UTILIZATION OF THE FOODSTUFFS 139 



all been absorbed from the intestine. This is true of eggs, milk, butter, 

 margarine, lard in fact, of fats generally which are fluid at the temperature 

 of the body and are not surrounded by membranes. However, even with other 

 fats, like bacon fat, which is inclosed in membranes, the utilization is com- 

 monly very complete. Thus, with 350 g. per day, most of which was bacon 

 fat (unrendered lard), only 45 g. appeared in the faeces. 



Carbohydrates also are well absorbed in the intestine, inasmuch as the 

 loss by the faeces from the ordinary articles of diet rises only to about ten 

 or eleven per cent at the highest, being as a rule smaller than this. It is 

 true of carbohydrates also that with finely prepared foods the utilization is 

 much better (0.8-3.2 per cent loss) than with coarse foods (6.9-11 per cent 

 loss). The digestibility of cellulose has already been discussed at page 110. 



By microscopic examination of the faeces J. Moeller has shown that healthy 

 men digest the starch of cereals and potatoes almost completely, even if the 

 starchy food is but imperfectly ground up. If, however, the starch is present 

 in the form of leguminous seeds or is eaten in green vegetables, it is passed out 

 undigested. The hard-walled cells of the ripe leguminous seeds appear not to 

 be digested at all, so that only that part of the starch which is liberated from 

 the cells by mechanical destruction of their walls is of any benefit in nutrition. 

 The starch of green leguminous plants, on the other hand, is just as completely 

 digested as that of cereals. The gluten layer of the latter behaves like the 

 leguminous seeds : their membranes, consisting of pure cellulose, are not digested, 

 and their contents, consisting of proteid and fat, are digested only so far as 

 they are set free by rupture of the cell membranes. 



The absorption of mineral constituents of the diet, calculated in percent^ 

 ages of the amount supplied, is generally rather poor. But we must remember 

 that the ash of the faeces comes mainly from the body itself, seeing that many 

 mineral substances are excreted through the intestinal wall. 



C. UTILIZATION OF A MIXED DIET 



The experiments which we have discussed so far relate chiefly to the 

 absorption of individual articles of food. We might suppose, however, that 

 a mixed diet, such as is ordinarily eaten by man, would be utilized more 

 advantageously than these experiments indicate ; and in fact it has been shown 

 that certain mixtures are absorbed better than their separate components. 

 But all the experiments on the utilization of a mixed diet which we have 

 as yet, go to show that animal nitrogen is absorbed better than vegetable 

 nitrogen. 



A few words remain to be added on the utilization of the total potential 

 energy of the diet. We assume here as before, that the total faeces represent 

 a residue of the ingested food. For a mixed diet Eubner, on the basis of 

 one experiment on the heat value of the faeces, estimates the loss at 8.11 

 per cent of the gross calorific value of the food. In those experiments with 

 a mixed diet in which the utilization of all the foodstuffs has been investigated, 

 the results show a loss in potential energy of 4.8 to 13.9 per cent. By direct 

 determinations of the heat value of the food, and of the faeces, Atwater has 

 found in 117 experiments with a mixed diet of easily digestible substances, 



