Collagen supplements commonly contain types I, II, III, IV, V, and X, or some combination of these. Each type is beneficial, but type II is the most common and has the most research behind it. Collagen supplements are usually made from the skin, bones, or cartilage of cows, pigs, chickens, and fish. The collagen protein itself comes in one of two forms: undenatured and hydrolyzed. Undenatured means the collagen proteins are still intact and have not been broken down into individual amino acids. This allows the body to absorb the proteins in their active forms without needing to process them. Hydrolyzing collagen means the proteins have been broken down into smaller pieces called peptides, which are simply groups of amino acids. Hydrolyzed collagen is also known as gelatin (the same stuff used to make jello), collagen hydrolysate, or collagen peptides.