Vitamins are essential biomolecules that act as both catalysts (meaning a substance that increases or decreases the rate of a chemical reaction) and substrates (meaning a molecule that is acted upon by an enzyme) in chemical reactions and are required in very small amounts for essential metabolic reactions in the body. With the exception of biotin, vitamin D and vitamin K, the body cannot manufacture vitamins — they must be obtained through food or nutritional supplementation. There are 13 known vitamins categorized as either fat-soluble (meaning absorbed through the intestinal tract with the help of lipids) and include vitamin A, vitamin D, vitamin E and vitamin K or water-soluble (meaning dissolves easily in water) and include eight B vitamins and vitamin C. The body stores fat-soluble vitamins for long periods of time while water-soluble vitamins (excluding vitamin B12) remain in the body for a short period of time and must be replenished more frequently.