Vitamins are biological nutrients that are required for a number of cellular and tissue-specific processes in your body. Vitamins are primarily attained from your dietary intake, with different vitamins originating from different types of food sources. Nearly all nutrients that are classified as vitamins are obtained through dietary means, which is indicative of the term essential, because the body cannot synthesize these compounds. There are only a few vitamins that are considered nonessential because the body is capable of synthesizing these nutrients. However, these so-called nonessential vitamins are still primarily obtained from your nutritional consumption, which technically makes them essential.
According to the U.S. National Library of Medicine, there are at least 13 essential vitamins. This list includes vitamins A, C, D, E, K, B1, B2, B3, B6, B12, pantothenic acid, biotin and folic acid. These "essential" vitamins are further divided into water-soluble and fat-soluble vitamins. Water-soluble vitamins are typically used immediately following ingestion or are eliminated as waste through the urine, with the only exception being vitamin B12 with is stored in the liver. Fat-soluble vitamins are stored in fat reserves in specialized cells called adipocytes.
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Nearly all vitamins are considered essential. However, some vitamins like vitamin D and biotin can be synthesized by the body, so they are not technically considered "essential". For example, vitamin D, which is important for calcium absorption and the maintenance of bone tissue, is synthesized by skin cells following exposure to ultraviolet B radiation from sunlight. Biotin is a unique vitamin because it is actually synthesized by the gastrointestinal bacteria that constitute your gut flora. While biotin is readily available from your diet, your gut flora usually produce enough of it that it is termed as nonessential.
Since most vitamins are necessary for normal biological processes, any sustained absence of any particular vitamin will result in a detrimental health condition. Vitamin A, also called retinol, is important for vision and tissue maintenance and deficiencies may result in skin rashes and night vision. Deficiencies in vitamin D, which is important for calcium absorption and bone metabolism, may result in an increased risk for osteoporosis, hypertension and cancer. Vitamin K is an important blood coagulation factor and nutritional deficiencies result in a decrease in blood clotting, leading to a higher risk of bleeding death.