Humans have specific enzymes to reduce glucose, galactose, and fructose. Most infants can break lactose into glactose and glucose, as can most adults (some adults have difficulty doing this chem cellular chem reaction.
If humans had the proper enzymes, they could digest cellulose, but the don't have it in their DNA.
Why are simple sugars required for metabolism? What makes them so important?
2 answers
I just wanted to add one thing. The importance of simple sugars has to do with their role in producing ATP. Without them, our cells would have a hard time producing the energy needed to carryout energetic unfavorable reactions.