It was just before the First World War when vitamin A was chemically identified as "fat soluble A" and in the 1920s studies were conducted on laboratory rats to see what happened when this component was left out of the diet.¹ A now familiar series of hypovitaminosis A changes occurredwidespread keratinisation of epithelia, decreased immune function, anaemia, xerophthalmia, and blindness. In the human population even subclinical deficiency of vitamin A leads to high levels of childhood morbidity and mortality. Astonishingly the ancient Egyptians knew that liver in the diet or the juice of cooked liver put into the eyes cured night blindness, but it took us about 5000 years to rediscover this medical fact.

Soon after, in the 1930s, similar dietary deprivation studies were performed with a view to asking what happens to the embryo when vitamin A is removed. In fact vitamin deficiencies, . . . [Full text of this article]