Here's why:

* Proteins are made up of amino acids, which are the building blocks of cells.

* Cells use proteins for a wide range of functions, including:

* Enzymes: Catalyze (speed up) chemical reactions within the cell.

* Structural components: Provide shape and support to cells and tissues.

* Hormones: Act as chemical messengers.

* Antibodies: Help the immune system fight off infections.

* Transport proteins: Move molecules across cell membranes.

While other food groups like carbohydrates and fats are also essential for various functions, protein is the primary source of the building blocks needed for cell construction and maintenance.