In this connection it is interesting to note that the heat value of 1 gram of human feces is very constant whether the person is on a meat diet or a medium mixed diet. Rubner1 gives the heat value of 1 gram of organic matter in the feces of a man on a meat diet at 6.403 cal., while on a mixed diet 1 gram varies between 6.061 and 6.357 cal. The average fuel value of feces is therefore 6.2 calories per gram of dry organic substance, and this changes only when there is a poor utilization of the food.1 According to Lorisch,2 one may calculate the approximate heat value of feces by reckoning the nitrogen therein as protein nitrogen and multiplying the amounts of "protein," "fat," and carbohydrate present by their usual heat value. The sum of these is said to give a rough estimate of the calorific loss through the feces.

1 Rubner: "Die Gesetze des Energieverbrauchs," 1902, p. 35.

After eating pumpernickel, bad bread, or string beans the waste of undigested residues of these substances may appear in the feces, changing its composition and lowering its percentage of nitrogen content.

In general, Prausnitz finds no difference between the digestibility and absorbability of animal and vegetable foods. Meat, rice, and bread from flour are all digested and absorbed. The ordinary feces indicate whether a given food is a small or a great feces builder, not how much or how little food has been used for the organism.

The value in such foods as cabbage, string beans, cauliflower, and the like lies, aside from their flavor, in the fact that their indigestible waste may enhance peristalsis in the intestine. Their food value is small, and if given to those with weak digestions, is dubious. Mendel3 points out that edible carbohydrate substances, like Iceland moss, agar-agar, Jerusalem artichokes, and inulin, are scarcely attacked by the digestive juices and therefore have little or no direct nutritive function. He4 also finds that the proteins of mushrooms are not digested in the organism.

The part played by bacteria in the composition of the feces has been variously estimated. Lissauer,5 working in Rubner's laboratory, showed that two-thirds of the fecal solids were soluble in alcohol. In the insoluble portion mucin, food protein, and the remnants of cast-off epithelial cells, as well as bacteria were found. When a diet of meat was given to a man food residues were almost entirely wanting in the feces. Though the quantity of bacteria may be of importance in the stools, it is an insignificant factor when compared with the total quantity of food ordinarily ingested. Lissauer finds the following percentages of bacteria in stools of the character noted below:

1 Rubner: v. Leyden's "Handbuch der Ernahrungstherapie," 1903, p. 32.

2 Lorisch: "Zeitschrift fur physiologische Chemie," 1904, xli, 308.

3 Mendel: "Zentralblatt fur Stoffwechsel," 1908, iii, 641.

4 Mendel: "American Journal of Physiology," 1898, i, 225.

5 Lissauer: "Archiv fur Hygiene," 1906, lviii, 145.

In man the minimal quantity of bacteria composing the stools was 2.53 per cent., the maximum 13.54 per cent., and the average was 8.7 per cent, of the total solids. Rubner has calculated that 1 gram of dry bacterial substance contains 0.114 gram nitrogen. Making use of these data, Lissauer has prepared the following table to illustrate the part which bacteria may play in the fecal nitrogen elimination of man:

It is evident that the quantity of bacterial nitrogen in the feces is small in comparison with the ordinary intake of nitrogen in the food. Though the feces apparently swarm with bacteria, it should be recalled that 4,000,000,000 weigh only 1 milligram.

It may be added that Osborne and Mendel1 report that 70 per cent, of the nitrogen of the rat's feces is due to bacteria.

1 Osborne and Mendel: "Journal of Biological Chemistry," 1914, xviii, 177.