Protein is composed of the amino acids essential for the building of new cells, including hair. Five amino acids are pretty relevant for hair growth: cystine, cysteine, methionine, arginine, and lysine.
Inadequate protein intake over a prolonged period can force the hair to enter the resting phase and shed a few months later. It is evident then that sufficient portions of protein-rich foods should form part of your daily diet. The best dietary protein sources are lean meats, fish, poultry, eggs, dairy products, soy, nuts, grains, and seeds. At least 15% of your daily calories should come from protein-rich foods.