Vitamins are compounds that our bodies must gather from the environment to ensure healthy growth and maintenance. Unlike minerals, which serve a similar function, vitamins can be natural found only in plant and animal tissue, though they can now be artificially synthesized and taken in pill form. There are 13 essential vitamins, each of which has unique benefits and dietary requirements.
The identification of each individual vitamin was accomplished relatively recently, and while our knowledge of their functions has advanced significantly, it is not complete. Pre-modern societies were often aware that certain foods could cure deficiency illnesses. The Egyptians knew that night blindness could be cured by feeding the patient liver, but they did not know that Vitamin A was the key to recovery. Much later, British sailors realized that scurvy could be prevented with citrus products. However, it was not until the biochemist Kazimierz Funk isolated vitamins in the early 20th century that the existence and importance of vitamins gained widespread acceptance.
There are two main groupings of vitamins, determined by the compounds' solubility. Four types of vitamin are fat-soluble (A, D, E and K) and nine are water-soluble (All 8 B vitamins and vitamin C). This distinction is important because fat-soluble vitamins are stored for long periods of time instead of being flushed rapidly from the body. These storable vitamins require both more and less vigilance; on the one hand, they do not have to be constantly worked into diets, but on the other hand they present the risk of building to toxic levels.
While overdosing on a particular vitamin is a possibility, the more significant risk comes from failing to get enough of a compound. Deficiencies in a vitamin can produce a whole range of breakdowns in bone, blood, and skin maintenance. Recognizing the importance of informing citizens about their dietary needs, the United States Department of Agriculture regularly publishes tables that reflect the medical consensus on the recommended levels for healthy functioning. Their advice used to boil down to one number, the Recommended Daily Allowance, but this has been expanded into four Dietary Reference Intakes (DRIs): the Estimated Average Requirement (EAR), the Recommended Dietary Allowance (RDA), the Adequate Intake (AI) and the Tolerable Upper Intake Level (UL).