Dietary supplement be obtained from the diet. The term is conditional both on the circumstances and on the particular organism. For example, ascorbic acid (vitamin C) is a vitamin for anthropoid primates, humans, guinea pigs and bats, but not for other mammals. Vitamin D is not an essential nutrient for people who get sufficient exposure to ultraviolet light, either from the sun or an artificial source, as then they synthesize vitamin D in skin. Humans require thirteen vitamins in their diet, most of which are actually groups of related molecules, "vitamers", (e.g. vitamin E includes tocopherols and tocotrienols, vitamin K includes vitamin
Vitamin C infection. The mega-dosing theory is to a large degree discredited. Modest benefits are demonstrated for the common cold. Benefits are not superior when supplement intakes of more than 1,000 mg/day are compared to intakes between 200 and 1,000 mg/day, and so not limited to the mega-dose range. The theory that large amounts of intravenous ascorbic acid can be used to treat late-stage cancer is - some forty years after Pauling's seminal paper - still considered unproven and still in need of high quality research. However, a lack of conclusive evidence has not stopped individual physicians from prescribing intravenous ascorbic acid