Thousands of chemical reactions take place during digestion. As soon as you put food in your mouth, an enzyme in your saliva called Amylase starts to break down sugars and other carbohydrates into simpler forms which human body can absorb. 

Hydrochloric acid in your stomach reacts with food to break it down, while enzymes cleave proteins and fats so they can be absorbed into your bloodstream through the walls of the intestines.