Collagen, also known as collagen protein, is a fibrous protein composed of three polypeptide chains twisted into a helix. It exists in the body as collagen fibers and dissolves into a gel when heated in water. It is abundant in higher animals, accounting for about one-third of their total protein content. It is mainly distributed in connective tissues; for example, collagen makes up 70% of the dry weight of skin. Collagen is a glycoprotein containing sugars and large amounts of glycine, proline, and hydroxyproline, but lacking cysteine and tryptophan. It is an incomplete protein with relatively low nutritional value, but it is crucial for the formation of skin, blood vessels, bones, tendons, teeth, and cartilage in both animals and humans, serving as the main matrix of these connective tissues.