Proteins are needed in diet because

Proteins are polymers of amino acids. These are divided into two categories; essential and nonessential amino acids. Essential amino acids cannot be synthesised in the body and therefore must be included in the diet. Proteins are used as structural components of tissues as channels, transporters, regulatory molecules and enzymes. Proteins can also be utilised as energy sources. Normally, this is a secondary function and becomes important only when there is not enough carbohydrate and fat in the body, e.g. during fasting.