Glucose is a kind of sugar that's found in nature on its own, as well as chemically bonded to other sugar units to form larger molecules. It's an extremely important source of energy for the human body, particularly for the brain cells. As such, your body stores glucose in the form of the large molecule glycogen.
Glucose Chemistry
Glucose is a small molecule called a monosaccharide, meaning single sugar unit. It consists of the elements carbon, oxygen and hydrogen, and it occurs on its own in nature as well as chemically bonded to other monosaccharide units. Table sugar, for instance, consists of glucose chemically bonded to the related monosaccharide fructose. Starch is a long chain of glucose units chemically bonded together. You take in glucose any time you take in starch, as well as in most sources of sugar.
Glucose in the Body
Once you absorb glucose from the food you eat into your bloodstream, your cells take up the glucose to provide for their energy needs. Cells can either burn glucose right away for immediate energy or store it for later use in the form of either glycogen or fat. Fat storage requires partially burning glucose to produce a molecule called acetyl-CoA and then building fat from the acetyl-CoA. Your cells make glycogen directly from glucose.
Glycogen Synthesis
Not all body cells store glycogen. The liver and muscles do, however; the liver stores glycogen for all the body cells, while the muscles store glycogen for their own use. When you have plenty of glucose available in the bloodstream, the liver and muscles synthesize glycogen using a series of enzymes. This requires a small amount of energy, but since you only make glycogen when glucose -- and therefore energy -- is plentiful, the liver and muscles easily produce and store glycogen.
Glycogen Breakdown
Muscles rely upon their glycogen stores during sports events or when you're active for a long period of time. Muscles can also rely upon glycogen for fuel if it's been a long time since you've eaten and there's not much glucose coming into the muscle from the bloodstream. If you're fasting and your blood sugar falls, the liver breaks down glycogen into glucose and releases it into the blood. This helps ensure that the brain and other body cells have a constant supply of sugar.
Considerations
Depending upon what you eat, you can encourage your body to store extra sugar as glycogen or as fat. Smaller meals that include plenty of fiber and protein in addition to starch and other sources of glucose encourage the liver and muscles to store excess glucose as glycogen. Meals that include lots of starch or sugar without fiber or protein tend to raise blood sugar quickly, which leads to hormonal signals that encourage your body to store extra sugar as fat.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
