Collagen is composed of a wide variety of subtypes (I through XIII), but it is undenatured or non-hydrolyzed type II collagen that is present in articular cartilage. Its main functions are resistance to intermittent pressure and joint support. Vitamin C contributes to normal collagen formation for the normal function of skin, bones, blood vessels and cartilage. Vitamin D contributes to the maintenance of normal bones and to the absorption and utilization of calcium and phosphorus.