Vitamins are essential nutrients that are active in metabolism and energy production and may function as enzymatic cofactors and antioxidants. With the exception of vitamin D, vitamins are not synthesized in the body and must be ingested regularly to maintain health. Vitamin deficiency or excess can lead to disease, and poor diet or disease can lead to vitamin deficiency. Vitamins may be either water soluble (B vitamins and vitamin C) or fat soluble (vitamins A, D, E, and K); fat-soluble vitamins are not cleared as readily from the body and thus present an increased risk of toxicity. Laboratory testing is used to assess the nutritional status (deficiency, sufficiency, and toxicity) of particular vitamins, including status in response to therapeutic supplementation.