Every cell of the human body requires energy to perform the metabolic functions that sustain life. Glucose is a small, simple sugar that serves as a primary fuel for energy production, especially for the brain, muscles and several other body organs and tissues. Glucose also serves as a building block for larger structural molecules of the body, such as glycoproteins and glycolipids. The human body tightly regulates glucose levels. Abnormally high or low levels result in serious, potentially life-threatening complications.
The brain normally relies almost exclusively on glucose to fuel its energy needs. Because of its high energy demands and inability to store glucose, the brain requires a constant supply of the sugar. The body possesses multiple mechanisms to prevent a significant drop in blood glucose, or hypoglycemia. Should such a drop occur, however, brain functions can begin to fail. Common brain-related symptoms of hypoglycemia include headache, dizziness, confusion, lack of concentration, anxiety, irritability, restlessness, slurred speech and poor coordination. A sudden, severe drop on blood glucose can lead to seizures and coma.
The skeletal muscles normally constitute approximately 30 to 40 percent of total body weight, although this varies based on sex, age and fitness level. The skeletal muscles utilize large amounts of glucose during exercise. Unlike the brain, the skeletal muscles store blood sugar in the form of glycogen, which is quickly broken down to supply glucose during physical exertion. Muscle tissue also normally absorbs large amounts of glucose from the bloodstream during exercise. Although skeletal muscles can utilize fat-derived molecules for energy production, depletion of glucose stores during prolonged exercise can lead to sudden fatigue -- commonly known as bonking or hitting the wall.
Fuel for Other Tissues and Organs
The various organs and tissues of the body have the capacity to utilize different fuels. In addition to the brain and skeletal muscles, some other important organs and tissues also rely on glucose as their primary or sole fuel. Examples include the cornea, lens and retina of the eyes, and the red and white blood cells. Interestingly, although the cells of the small intestines are responsible for absorbing glucose from food and passing it into the bloodstream, they primarily use another molecule called glutamine for fuel. This leaves more glucose for other organs and tissues that are more reliant on the sugar.
In addition to its role in energy production, the human body utilizes glucose along with other substances to manufacture other important structural molecules. For example, the glycoprotein collagen consists of a protein backbone plus simple sugars, including glucose. Collagen is an essential structural molecule found in skin, muscles, bones and other body tissues. Other glycoproteins play important roles in the development and maintenance of the nerves of the body. Glycolipids, which consist of fat and sugar building blocks, are fundamental components of the membranes that surround the individual cells of the body, as well as structures within these cells.
Hypoglycemia and Hyperglycemia
A significant drop in blood sugar typically causes symptoms of hypoglycemia relatively quickly, because of the brain's exquisite dependence on a constant glucose supply. A high blood glucose level, or hyperglycemia, may or may not cause obvious symptoms. In people with type 1 diabetes, who have little to no production of the blood-sugar-lowering hormone insulin, the combination of high blood sugar and lack of insulin often leads to signs and symptoms, including: -- excessive thirst and hunger -- unintentional weight loss -- lack of energy -- increased urination
In people with type 2 diabetes or its predecessor prediabetes, these signs and symptoms often do not occur or are not significant enough to become obvious. For this reason, many people with these conditions often go undiagnosed for many years. Despite the lack of symptoms, however, persistent hyperglycemia can cause serious complications, including heart and kidney disease, nerve damage, and eye conditions that might lead to blindness.
Warnings and Precautions
Because glucose serves so many important functions in the body, discuss any concerns about your glucose levels with your doctor. This is especially important if you have risk factors for prediabetes and type 2 diabetes, including: -- age older than 40 -- above-normal body weight -- inactive lifestyle -- parents or siblings with diabetes
Seek immediate medical attention if you develop any signs or symptoms of hypoglycemia or hyperglycemia. If you have diabetes, follow your diet, exercise and medication plans carefully. Do not stop taking your medication or change the dosage unless your doctor tells you to do so.
Reviewed and revised by: Tina M. St. John, M.D.
Is This an Emergency?
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