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Vitamins are a group of organic compounds necessary for the human body to function. Vitamins are called micronutrients because they are needed in much smaller quantities than macronutrients (e.g., carbohydrates, lipids, proteins).

Vitamins do not provide energy (calories) when broken down but are active throughout the body and function in different ways. Some are coenzymes, compounds that make chemical reactions within the body possible. Others act as hormones, signaling cells or tissues to perform a certain function. For instance, vitamin D signals the intestines to absorb more calcium to support bone growth. A few of the vitamins are antioxidants, which donate electrons to stabilize highly reactive chemicals called free radicals. If antioxidants are absent, free radicals take electrons from other chemicals in cells, reducing their ability to function and causing tissue damage.

Vitamins were first discovered in the early 20th century, when physicians and scientists began to recognize that chemicals in certain foods cured or alleviated what were then common medical conditions (e.g., beriberi, scurvy, rickets). Further study isolated the chemical compounds from the foods, which were named vitamins. The term vitamin is from “vita,” the Latin word for life, and “amine,” which is a chemical compound that includes nitrogen, an element in many of the first vitamins discovered. They were given a letter designation in order of discovery and since then many have also been given names that relate to their chemical formulas.

Vitamins are essential nutrients, meaning they are required for the body to function but must be obtained from dietary sources, or in some cases from supplements. The body makes some vitamins, but not enough to satisfy the body’s needs. The 14 compounds generally recognized as vitamins are the following:

• Vitamin A (retinol, retinal or retinoic acid)
  
  
• B vitamins. Due to similarity in their chemical structures, the B vitamins were once thought to be a single compound. They are now divided into nine commonly recognized vitamins that are grouped as B complex vitamins. These include:
  
  
• B1 (thiamin)
  
  
• B2 (riboflavin)
  
  
• B3 (niacin, nicotinic acid, nicotinamide or niacinamide)
  
  
• B5 (pantothenic acid)
  
  
• B6 (pyridoxine, pyridoxal or pyridoxamine)
  
  
• B9 (folate or folic acid)
  
  
• B12 (cobalamin)
  
  
• Biotin (sometimes called B7)
  
  
• Choline
  
  
• Vitamin C (ascorbic acid)
  
  
• vitamin D (calciferol)
  
  
• Vitamin E (tocopherol or tocotrienol)
  
  
• Vitamin K (phylloquinone, menaquinone, menadione or naphthoquinone)

Other compounds considered vitamins at one time have been reclassified. Some have been discovered to be nonessential (the body can make them). Others were not needed by the body or moved to other groups of chemicals, such as fatty acids.

The chemistry of vitamins is complex. Many are not one but actually several related chemical compounds. In addition, some are not obtained from food in a usable form. Instead, they are precursors or provitamins, which must undergo chemical reactions within the body before they can function. One example of this complexity is vitamin A. It includes three functioning forms (retinol, retinal and retinoic acid) and a storage form (retinyl ester). It can also be made from carotenoid precursors, such as beta carotene.

To help consumers and medical professionals determine if they are getting enough nutrients to satisfy the needs of the body, the U.S. National Academy of Sciences established dietary reference intakes (DRIs). They are based on age and gender (including whether women are pregnant or breastfeeding). Adequate intakes (AI) are estimated for some vitamins (B5, biotin, choline, D and K). This is the amount of each vitamin believed to be necessary to maintain health, but there is some scientific uncertainty. For infants (0 to 12 months old), AI expresses the average intake of healthy, breastfed children. Many of the vitamins also have tolerable upper intake levels (UL). This is the maximum daily intake of vitamins a patient may consume without risking damage to his or her health. These recommendations are measured in micrograms per day (mcg/day) or milligrams per day (mg/day). The following table provides the DRIs for adult men and women.

DRIs for Adults (19-50 years)

In addition to milligrams and micrograms, some nutritional labels may show the amount of some vitamins in international units (IU). IUs are different for each vitamin and do not convert consistently to other units of measure. For example, one IU equals:

• 0.3 micrograms of retinol (a form of vitamin A)
  
  
• 0.025 micrograms of cholecalciferol (a form of vitamin D)
  
  
• 0.667 milligrams of alpha-tocopherol (a form of vitamin E)