It is a decomposition reaction friend. Because, as it enters our alimentary canal, the food what we have swallowed it will be a complex food. The complex food breaks up into simplest food to give two or more products like Amino acids, proteins.

See here, the food is a reactant, it decomposes to form two or more products (Amino acids and Proteins).So as you have studied this concept in the part of Chemistry.

Reactant decomposes to form two or more products is called Chemical Decomposition. This is what happened in our human digestive system. In Chemistry, it is regarding to the element decomposes into two or more products, whereas here, as per the question, for biology, element resembles the food and decomposes refers to the breakups and two or more products refers to the Amino acids and Proteins.

Hope you have understand the concept friend..

Decomposition Reaction! See it's simple Food Particles which are complex breaks into simple And that is what is the definition of decomposition reaction