Vitamins are essential organic compounds required by organisms. A vitamin is classified as such when it cannot be synthesized in sufficient quantities by an organism and must be obtained from dietary sources. Thus, ascorbic acid is classified as vitamin C for people, but not as a vitamin for most other animals because they can manufacture it in their bodies. Vitamins are also classified by their biological and chemical activity, not their structure, which gives rise to a number of vitamer compounds. Vitamins are also divided into water-soluble and fat-soluble forms, as well as natural and synthetic types.
Vitamin A
Vitamin A consists of six vitamer compounds that have similar or interchangeable biological and chemical activities. These compounds include retinal, retinol and four carotenoids that are converted to retinol, the best known being beta-carotene.
Vitamin B-Complex
Vitamin B consists of eight vitamer compounds that are all involved in metabolism and co-enzyme production, among other functions. These compounds include thiamine-B1, riboflavin-B2, niacin-B3, pantothenic acid-B5, pyridoxine-B6, biotin-B7, folate-B9 and cobalamin-B12. The daily requirements of B vitamins vary dramatically.
Vitamin C
Vitamin C, or ascorbic acid, is not considered a vitamin in the vast majority of other mammals.
Vitamin D
Vitamin D is a collection of five vitamers known as secosteroids. Of these, vitamins D2 and D3 are the most essential to human physiology. Vitamin D2 is known as ergocalciferol, whereas vitamin D3 is named cholecalciferol. Vitamin D3 is produced in the skin in response to sunlight, which causes some researchers to consider it more as a hormone, according to "Vitamins: Fundamental Aspects in Nutrition and Health."
Vitamin E
Vitamin E is a collection of eight vitamers, divided into four tocopherol types and four tocotrienol types. Tocopherols are the best-studied group and are referred to as alpha, beta, gamma and delta types. The different types of vitamin E provide benefits that vary from each other.
Vitamin K
Vitamin K represents a group of five vitamers essential for coagulation and blood clotting. Vitamins K1 and K2 are derived from plant and animal sources, respectively, while K3, K4 and K5 are synthetic types that are added to commercially produced foods.
Natural Vs. Synthetic
Until the 1930s, when the first commercial yeast extract and synthetic vitamin C supplements were sold, vitamins were obtained solely through foods, as cited in the "Doctor's Complete Guide to Vitamins and Minerals." All vitamins are now available as semi-synthetic or synthetic supplements, but some have different names. For example, vitamin B9 from food is called folate, but in synthetic form is known as folic acid. Vitamin B12 is known naturally as cobalamin, but synthetically as cyanocobalamin or hydroxocobalamin. Vitamin B3 is known naturally as niacin, but a synthetic counterpart that does not cause "niacin flushing" is called niacinamide.
Fat-Soluble vs. Water-Soluble
Vitamins A, D, E and K are considered fat-soluble because fats are needed in the digestive tract for them to be properly absorbed. As well, they are stored in the body for longer periods of time. All the other vitamin types are considered water-soluble and are readily absorbed without food, but excess amounts are quickly excreted in the urine. An exception to this rule is B12, which can be stored in the body for at least a few years, according to "Nutritional Sciences."
References
- "Vitamins: Fundamental Aspects in Nutrition and Health"; G. Combs; 2008
- "Doctor's Complete Guide to Vitamins and Minerals"; Mary Dan Eades and Philip Lief; 2002
- "Nutritional Sciences"; Michelle McGuire; 2007
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