Introduction

Nothing changes quicker than opinion. This remark may be said to apply truly to every subject, but to none with greater force than that of diet. We must all eat to live, and one would have thought that by this time such a universal habit would have become quite automatic, or at least encompassed by such stereotyped rules that there was no room for doubt as to its practice.

To some extent, indeed, this statement is true so far as it applies to large sections of people, for custom or national prejudice has ordained that certain alimentary substances should be eaten in a particular manner, so that dietetic habits are formed which appear to have at least a semblance of physiological sanction. But probably for this very reason, and because diet is a question for the individual and not for the nation, its details being regulated more with reference to the temperament than to a fixed standard, there is no general consensus of opinion as to the suitability or otherwise of any system which might with slight variations be made applicable to all. On the contrary, so many theories have of late been presented for our acceptance, that the time is ripe for reviewing the whole situation and endeavouring to arrive at some rational decision which may guide us in the application of dietetic principles in everyday life. For nineteen centuries or more the religious world has attempted to find a common basis of belief for the conflicting issues before it without avail. But for a much longer period of time it has decided on a common basis of action, which is briefly formulated in the statement that religion is conduct.

In the belief that there is a possibility of determining some such simple rule of action to guide us in the realm of nutrition, I am setting forth, not without some trepidation, to make the attempt.

The Alimentary Principles

In the largest sense of the term, we are what we eat. Food is not only the means of giving us heat and energy, but is the building material from which our bodies are constructed, and by means of which all dilapidations are repaired. A continual process of waste is taking place in the effort to maintain life, and to repair this tissue-waste the intake of material becomes an absolute necessity. Protoplasm is the physical basis of life, and, in addition to its other properties, its essential characteristics are (1) its power of assimilation, i.e., its ability to convert into its own substance nutrient material conveyed to it; (2) its power of excretion, i.e., its ability to expel waste materials produced by its other activities. This exchange of material or intra-molecular rearrangement is termed metabolism, the building up or assimilative process being designated anabolism, the breaking down or disintegrating process receiving the title of katabolism.

In the aggregate, this exchange of material assumes quite considerable proportions, as a healthy full-grown man of, say, 70 kilograms in weight, each day of twenty-four hours -

(1) Excretes about 2,400 grams of water, i.e., 1,300 c.c, by the kidneys, about 600 c.c. by the skin, about 400 c.c. by the lungs, and nearly 100 c.c. by the faeces.

(2) Exhales by the lungs a quantity of carbon dioxide containing about 600 grams of oxygen and from 230 to 260 grams of carbon.

(3) Loses another 20 or 30 grams of carbon by the faeces and skin, and throws off from 22 to 23 grams of different mineral salts, more than half of which is NaCl.

(4) Eliminates, chiefly in the form of urea in the urine, from 15 to 18 grams of nitrogen.

(5) And dissipates a quantity of energy which, calculated in heat, amounts to 2,400 or more calories.

It can be demonstrated that a certain proportion of this output is derived from the breaking down of the body fat, the muscular and other tissues, and hence a brief consideration of this extensive expenditure will indicate the necessity for the provision of an adequate amount of daily nourishment in the form of food to balance the account.

The elements just mentioned, with hydrogen, constitute, according to Rubner, 95.6 per cent, of the human body, while the other 4.4 per cent. consists of sulphur, phosphorus, silicon, iodine, fluorine, chlorine, sodium, potassium, calcium, magnesium, manganese, lithium, iron, and occasionally others. Few of these occur in the free state. Oxygen and nitrogen (to a slight extent) are found dissolved in the blood-plasma; hydrogen is formed by putrefaction in the alimentary canal. These, however, are exceptions, and it is the rule to find the elements in the body combined with one another to form compounds.

Whilst it is necessary, therefore, to supply all the above elements in order adequately to balance the account, they must be provided in such a form that they can be utilised by the body. The designation applied to the form most suitable for this purpose is "proximate alimentary principles," and these are conveniently grouped as follows: -