Collagen powder may include vitamin C due to the role of vitamin C in collagen production in our bodies, namely by adding a hydroxyl group to proline to form the amino acid hydroxyproline, a component of collagen. However, most people are not deficient in vitamin C, so this addition does not necessarily provide extra benefit. Furthermore, supplements meant for skin often combine collagen with hyaluronic acid, a component of skin, joints, and eyes that helps maintain hydration and lubrication.