Webster's New World Medical Dictionary

vital
Necessary to maintain life. For example,
breathing is a vital function.
vitamin
An organic substance that naturally
occurs in plants or animal tissue that is essential for
normal metabolism of the body and to life. Vitamins
play a part in dozens of crucial activities in the body:
Some are antioxidants, preventing oxidation of cells
and potentially preventing cancer; others permit or
deny chemical reactions involved in sight, brain
function, metabolism, nucleic acid synthesis, and
the like. All vitamins are either available in food or
can be made within the body. However, many peo-
ple do not eat a diet that contains the minimum daily
requirements of certain vitamins. Nutritionists sug-
gest that the best way to ensure appropriate doses of
vitamins is to eat a healthful diet, particularly one
that is rich in green, leafy vegetables and carotene
compounds. These foods offer many benefits that
vitamin supplements cannot, including fiber, and
probably include vitamin-like substances that have
not yet been isolated. Lack of specific vitamins can
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lead to deficiency syndromes, such as rickets,
beriberi, and anemia. Overconsumption of certain
vitamins can also have consequences, ranging from
minor to life threatening. Some vitamins are water
soluble, and any excess is simply excreted in the
urine. Others are fat-soluble and may build up in
the body, potentially reaching dangerous concentra-
tions. Vitamins may also interact with prescription
and over-the-counter drugs, making them more or
less potent. For these reasons, it is important to
consult a physician before adding vitamin supple-
ments to a daily regimen. See also Appendix C,
“Vitamins.”