THE ABSORPTION AND USE OF FOODS 455 



heat in keeping itself at a proper temperature the year round, 

 and so the extra amounts of food we have to eat on account of 

 the inefficiency of our bodily machines are not wholly wasted 

 after all. 



The Relative Food Values of Proteins, Carbohydrates and Fats. 

 Disregarding the use of protein as a tissue-repairer, and consider- 

 ing all three varieties of foods simply as furnishers of energy, we 

 may inquire whether any one of them is superior to the others, or 

 whether any particular proportion of the three food-stuffs is 

 specially desirable. From the purely mechanical standpoint there 

 is evidently no choice among them; the Body requires 2,400 or 

 more Calories of energy each day; each food-stuff yields definite 

 amounts of energy; therefore all we have to do to supply the Body's 

 requirement is to eat enough grams of one or the other food-stuff, 

 or of a mixture of them. The answer to the question goes back, 

 then, to other considerations than that of the energy content of the 

 foods. The first of these is the matter of relative digestibility and 

 absorbability ; it is of little avail to eat a food if it fails to be prop- 

 erly digested and absorbed. Experiments have shown that carbo- 

 hydrates, exclusive, of course, of cellulose, are the most completely 

 absorbed of all foods, 97 per cent of the amount eaten finding its 

 way into the Body; fats come next in order, 94.4 per cent being 

 absorbed; proteins are taken up least completely of all, the Body 

 getting only 92.6 per cent of the protein eaten. There are also dif- 

 ferences of digestibility and absorbability of different foods within 

 the same class; the protein of lean meat, for example, being more 

 readily digested and absorbed than that of beans and peas. Cheese, 

 which contains the highest per cent of protein of any common food, 

 has a reputation, perhaps undeserved, for indigestibility. Graham 

 bread is, by many, supposed to be more nutritious than white. 

 It is true that graham flour contains a higher percentage of protein 

 than does white flour, but the extra protein of the graham flour is 

 in the bran, whence the human digestive process fails to extract it; 

 so as a matter of fact white bread yields more actual nourishment 

 to the Body than does graham. Some fats are much more di- 

 gestible than others ; olive oil and pork fat, for example, are more 

 completely utilized by the Body than is mutton fat. Fat of any 

 sort, taken in the meal with other foods, seems for some reason to 

 delay the whole digestive process, and the delay is greater the 



