Our bodies are constantly in motion. The heart and lungs move with every breath. Every thought causes some change in the brain. Whenever any part of the body loses its power of motion, it dies. All this motion, whether voluntary or involuntary, results in the gradual wasting away of the flesh, blood, and bones of which the body is composed. "We begin to die as soon as we begin to live." If the worn-out materials be not replaced, we die. One great object of food is to supply this waste. The demand for building material is greatest when the body is in a state of activity. Until the human body has attained its complete growth, there should be a constant supply of material for new growth, as well as for repair. In maturity, or when, from bodily inactivity, there is less waste, a smaller supply will suffice. Food taken at regular intervals supplies means of growth, and repairs the worn-out tissues.


The temperature of the living human body is about 98°. In hot or cold climates, in summer or winter, though the temperature of the external parts may vary, the internal temperature is the same; and if not maintained within a few degrees of this point, death invariably follows. The source of this animal heat, so independent of outside circumstances, must be from within. To keep up this internal heat or fire, a constant supply of fuel is necessary. This fuel is supplied by our food. To furnish material for growth and repair, and to provide fuel for the warmth of the body, is the twofold object of food.

Animal Combustion

The process by which food maintains the motion and warmth of the body is a kind of combustion, and has often been compared to the combustion carried on in the steam-engine (see Youmans's Chemistry). We can have no combustion without oxygen; therefore oxygen is the first important element of food. The air is our great source of supply of oxygen, and a volume might be written on the necessity for pure air and perfect ventilation. We breathe oxygen from the air into our lungs, and exhale carbonic acid. There must therefore have been some internal union of carbon or hydrogen with oxygen, and such a union always produces heat. The carbon and hydrogen are obtained from our food, and are important elements. They are necessary for animal combustion.

Food is taken in a natural or in a prepared state, and, after undergoing certain processes of digestion and assimilation, becomes a part of our bodies for a time, and then is burned in the body, the process resembling somewhat the burning of wood and coal in our grates. But this union of carbon and oxygen, "instead of taking place in one spot and so rapidly as to be accompanied by light, as in the case of the grate fire, takes place in each drop of the blood, and so slowly and continuously as not to be noticed." The force and heat absorbed from the sun by the vegetable in growing, and stored in its starch and sugar, are set free, by the decomposition of the vegetable, into carbonic acid and water again. These are given out, partly as heat, keeping the body temperature at 98°; and partly in other forms, - in that of mechanical motion, etc.

All the external or internal work of the body is done by the force and energy of the food which is burnt therein. The greater the amount of work to be done, the greater must be the supply of fuel. The fire is constantly burning. "The smoke passes out in exhalation, inhalation is the bellows to furnish more oxygen," and food supplies the fuel. The kidneys are the grates through which the ashes are removed. But if we are "a house on fire," why are we not consumed? Because, lest this internal fire burn too freely, the oxygen of the air is diluted with nitrogen, which is incombustible. The blood, bones, and muscles of the body are composed largely of nitrogen, sixteen per cent of that element being present; and this prevents the complete burning up of the structure. "What the iron is to the stove, the nitrogenous tissues are to the body."

But the stove wears out in time, and so our bodies are constantly wasting away; and these nitrogenous elements must be supplied by our food.

Food, to accomplish its purpose fully, should consist of these four elements: oxygen, to support combustion, - obtained from the air; carbon and hydrogen, to furnish fuel, - obtained from water and carbonaceous food; nitrogen, to build up and repair the tissues of the body, - obtained from nitrogenous food.

Food, in the form in which it is eaten, cannot sustain life. It must be converted into a fluid that can pass through very small channels into the blood. Then it must be mixed with the air, and undergo certain changes, before it can replace the worn-out elements of the body. To prepare food so that it can most readily be assimilated, that is, made like our bodies, should be the chief purpose in cooking. To do this, three things are essential: 1st. The food selected should be of the right material, and properly proportioned; 2d. It should be cooked in the most digestible and attractive manner; 3d. It should be adapted to the various circumstances of age, occupation, climate, and state of health.

Food, to be of the right material, should contain all the elements that our bodies contain. It is of primary importance, then, in studying food, to understand first the composition of the human body.