There are many types of sugars, which are the simplest type of carbohydrate. While too much dietary sugar poses a number of health risks, the simple sugar glucose serves a critical role in the human body. Glucose serves a primary fuel to generate energy that the body's cells use to carry out their metabolic and biological functions. Glucose is particularly important for the brain, red blood cells and muscle cells during exercise.
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Biological Fuel Source
The primary function of glucose is to serve as a biological fuel source for the body. All cells of the body are capable of using glucose to generate energy. Through a series of complex biochemical reactions, the breakdown of glucose yields high-energy molecules called adenosine triphosphate (ATP). ATP molecules then provide the energy to drive the cellular activities that ultimately keep the body functioning. While many types of body cells can use nutrients other than glucose to generate energy, some rely exclusively or almost exclusively on glucose.
Brain and nerve cells normally rely exclusively on glucose as their fuel source. The brain is a relatively large organ with high metabolic rate. A typical adult brain utilizes roughly 120 grams of glucose each day. Because brain cells cannot store glucose, a constant supply must be provided from the blood stream. During periods of prolonged starvation, the brain can switch to using breakdown product of fats (ketones) for fuel.
Mature red blood cells also rely exclusively on glucose for fuel because these cells lack the internal machinery to generate energy from any other nutrient source. Other cells that rely almost exclusively on glucose to generate high-energy ATP molecules include:
- the lens of the eye
- some retinal cells (the vision-sensing tissue at the back of the eye)
- cells of the innermost portion of the kidneys
The skeletal muscles of the body are capable of utilizing both fatty acids and glucose for fuel. At rest and with limit physical activity, most of the energy to fuel the skeletal muscles comes from the breakdown of fatty acids. However, with moderate to strenuous physical activity, the breakdown of glucose becomes a prominent source the energy used to power the skeletal muscles.
The glucose needed by the body comes from dietary sources, body stores of glycogen (the storage form of glucose) and manufacturing it from other molecules, a process called gluconeogenesis. Since the body can only store enough glycogen to fuel its energy needs for roughly one-half day, it's important to include healthy sources of glucose in a well-balanced diet.
Reviewed and revised by: Tina M. St. John, M.D.