Blood glucose, also known as blood sugar, is an energy source for muscles and other tissue cells that comes from food and your liver. Sugar enters the bloodstream, gets absorbed, and enters the cells with the help of insulin. If your blood glucose levels are low (such as from a lack of eating), stored glycogen in the liver is broken down into glucose to keep your glucose levels within a normal range.
Source:
http://www.mayoclinic.org
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