But we now know (and many, indeed, have recognized it for many years) that the nutrition of the animal body, and especially of man's body, is not so simple a matter as this method would suppose. It is necessary not merely to supply the proteids, fats, starches, and sugars, in correct weight and bulk, but also certain qualities and substances in food, much more subtle and difficult to estimate precisely, which are required in order to maintain health. There are elaborate chemical compounds present in really "fresh" meats and vegetables which seem to be absolutely necessary in order to keep man (and some of the higher animals) in health, and not only that, but it is ascertained that without them he cannot be properly nourished, but dies! These subtle, highly complex bodies seem to be present in very small quantities in good fresh food, and yet are absolutely necessary though so minute in amount. The failure of a diet consisting exclusively of tinned meats and preserved foods is due as much to this as to the nausea set up by it--of which I have written on a former occasion ("Science from an Easy Chair," Second Series, 1913, p. 171, "Food and Cookery").
Let us take an example. A distinguished medical chemist, Mr. Gowland Hopkins, has recently published an account of some experiments in which he fed young rats on a purely chemical, or "artificial" diet. He gave them, in proper proportions, chemically purified casein or curd, starch, sugar, lard, and salts, mixed into a thin paste with water, of which they had an abundant separate supply. Young rats fed with abundant natural foods of mixed substances, such as cheese, bread, egg, bits of meat and vegetable, and water, grow rapidly; they double their weight in twenty days. The young rats fed by Mr. Hopkins upon the artificial pure food--though supplied with it and taking it in abundance--did not increase in weight, and most of them died before the twentieth day! The curious and important fact was established (by careful and repeated experiment) that if a teaspoonful of milk was added to the artificial food (less than one twenty-fifth of the solid matter of their daily food) the young rats did as well as on "natural" food, doubled their weight in twenty days, and grew up to be strong and healthy rats. It was made clear that something was obtained by the rats from the small quantity of milk--something necessary for carrying on their nutrition, something the importance of which was not its quantity but its peculiar quality, which was absent in the artificial diet, but present in the mixed diet of varied materials which a young rat naturally gets. It seems that some highly elaborated proteid is necessary, if only in minute quantity, to set nutrition going, and that this is furnished by the teaspoonful of milk. Here, then, we have a case in which the simple rough conclusions as to all that is necessary in diet being the proper quantities of flesh-forming and heat-giving substance, are found to be erroneous.