their blood sugar levels are dropping too low. They can treat the condition quickly by eating or drinking something with sugar in it such as candy, juice, or nondiet soda. Taking glucose tablets or gels (available in drug stores) is another convenient and quick way to treat hypoglycemia.

People with IDDM are most vulnerable to severe insulin reactions, which can cause loss of consciousness. A few patients with long-standing insulin-dependent diabetes may develop a condition known as hypoglycemia unawareness, in which they have difficulty recognizing the symptoms of low blood sugar. For emergency use in patients with IDDM, physicians often prescribe an injectable form of the hormone glucagon. A glucagon injection (given by another person) quickly eases the symptoms of low blood sugar, releasing a burst of glucose into the blood.

Emergency medical help may be needed if the person does not recover in a few minutes after treatment for hypoglycemia. A person suffering a severe insulin reaction may be admitted to the hospital so that blood sugar can be stabilized.

People with diabetes can reduce or prevent episodes of hypoglycemia by monitoring their blood sugar levels frequently and learning to recognize the symptoms of low blood sugar and the situations that may trigger it. They should consult their health care providers for advice about the best way to treat low blood sugar. Friends and relatives should know about the symptoms of hypoglycemia and how to treat it in case of emergency.

Episodes of hypoglycemia in people with IDDM may become more common now that research has shown that carefully controlled blood sugar helps prevent the complications of diabetes. Keeping blood sugar in a close-to-normal range requires multiple injections of insulin each day or use of an insulin pump, frequent testing of blood glucose, a diet and exercise plan, and guidance from health care professionals.

Other Causes of Hypoglycemia
Hypoglycemia in people who do not have diabetes is far less common than once believed. However, it can occur in some people under certain conditions such as early pregnancy, prolonged fasting, and long periods of strenuous exercise. People on beta blocker medications who exercise are at higher risk of hypoglycemia, and aspirin can induce hypoglycemia in some children. Drinking alcohol can cause blood sugar to drop in some sensitive individuals, and hypoglycemia has been well documented in chronic alcoholics and binge drinkers. Eating unripe ackee fruit from Jamaica is a rare cause of low blood sugar.

Diagnosis
To diagnose hypoglycemia in people who do not have diabetes, the doctor looks for the following three conditions:

For many years, the oral glucose tolerance test (OGTT) was used to diagnose hypoglycemia. Experts now realize that the OGTT can actually trigger hypoglycemic symptoms in people with no signs of the disorder. For a more accurate diagnosis, experts now recommend that blood sugar be tested at the same time a person is experiencing hypoglycemic symptoms.

The doctor will also check the patient for health conditions such as diabetes, obtain a medication history, and assess the degree and severity of the patient's symptoms. Laboratory tests to measure insulin production and levels of C-peptide (a substance that the pancreas releases into the bloodstream in equal amounts to insulin) may be performed.

Reactive Hypoglycemia
A diagnosis of reactive hypoglycemia is considered only after other possible causes of low blood sugar have been ruled out. Reactive hypoglycemia with no known cause is a condition in which the symptoms of low blood sugar appear 2 to 5 hours after eating foods high in glucose.

Ten to 20 years ago, hypoglycemia was a popular diagnosis. However, studies now show that this condition is actually quite rare. In these studies, most patients who experienced the symptoms of hypoglycemia after eating glucose-rich foods consistently had normal levels of blood sugar–above 60 mg/dl. Some researchers have suggested that some people may be extra sensitive to the body's normal release of the hormone epinephrine after a meal.

People with symptoms of reactive hypoglycemia unrelated to other medical conditions or problems are usually advised to follow a healthy eating plan. The doctor or dietitian may suggest that such a person avoid foods high in carbohydrates; eat small, frequent meals and snacks throughout the day; exercise regularly; and eat a variety of foods, including whole grains, vegetables, and fruits.

Rare Causes of Hypoglycemia
Fasting hypoglycemia occurs when the stomach is empty. It usually develops in the early morning when a person awakens. As with other forms of hypoglycemia, the symptoms include headache, lack of energy, and an inability to concentrate. Fasting hypoglycemia may be caused by a variety of conditions such as hereditary enzyme or hormone deficiencies, liver disease, and insulin-producing tumors.

In hereditary fructose intolerance, a disorder usually seen in children, the body is unable to metabolize the natural sugar fructose. Attacks of hypoglycemia, marked by seizures, vomiting, and unconsciousness, are treated by giving glucose and eliminating fructose from the diet.

Galactosemia, a rare genetic disorder, hampers the body's ability to process the sugar galactose. An infant with this disorder may appear normal at birth, but after a few days or weeks of drinking milk (which contains galactose), the child may begin to vomit, lose weight, and develop cataracts. The liver may fail to release stored glycogen into the blood, triggering hypoglycemia.

Removing milk from the diet is the usual treatment.
A deficiency of growth hormone causes increased sensitivity to insulin. This sensitivity occurs because growth hormone opposes the action of insulin on muscle and fat cells. For this reason, children with growth hormone deficiency sometimes suffer from hypoglycemia, which goes away after treatment.

People with insulin-producing tumors, which arise in the islet cells of the pancreas, suffer from severe episodes of hypoglycemia.

To diagnose these tumors, called insulinomas, a doctor will put the patient on a 24- to 72-hour fast while measuring blood levels of glucose, insulin, and proinsulin. High levels of insulin and proinsulin in the presence of low levels of glucose strongly suggest an insulin-producing tumor. These tumors are usually benign and can be surgically removed.

In rare cases, some cancers such as breast cancer and adrenal cancer may cause hypoglycemia through secretion of a hormone called insulin-like growth factor II. The treatment is removal of the tumor, if possible.