Vitamins are typically grouped into two classes: water-soluble vitamins and fat-soluble vitamins. The fat-soluble vitamins include vitamins A, E, D, and K and are stored in the liver and fatty tissues, says Colorado State University. These vitamins are stored by the body for longer periods of time and excreted slower than the water-soluble vitamins. Deficiencies of these vitamins are rare but can occur if the diet is not adequate or is not adequately supplemented.
Vitamin A
The term vitamin A can actually refer to a number of substances. Retinal and retinol are typically called preformed vitamin A, according to the Linus Pauling Institute at Oregon State University. Retinal is then transformed by the body into retinoic acid. Beta-carotene and other carotenoids can also be made into retinol by the body, says the Institute. Vitamin A helps with vision, helps ensure normal functioning of the immune system and helps transport iron to red blood cells to make hemoglobin. The Linus Pauling Institute states that retinol and retinoic acid are crucial for embryonic development, but an excess of vitamin A can cause birth defects.
Vitamin E
Vitamin E, a vitamin with antioxidant properties, also exists in a variety of forms. MayoClinic.com lists these forms as: alpha, beta, gamma and delta tocopherol; and alpha, beta, gamma and delta tocotrienol. In humans, alpha-tocopherol is the most active form. Deficiency of this vitamin is rare, but when deficiency does occur it is typically because an individual does not absorb fat well through the gut, says the MayoClinic.com. Vitamin E helps prevent cell damage that occurs from free radicals, which can harm cells and may lead to cancer, states the National Institutes of Health. Individuals who are on blood-thinning medications should use caution when supplementing with this vitamin, as increased risk of bleeding has been noted with vitamin E.
Vitamin D
Two forms of vitamin D are important for humans, says MayoClinic.com: ergocalciferol, or vitamin D2, and cholecalciferol, or vitamin D3. The main functions of this vitamin is to maintain calcium and phosophorus levels in the blood and to help the body absorb calcium, according to the MayoClinic.com. Vitamin D may help protect the body from osteoporosis, high blood pressure and autoimmune diseases. Although many foods are fortified with the vitamin and the sun helps the body produce vitamin D, many people stay out of the sun or wear high levels of sunblock, leading to vitamin D deficiencies, according to Dr. Michael Holick, a biochemist and endocrinologist. Holick believes that in the near future, the National Institute of Medicine will recommend higher doses of vitamin D to adapt to changes in lifestyle and sun-related behaviors.
Vitamin K
Vitamin K can be naturally found in the body, as the bacteria in the intestines make this vitamin, according to the University of Maryland Medical Center. The main function of this vitamin is helping blood clot, and it also strengthens bones by helping the body use calcium properly. High levels of vitamin K have been shown to reduce the risk of fractures and osteoporosis, says the University of Maryland. Deficiency is rare but can occur if antibiotics are taken for a prolonged period, killing off the intestinal bacteria and the vitamin K source.
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