In the body, glucose can be obtained from:
- sucrose (table sugar) a more complex sugar (essentially a combination of a glucose and fructose molecule)
- starches, a combination of a long chain of glucose molecules (which can be broken down by digestive enzymes)
- fructose (found in fruits and a part of sucrose) by metabolizing it through the liver.
Unlike other sugars, glucose is used directly by cells to produce energy by combining it with oxygen to form ATP in the mitochondria. Cells then take ATP and shed a phosphate group to directly extract energy.
Cells can extract energy from glucose in the absence of oxygen by producing lactic acid. However, lactic acid is poisonous until it is recombined with oxygen. Lactic acid is directly responsible for muscle fatigue and too high a buildup can be fatal.
The measurement of blood sugar is actually a measurement of dissolved glucose in the blood, as other sugars cannot be directly metabolized by cells.