CHEMISTRY OF DIGESTION AND NUTRITION. 305 



The experience of mankind, as well as the results of experimental investiga- 

 tion, shows that the healthy diet is one composed of proteids, fats, and carbo- 

 hydrates. The proportion in which the fats and the carbohydrates should be 



taken and, to a certain extent, this is true also of the proteids may be 



varied within comparatively wide limits, in accordance with the law of " iso- 

 dynamic equivalents." This is illustrated by the following " average diets " 

 calculated by different physiologists to indicate the average amount of food- 

 stuffs required by an adult man under normal conditions of life : 



Average Diets. 



In Voit's diet, which is the one usually taken to represent the daily needs 

 of the body, it will be noticed that the ratio of the nitrogenous to the non- 

 nitrogenous food-stuffs is about as 1 : 5. It must be remembered, in regard to 

 these diets, that the amounts of food-stuffs given refer to the dry material : 118 

 grams of proteid do not mean 118 grams of lean meat, for example, since 

 lean meat (flesh) contains a large proportion of water. Tables of analyses of 

 food (one of which is given on page 216) enable us to determine for each par- 

 ticular article of food the proportion of dry food-stuffs contained in it, and in how 

 great quantities it must be taken to furnish the requisite amount of proteid, 

 fats, or carbohydrates. There is, however, still another practical consideration 

 which must be taken into account in estimating the nutritive value of articles 

 of food from the analyses of their composition, and that is the extent to which 

 each food-stuff in each article of food is capable of being digested and absorbed. 

 Practical experience has shown that proteids in certain articles of food can be 

 digested and absorbed nearly completely when not fed in excess, while in other 

 foods only a certain percentage of the proteid is absorbed under the most favor- 

 able conditions. This difference in usableness of the food-stuffs in various 

 foods is most marked in the case of proteids, but it occurs also with the fats 

 and the carbohydrates. Facts of this kind cannot be determined by mere 

 analysis of the foods ; they must be obtained from actual feeding experiments 

 upon man or the lower animals. In general, it may be said that in meats from 

 2 to 3 per cent., in milk from 6 to 12 per cent., and in vegetables from 10 to 

 40 per cent, of the proteid escapes absorption. The greater value of the meats, 

 then, as a source of proteid supply consists not only in the greater average per- 

 centage of proteid contained in them as compared with the vegetables, but also 

 in the fact that their proteid is more completely absorbed from the alimentary 

 canal, less being lost in the feces. Munk l gives an interesting table showing 

 how much of certain familiar articles of food would be necessary, if taken 

 alone, to supply the requisite daily amount of proteid or non-proteid food; his 



1 Weyl's Handbuch der Hygiene, 1893, vol. iii., part i. p. 69. 

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