The human body needs vitamins for many processes, including growth, cellular metabolism, digestion and nerve function. There are 13 vitamins humans absolutely need for health. These are usually obtained from the diet. While most vitamins, such as vitamins E and C, are ingested in their final form, the body makes several vitamins from precursors that are consumed as part of food. These include vitamin A, vitamin D and niacin, one of the B vitamins.
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The active form of vitamin A is retinol, a compound found rarely in foods. The body synthesizes the vitamin from precursors in the diet, called carotenoids. The most commonly available carotenoid is beta-carotene, which the body may store in fat cells until it is needed. Beta-carotene is provided by egg yolks, liver, fish oil, dairy products and many vegetables. Carrots in particular contain abundant beta-carotene, which gives them their orange color; the orange is masked by chlorophyll in green vegetables that are also good sources of beta-carotene. Vitamin A is essential for vision, and for development of many organs in infants and children.
Vitamin D is made in the skin during exposure to the sun. When sunlight strikes the skin, a complex series of chemical reactions converts a form of cholesterol into vitamin D. To provide the body with adequate amounts of vitamin D, some cholesterol must either be present in the diet or be made by liver cells. Vitamin D is important for calcium metabolism and healthy bones. It also helps regulate blood sugar and supports cells of the immune system.
An amino acid called tryptophan is a precursor for several important compounds, including niacin or vitamin B3. Tryptophan is an essential amino acid that the body is unable to make; it must be obtained from food. Many foods provide tryptophan, including cheese, eggs, fish, milk, nuts, poultry and soy products. Liver cells use several other factors, including iron and riboflavin to convert tryptophan into niacin, which is needed for red blood cell development, energy production and other body processes.