Vitamin A deficiency and excess.

Vitamin A 129 dubious but in practice the results are very good-partly in fact because errors tend to be compensated ! Correction procedures devised specially to meet difficulties over vitamin A assays are actually theoretically safer in most of the many other situations in which they have been used ! I suppose there are lessons here as well as an element of irony. The problem of the site and manner of conversion of p-carotene into vitamin A was another problem which had its ups and downs, but the importance of processes occurring in the lining of the gut is beyond doubt. There have been interesting quantitative aspects of the storage of esterified vitamin A in the liver and its release as alcohol into the blood. The human requirement for vitamin A has been another problem with its own chequered history of good work done under difficulties, and the Sheffield wartime experiment on volunteers has its niche in the history of the subject. The deficiency syndrome was at first (perhaps necessarily) oversimplified, but as time has gone on appreciation of its complexities has grown. Hypervitaminosis A has in some respects been as revealing as hypovitaminosis, and experiments with tissue cultures have enriched notions based on experiments with intact animals. Links with the incidence of congenital abnormalities have shown that vitamin A can be teratogenic as well as indispensable. The variety of deficiency signs shown by different species has enlarged and complicated our outlook. My colleagues, Howell, Pitt and Thompson, have studied vitamin A acid (retinoic acid) with results which have not lost any interest to me through familiarity. I am left with a feeling that despite the enormous-and in many ways deeply satisfyingbody of knowledge about vitamin A we are still short of essential biochemical information. I t is not my place in an introduction to speculate about the possibility of new light on the mode of action of vitamin A. I am here to listen. I am convinced, however, that the intellectual consummation of half a century of research has still to come. When it does we may be surprised at the sign-posts on the road which escaped our attention but were there all the time!