The importance of sunlight as a source of vitamin D for man

Abstract

Since Lord Kelvin expressed this view as the ideal to which scientists should aim there has been an assumption, particularly in modern times, that this aim is achieved. In nutrition, however, there are severe limitations on the extent to which achievement of this aim is possible. In the case of vitamin D nutrition it was over 50 years before even moderate progress, in the form of an assay for vitamin D status, could be made. This long delay before nutritionists had a means of measuring what they were talking about has caused a number of problems. For example there are gaps in our knowledge of vitamin D of which most nutritionists are unaware, because previous generations of investigators either were unable to obtain or did not realize the necessity of obtaining crucial information. Further complications have arisen because of the prolonged period taken to understand the physiology and biochemistry of vitamin D and as a result a number of erroneous attitudes have become established. During this period there were times with very little development in these areas of vitamin D which were taken by some people to mean that a full understanding of these subjects had been reached. In general it is not realized that our views on various aspects related to vitamin D are not on as sound a basis as is believed. In addition it is not appreciated that the recent advances in our knowledge of vitamin D physiology require previous results to be reconsidered. In the last two or three years my colleagues at the Dunn Nutritional Laboratory, Dr D. Fraser and Dr M. Davie, and I have been developing a new approach to nutritional aspects of vitamin D and in this article I would like to describe some rcsults which have caused us to reconsider the need fordietary vitamin D. In thecourseof this, I will trytoillustraie some of the points made above.

‘When you can measure what you are speaking about and express it in numbers, you know something about it, and when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind’.