Technically, all sugars have the same chemical formula of C6H12O6. However, the means by which these atoms are assembled to form the basic sugar molecule varies from one type of sugar to another.
For whatever the reason, the human body has evolved to the use the Glucose form of sugar. Consequently, the body through a process known as the Carbohydrate Cycle converts all the other sugars into Glucose. The Glucose is then transported via the blood system to the various cells of the body to be used as a source of energy.
For more on Glucose see http://en.wikipedia.org/wiki/Glucose