The unabsorbed parts of the food are discharged from the body in the faeces, which do not, however, consist solely of such matter but also contain products derived from the intestine. The undigested residue consists of the remains of tendon, ligament, elastic fibre, blood vessels, muscle fibre and other cells of animal food; of fat, cholesterin, and the cleavage products of protein; of vegetable fibre (cellulose), starch granules, and derivatives of chlorophyll; of calcium and magnesium salts of fatty acids, and magnesio-ammonium phosphate. The metabolic products in the faeces, derived from the intestines, are epithelial cells, mucus, and the residue of the digestive juices.

We may find out how much of any given food is absorbed by a study of the composition of the dejecta, because any portion of it not taken up by the gut will be found in them. It is clear that this method of determining the absorbability or digestibility of foods is not quite accurate unless we can recognize how much of the faeces is composed of the metabolic products, and this cannot at present be done. Nevertheless the results are sufficiently near to be of great value : no carbo-hydrate is excreted from the intestine and therefore all the vegetable fibre in the faeces is derived from the food : the amount of nitrogen passed out as metabolic products, such as digestive juices and epithelial cells, is small and can be allowed for; and the same applies to the non-fatty ethereal extractives of the faeces, which add to the apparent amount of fat. It is of obvious importance in the study of dietetics to know how much of the food ingested is actually taken up and made available for the needs of the body. The following table (from Atwater) summarizes the results obtained from a number of ordinary foods. These figures are obtained by experiments in which a healthy individual was fed upon one food-stuff only, with condiments, usually for at least three days. Allowance is made for the metabolic products and the figures are therefore higher than those of many published tables.

Magnus Levy found that on the average 8 per cent of the total dry substance of milk was lost, 7 per cent of meat, and 5 per cent of bread. On pure vegetable foods the loss was 10 per cent.

These figures show that the protein and fat of animal foods are much more completely absorbed than those of vegetables, with which there is a loss in the faeces of 15 to 25 per cent of the protein. In calculating the value of a vegetable diet allowance should be made for this, and more than the apparent needs supplied. The reason for the loss is that the nitrogenous matter of vegetables is mixed with masses of cellulose and starch which render it difficult for the digestive juices to reach the protein and dissolve it. Observations on the digestibility of foods in vegetarians have shown that the waste does not diminish in those who have been accustomed to such a diet for years. Hence on a purely vegetarian diet, to quote Chittenden, " an excess of work is thrown upon the alimentary organs which not only causes discomfort but is a physiological loss, entailing the working over by the system of large quantities of material in order that the required amount of protein matter may be obtained." This does not mean that it is desirable to take our carbo-hydrates entirely in the more absorbable forms of fine meal or sugar, for a moderate admixture of vegetables with other food is an advantage on account of the stimulus to the alimentary movements which their bulk and fibrous nature afford. Of the animal foods, those meats which have short and delicate fibres are the most digestible, such as the breast of young chickens : strong and tough fibres resist disintegration. Again, muscle fibres which are intimately mixed with fat, as are those of the eel and lobster, are not so easily attacked by the digestive juices.

Animal Foods.

Percentages Digested.

Protein.

Fat.

Carbohydrate

P. ct.

P. ct.

P. ct.

Beef and Veal.

100

95

-

Mutton

100

95

-

Pork . . .

100

95

__

Fish and Oysters .

100

95

Milk . . .

100

96

100

Cheese

100

95

100

Butter

-

96

Oleomargarine

-

95

-

Tallow. . .

95

-

Lard .

95

-

Oils . . .

95

-

Eggs . . .

100

98

Vegetable Foods.

Percentages Digested.

Protein.

Fat.

Carbohydrate

P. ct.

80 per cent assumed for all.

95 per cent assumed for all.

Wheat flour, fine

85

Wheat flour, medium

81

Wheat flour, coarse .

75

Rice .

85

Macaroni.

85

Rye flour.

78

Maize meal .

85

Potatoes .

75

Cabbages, turnips, etc .

80

Beans

85

Peas .

85

Similar considerations apply to vegetable foods : thus RUbner found that the digestibility of starch depends upon the ease with which the juices can reach it; with fine meal the loss of carbohydrate was only 1.1 per cent, with coarse meal 26 per cent, with whole corn 7.4 per cent, and in carrots 18 per cent. Such high figures as this last do not occur when other foods are taken as well.

Indeed, results obtained by studying the digestibility of one food at a time, valuable as they are, do not necessarily apply to a mixture of foods, and we must now look at the results which have been obtained on various dietaries.

Atwater found the absorption of an ordinary mixed diet, in a large number of observations, to be as follows : -

Protein, 90-93 per cent, Fat, 95-96 per cent, Carbo-hydrate, 97-98 per cent, Calories used, 90-93 per cent.

The digestibility is equally good with large amounts of food. In a patient in bed undergoing special feeding upon an enormous and varied diet Hale White and the writer found the utilization of protein and of fat to be 96 per cent.

Muscular work does not diminish the digestibility of foods, for in training Atwater and Sherman found the following absorption figures : -

Protein, 93 per cent, Fat, 93 per cent, Carbo-hydrate, 99 per cent.

On a fruitarian diet the digestibility is lower, as we should expect from the studies on individual foods. Jaffa found that the average of 30 digestion experiments on fruitarians gave : -

Protein, 75 per cent, Fat, 86 per cent, Sugar, starch, etc., 95 per cent, Fibre, 79 per cent, Calories, 86 per cent.

These figures are not markedly different in childhood, for a child of seven years, feeding on apples, bananas, oranges, dates, honey, olive oil, almonds, pignolias, and walnuts, digested the following proportions : -

Protein, 82.5 per cent, Fat, 87 per cent, Sugar, starch, etc., 96 per cent, Fibre, 80 per cent, Calories, 87 per cent.

The results of digestion experiments as a whole may be summarized in the following words (after Atwater). Animal protein is readily and completely digested, that of vegetables less completely. Of potatoes and beans, for instance, one third may escape digestion and thus be useless as nourishment. Fats are on the whole well digested. The carbo-hydrates in general are digestible except the crude fibre or cellulose, but the amount of this in an ordinary diet is small.

The digestive powers of different individuals differ less than is commonly supposed. We are here referring to the utilization of the food and not to the length of time which it remains in the stomach or the symptoms it may excite there, for the relative ease of digestion and the time it takes vary in different individuals to a considerable degree. Flavouring materials appear to have but little effect upon the utilization of the food.