BEFORE entering on the consideration of questions of Diet and Dietetics, it is of the first importance that the processes of digestion and assimilation of food in the body should be thoroughly understood, as well as the composition and the exact values of those foods which serve to build up the body after wear and waste, and to maintain it in a condition of health. I will, therefore, commence by giving a brief description of the constitution and dietetic values of the various kinds of food which form the mixed diet of an ordinary European, and also some account of the processes of digestion, absorption, and excretion.

The human body is composed of the following elements: - Carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, chlorine, iodine, potassium, calcium, magnesium, and iron. The first four are present in far larger proportion than the rest. In order that the body may be reconstituted and nourished, all these elements must be represented in the food of man.

Food is composed of organic and inorganic materials. The organic materials are furnished both by the animal and the vegetable kingdom, and are composed of the following elements: - Carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus. Of these, oxygen is necessary for the oxidation of the tissues, that is to say, for their combination with oxygen in the processes of life and function; carbon is necessary for the production of heat, which is caused by the combination of carbon with oxygen to form carbonic acid gas; hydrogen is necessary in order to combine with oxygen and form water; and nitrogen is all-important, as it is the essential element in the composition of the living tissues of the nerves, muscles, brain, and blood, as well as of the secretions and juices of the body. Its presence is also necessary in all the vital processes, for without it no energy can be produced, nor can any of the changes take place which are characteristic of the living state in the body.

Foods are divided into nitrogenous and non-nitrogenous, according as they contain the element of nitrogen or not. In the following table the various principles of food are classified and arranged: -

The albuminates contain about 15 per cent, of nitrogen. The following analysis by Malder clearly explains their composition: -

Albumen is composed of carbon, oxygen, hydrogen, nitrogen, with some sulphur and phosphorus. It exists in its purest form in the white of an egg, and is characterised by being coagulable by heat. Albumen is an important constituent of all flesh foods, and of eggs and milk. It is also present in a great number of vegetable products, and is found in wheat, oats, Indian corn - hence in bread, oatmeal, and Indian meal - in barley, rye, rice, buck-wheat, beans, peas, lentils, bananas, potatoes, almonds, and nuts, and in small quantities in carrots, parsnips, turnips, and artichokes. It is present in its most digestible form in the flesh of animals. Some vegetables contain albumen in large quantities; and, as constantly insisted upon by vegetarians in support of their views, there is a larger amount of albumen in a pound of peas than in a pound of beef.

Fibrin is almost identical with albumen, but it contains more oxygen and sulphur. It is a constituent of the blood, and undergoes spontaneous coagulation out of the body.

Casein is a component of milk, from which it is thrown down by the action of an organic acid, such as rennet. It is casein which constitutes the curd of milk, the curdling being effected by the production of free lactic acid during the process of the souring of the milk. Casein is also the basis of cheese, and in this form it is a highly nitrogenous food. Besides the four elements, carbon, oxygen, hydrogen, and nitrogen, casein also contains sulphur, but no phosphorus; and it is remarkable for the large quantity of phosphate of lime which it is capable of holding bound up with it, and for the tenacity with which it retains it. (Pavy.)

Gluten is the tenacious, sticky material which is left when flour is kneaded with water and afterwards washed to remove the starch. According to the report of the Paris Gelatine Commission, which sat for ten years making continual researches on the value of the albuminates and gelatine as articles of food, it is stated that gluten is alone necessary to support life. This assertion has since been disputed. To gluten, however, bread, the staff of life, owes its high nutritive qualities.