Collagen is a group of naturally occurring proteins. In nature, it is found exclusively in animals. It is the main protein of connective tissue. It is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content.
In muscle tissue it serves as a major component of endomysium. Collagen constitutes 1% to 2% of muscle tissue, and accounts for 6% of the weight of strong, tendinous muscles. Gelatin, which is used in food and industry, is derived from collagen.