Proteins, fats, and carbohydrates constitute the organic section of the alimentary principles, and are obtained, at least in the first instance, from the vegetable kingdom. Our bodies after death crumble into dust and mingle with the soil, thus supplying it with nitrates. The atmospheric air contains carbonic acid, likewise derived from the activities of living animals. Plants alone are able to utilise carbon and with the aid of water build it up into carbohydrates. These are eaten by animals, and after being used by them are excreted in the form of carbonic acid and water. Animals after their death therefore become the food of plants, and so it is only right and proper that they in their turn should become the food of animals. Plants again can extract nitrogen from the soil and, assisted by the carbon in the air, build up complex vegetable proteins, which being consumed by animals and broken down into simpler products again become the food of animals.

The power possessed by plants of building up complex organic compounds out of simpler chemical elements is the key to the claim made by vegetarians that plant foods alone are endowed with "vitality," whilst flesh foods are simply decaying devitalised tissue utterly useless for body-building purposes. Experience not only teaches that this argument is quite erroneous, but evidence is accumulating in favour of the existence in animals of a similar synthetic power.

The inorganic section consists of water and mineral salts, but practically the only one of the latter consumed as such is common salt (chloride of sodium), all the others being ingested in intimate association with the organic foods.

The organic nutrients are again divided into nitrogenous and non-nitrogenous, the proteins belonging to the former and fats and carbohydrates to the latter section.