Vitamin metabolism and requirements: some aspects reviewed.

Current knowledge concerning the metabolism and requirements of thiamine, riboflavine, vitamin B6, vitamin A, and vitamin C in the human is reviewed. The requirements of these nutrients appear to be the results of their excretory losses in the urine and faeces and through metabolic degradations. Although influenced by numerous factors, the catabolism appears in general to be the result of actions by degradative pathways unrelated to the biochemical or physiological functions of the vitamins. As a consequence, these excretory and catabolic losses cause continuous obligatory requirements for the vitamins that must be provided by dietary means.