vitamin

Any of various chemically-unrelated organic (carbon-containing) compounds that are necessary in small quantities for the normal functioning of the mammalian body. Many act as coenzymes, small molecules that enable enzymes to function effectively. Vitamins must be supplied by the diet because the body generally cannot make them. Deficiency of a vitamin may lead to a metabolic disorder (‘deficiency disease’), which can be remedied by sufficient intake of the vitamin. Vitamins are generally classified as water-soluble (B and C) or fat-soluble (A, D, E, and K).

Vitamin A (retinol) is found in milk, cheese, butter, eggs, liver, kidney, oily fish (such as herring), and cod liver oil. However, the chemical carotene (found in most plants) can be converted to vitamin A in the human body. This vitamin is needed to make a chemical used by rod cells in the retina of the eye that respond in dim light. Shortage of this vitamin results in poor vision, especially at night.

Vitamin D is found in the greatest amounts in fish, but is present in tiny amounts in milk, cheese, butter, eggs, and liver. However, it can be made in the human skin when it is exposed to sunlight and this is sufficient for most people. It is needed to maintain bone in the body. Without the vitamin, bones are weak and can bend (rickets). Young children, pregnant women, and women breastfeeding a baby need much more than most other people. People who are housebound and do not get into the sun may become short of this vitamin.

Most people have balanced diets and so a real need to obtain extra vitamins from tablets is not common. Some vitamins are poisonous in high doses and there have been cases of people dying as a result of taking excessive amounts of vitamin pills.