Thirteen vitamins are necessary for health; four-A, D, E, and K-are fat-soluble, and the remainder are water-soluble.
The distinction between fat- and water-soluble vitamins is important, because although fat-soluble vitamins are more readily stored in the body, they are likely to be poorly absorbed in gastrointestinal disorders of fat malabsorption.
Small amounts of some vitamins can be synthesized endogenously-vitamin D from precursor steroids; vitamin K and biotin by the intestinal microflora; and niacin from tryptophan, an essential amino acid-but the rest must be supplied in the diet.
A deficiency of vitamins may be primary (dietary in origin) or secondary (because of disturbances in intestinal absorption, transport in the blood, tissue storage, or metabolic conversion).