20 THE PRESIDENTIAL ADDRESS 
substances consumed in almost infinitesimal amounts may, each in 
its way, be as essential as those which form the bulk of any adequate 
dietary. This complexity in its demands will, after all, not surprise 
those who have in mind the complexity of events in the diverse 
living tissues of the body. 
My earlier reference to vitamins, which had somewhat different 
bearings, was, I am sure, not necessary for a reminder of their nutri- 
tional importance. Owing to abundance of all kinds of advertise- 
ment vitamins are discussed in the drawing-room as well as in the 
dining-room, and also, though not so much, in the nursery, while 
at present perhaps not enough in the kitchen. Unfortunately, 
among the uninformed their importance in nutrition is not always 
viewed with discrimination. Some seem to think nowadays that if 
the vitamin supply is secured the rest of the dietary may be left to 
chance, while others suppose that they are things so good that we 
cannot have too much of them. Needless tosay, neither assumption 
is true. With regard to the second indeed it is desirable, now that 
vitamin concentrates are on the market and much advertised, to 
remember that excess of a vitamin may be harmful. In the case of 
that labelled D at least we have definite evidence of this. Neverthe- 
less the claim that every known vitamin has highly important 
nutritional functions is supported by evidence which continues to 
grow. It is probable, but perhaps not yet certain, that the human 
body requires all that are known. 
The importance of detail is no less in evidence when the demands 
of the body for a right mineral supply are considered. A proper 
balance among the salts which are consumed in quantity is here of 
prime importance, but that certain elements which ordinary foods 
contain in minute amounts are indispensable in such amounts is 
becoming sure. ‘To take but a single instance: the necessity of a 
trace of copper, which exercises somewhere in the body an indis- 
pensable catalytic influence on metabolism, is as essential in its way 
as much larger supplies of calcium, magnesium, potassium or iron. 
Those in close touch with experimental studies continually receive 
hints that factors still unknown contribute to normal nutrition, and 
those who deal with human dietaries from a scientific standpoint 
know that an ideal diet cannot yet be defined. This reference to 
nutritional studies is indeed mainly meant to assure you that the 
great attention they are receiving is fully justified. No one here, 
I think, will be impressed with the argument that because the human 
race has survived till now in complete ignorance of all such details 
the knowledge being won must have academic interest alone. This 
line of argument is very old and never right. 
One thing I am sure may be claimed for the growing enlighten- 
ment concerning human nutrition and the recent recognition of its 
