The conception of deficiency diseases,
particularly of the nutritional group is quite new.
The discovery of the vitamins has opened a wide
field in medical research. Gradual realization
that many human diseases were not produced by
infections or toxins but by vitamin deficiencies
led to the conception of various deficiency
diseases. Until recently the vitamins were only
of clinical interest in the treatment of certain
types of deficiency diseases but it is now
realized that in large doses vitamins can be used in
other conditions therapeutically.
The next stage in the history of vitamin
research consisted of distinguishing individual
vitamins, their isolation, analysis and synthesis.
Vitamins A and D apparently exist only in the
bodies of vertebrates. Vitamin A can, however, be
formed in the manmalian body from certain red and
yellow pigments, carotenoids, which are to be found
in many plants, micro organisms and vertebrates.
Such substances are termed "provitamins".