Vitamins and minerals are a vital part of a healthy diet. If a person eats a variety of foods, the likelihood of developing a deficiency of these nutrients is very small. However, people who follow restrictive diets may not get enough of a particular vitamin or mineral. For example, strict vegetarians may become deficient in vitamin B12, which is available only in animal products. On the other hand, consuming large amounts (megadoses) of vitamin and mineral supplements, without medical supervision, may have harmful (toxic) effects. Source: The Merck Manual Home Edition- Category ID : 76682