A food has been defined by C. Voit as a "substance which can cause the addition of a necessary element to the body or prevent or diminish the discharge of such material." To this is added that the substance must not be injurious or cause a greater loss of energy than it brings in.

This definition includes oxygen, water and mineral salts, as well as protein, fat, and carbo-hydrate. It expresses the conception of food as matter supplied to the body, without verbal mention of the potential energy with which some of that matter must be endowed. Those foodstuffs which yield energy are, however, clearly designated in the words "substance which can . . . prevent or diminish the discharge of material "from the body. We have discussed the twofold function of the food in the last chapter and have seen that oxygen, water, salts, and that amount of protein which is needed to replace the wear and tear of tissues, supply the material needs of the body, whilst fat, carbo-hydrate and any excess of protein furnish energy for the activities of the muscles and other organs. It may seem unfortunate that the same term should be used for that which furnishes material and that which furnishes energy. But a separation of the two classes offers difficulties, for the energy yielding foods, especially protein, also provide material for the structure of the tissues, and any definition restricting the term food to substances capable of giving up energy to the organism would exclude water and salts, without which life cannot be supported. Oxygen is not commonly regarded as a foodstuff, and may be excluded by limiting the definition to materials taken in from the alimentary canal. We shall find occasion to discuss the part played by water and salts in the diet of man. The science of dietetics is, however, chiefly occupied with the study of those foods which are oxidized in the body and provide power for the activity of the muscles, glands, and other organs.

We have considered, in the previous chapter, the three classes of food-stuffs, protein, carbo-hydrate, and fat, and have referred to the decompositions which they undergo in the tissues. Each of them when oxidized by burning the dry material gives out a definite quantity of heat for every gramme burnt. This is known as the heat value, or caloric value, and is expressed in kilocalories. A kilocalorie, large calorie, or Calorie,1 is a thousand small calories. A small calorie is the amount of heat required to raise a gramme of water through 1° Centigrade. This unit is inconveniently small for physiological values. The caloric value of any food can be determined by a calorimeter, and is a measure of the energy which is given out by the complete oxidation of the substance under any circumstances. That is to say a gramme of fat oxidized in the body to carbon dioxide and water yields exactly the same energy, manifested as work or as heat, as if it were burnt in the calorimeter. The following table gives the heat evolved by a few representative food-stuffs when burnt in a bomb calorimeter.

1 When the word calorie is used alone it should, strictly speaking, be spelt with a capital letter if large calories are denoted. This rule is, however, commonly observed in the breach, and in works on metabolism the term calorie may always be presumed to refer to large or kilo-calories.

1 gramme of casein . . . yielded 5.9 kilocalories.

1 „ egg albumen . . ,, 5.7 „

1 „ protein (average). . „ 5.7 ,.

1 ,, animal fat „ 9.5 ,,.

1 „ butter fat ,, 9.2 „.

1 „ cane sugar „ 4.0 „.

1 „ sugar of milk . . „ 3.9 ".

1 ,, grape sugar . . ,, 3.7 „.

1 „ starch „ 4.2 „.

(From Hammarsten's Physiol. Chem., Transl. by Mandel).

The carbo-hydrates and fats are completely burnt in the body, and therefore give up all their available energy. With protein the case is different, for the nitrogen it contains is excreted as urea and other bodies which are not completely oxidized and therefore have themselves a heat value. By subtracting the value of the nitrogenous excreta from that of the protein in the food a figure is reached which is a measure of the energy actually yielded to the body, and this is known as the "physiological heat value" of protein. Taking the average of the heat of combustion of different carbo-hydrates, fats, and proteins, and allowing for the incomplete oxidation of protein the physiological caloric values according to Rubner are -

1 gramme of protein . . . . . - 4.1 calories.

1 „ carbo-hydrate . . . . - 4.l ,,.

On a vegetable diet the physiological heat value of protein is a little lower than the above, because absorption is less complete, and is given at 3.9. The value of animal protein is, however, about 4.2, so that on a diet of which 60 per cent of the protein was derived from animal sources and 40 from vegetable the figure would remain at that given by Rubner, namely 4.1.

From these figures it is possible to calculate approximately the fuel value of any diet if the composition of its constituents be known. A more accurate method is to determine in a calorimeter the heat of combustion of the actual foods and that of the dried excreta, the difference between the two giving the energy which the body has retained. This has been done in some recent researches. For calculating the value of diets in medical work, Rubner's averages are sufficiently accurate. We can therefore ascertain by these means how much energy is taken in by an individual over any period of time. This energy is used in the activities of the body or else stored up in fat, protein, or glycogen, to be available for use in the future. Careful experiments have shown that there is a balance between the energy taken in as food and that given out by the body in the form of work or heat, and that the body has no other source of energy but the food.

We see then that the food-stuffs, except water and salts, are substances which are capable of being oxidized to give up energy. But if this were all, any combustible substance, such as coal or wood, would be available as a food-stuff, and this is not the case. A food must therefore possess certain other qualifications. Of these the chief is that it must be capable of digestion, that is to say, of being broken down by the chemical activity of the digestive juices into such a form as can be absorbed, and further, the substances absorbed must be such as can be oxidized in the tissues.

In health the process of digestion is carried on without any conscious sensation except of well-being. In ill-health unpleasant sensations arise which we call indigestion. These feelings are almost always connected with that part of the digestion which takes place in the stomach. In a person of "weak digestion" a food may be indigestible in the sense that it gives rise to pain from the stomach, but may nevertheless be successfully dealt with by the juices of the small intestine, where the major part of digestion takes place. For this reason the term digestibility is often limited to mean digestibility in the stomach without undue delay or painful sensation, and the length of time which food remains in the stomach is taken as a measure of its "digestibility." This criterion is of use in disease but of much less value in health, and it is a mistake to conclude that foods which remain long in the stomach should be avoided by normal people on that account. In its true sense the term " digestibility " refers to the whole process of digestion, and an indigestible food is one a great part of which passes out in the faeces without having been disintegrated and absorbed. Most ordinary foods are in this sense digestible, the least so being vegetable foods containing much fibre. A table showing the digestibility or absorbability of a number of food-stuffs has been given in the last chapter under the heading of absorption.

The following table, representing the experiments of various authors and compiled from Hutchison's Food, and the Principles of Dietetics, gives the length of time which various foods were found to remain in the stomach.

Beef, raw ....... 2 hours.

„ boiled . . . . . . 3 „

„ half roasted . . . . . 3 ,,.

„ roasted . . . . . . . 4 ,,.

The digestibility of mutton has been found to be about the same as that of beef. Pork requires a longer time than mutton and beef, and veal appears to occupy an intermediate position.

Bread, 2 1/2 ozs. . . . . . 2 1/3 hours.

Eggs, two, lightly boiled . . . 1 3/4 ,,.

„ raw . . . . . . 2 1/4 „.

„ poached with butter . . . 2 1/2 „.

„ hard boiled ...... 3 „

„ as an omelette . . . . 3 „

Fish, 7 ozs. ....... 2 1/2 „

Salt Fish....... 4.

Apple, raw, 5 1/3 ozs. . . . . 3 1/6 ,,.

Cabbage, 5 1/3 ozs. . . . . . . 3 „

Cauliflower, 5 1/3 ozs. . . . . 2 1/4 „

Potatoes, 5 1/3 ozs. ...... 2-2 1/2 „

Lentils, boiled, 5 1/3 ozs. . . . . . 4 „

Peas, 7 ozs. . . . . . . . 4 1/4 „

Rice, boiled, 2 1/2 ozs. . . . 3 1/2 „

These figures must be regarded as examples and too much reliance must not be placed upon them. The time taken for dealing with a simple food-stuff does not necessarily apply to that food-stuff when taken with others. Individuals also differ widely in their digestive powers. We may expect the skiagraphic method to furnish us with exact information as to the length of time which meals remain in the normal and the diseased stomach.