The adult man in full health recuperates daily by his food what he loses by his functions. In the normal state the two conditions balance one another but if by an internal or external cause an inequality is produced, the individual happens to lose more than he gains or to accumulate more than he loses.

If the nutrition is abnormal or such cells charging themselves with albumin, with fat, with nitrogenous matters incompletely used up or oxidized, if the organs of dissimilation are choked up or exhausted or if the eliminating filters of materials harmful to life only work imperfectly the individual can become ill.

If food no longer provides the necessary quantities of nitrogen, iron, phosphorus, potash, lime, magnesia, etc or if some of the rare and specific matters of significance which have escaped us for a long time such as iodine, bromine, arsenic, manganese, etc diminish or disappear from the aliments and the organs, morbid diathesis, predisposition, pathological state, acute or chronic disease are established.

If the oxidations are not sufficiently assured through want of the oxidizing agents, if the other ferments of the system are too strong or too weak proportionally, or rendered inactive, then the faulty temperament, morbid diathesis, predisposition, pathological state, acute or chronic disease are established, sometimes quickly and more often gradually.

Attention to the rules of diet is the method of alimentation specially designed in these cases to nourish the individuals predisposed or ill, so as to cooperate with medical advice properly given, in re-establishing the organs and functions in their normal state.

How can we escape from the numerous causes of decay which we have just pointed out, and how can we normally regulate our food? Theory, formulae, would be, a priority, powerless to resolve this too complex problem. We will, first of all, seek its practical solution in the examination of facts and statistics, and verify it methodically afterwards.

In all countries and at all times, man has obtained from the three kingdoms vegetable, animal and mineral the nourishment which is indispensable to him. Without doubt, in extreme climates, the Laplander or Greenlander feeds himself almost entirely on the flesh and fat of the fish or cetacea which he catches. The civilized man of our country, rich or poor, forms his nourishment of meat, milk, bread, fruits, water, and of different salts which his foods bring him, or which he adds to them.

In thus varying his food, and borrowing it at once from animals, plants, and minerals, he obeys, as we shall see, an instinct which guides him more surely than his reason. Of all material mechanisms, that of the animal is the most complex there enters normally into his constitution seventeen or eighteen simple bodies hydrogen, oxygen, sulphur, fluorine, chlorine, bromine, iodine, nitrogen, phosphorus, arsenic, carbon, silicon, potassium, sodium, calcium, magnesium, iron and perhaps copper, manganese, aluminium, boron and vanadium.

All these elements (and perhaps those which we do not yet recognize), associated in a very complex manner, are like the special pieces, the wheels or constituent parts of complex principles without which the life of certain organs, and the general life of the individual, remain unrealizable.

It appears then very improbable, if not impossible, that a single alimentary substance, be it milk, flesh or bread, and even that a single kingdom exclusively, either of animals or plants, could furnish all the elements at once, at least in relative acceptable quantities and in a suflicient weight.

Carnivorous animals, can feed themselves indefinitely on meat and do without vegetables by reason of the aptitude they have of transforming into ammonia a notable quantity of their nitrogenous aliments and of thus alkalizing their blood. But man only possesses this faculty to a relative extent. The herbivorous man or the vegetarian can certainly live solely on herbs or vegetables but it is only by accumulating a mass of nourishment so that a considerable portion of it remains unused. It is rejected after having furnished the necessary principles, owing to the superabundance of some and to the in-utilization of others.

Having neither the aptitude of the carnivore, nor the digestive capacities of the herbivora, man at all times and among all peoples has had recourse, in order to nourish himself, to a mixed diet at once vegetable, animal and mineral. The most natural food for our species after milk, bread, even when water and salt have been added to it, does not indefinitely suffice, as the experience of the Englishman Stark, who was the victim of it, has demonstrated.

In a general way, we observe that our organs have above all, while destroying themselves by the very discharge of the functions, the need of drawing from nourishment the materials of which they are constructed, or those which resemble them the most by their constitution.