Digestion reduces the complex carbohydrates to simple sugars, which act as fuel in our body. Similarly fats and proteins are complex structures in foods which are digested into fatty acids and amino acids. Fat is the second most important source of energy. The first thing we need from a nutritional formula is energy. If you are designing such a formula, you have to decide what fuels you are going to add to supply energy.
Living cells are glucose-burning machines.
Animals take advantage of the ability of plants to manufacture sugar and other nutrients. This energy is released as the energy-supplying molecules are dismantled by oxidation. Food-derived energy allows us to move, to do work by muscle contraction, and to keep warm. Body heat is generated by the metabolic activity of every cell. Carbohydrates and fats are the principle sources of energy , although amino acids may be utilized as energy.