508 THE HUMAN BODY 



specially desirable. From the purely mechanical standpoint there 

 is evidently no choice among them; the Body requires 2,500 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 digesti- 

 bility and absorbability; it is of little avail to eat a food if it fails 

 to be properly digested and absorbed. Experiments have shown 

 that carbohydrates, 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 differences 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 in- 

 digestibility. 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 doe? 

 graham. The special importance of graham flour or of whole 

 wheat is in the roughage it contains. Some fats are much more 

 digestible 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 

 more fat is present. For this reason it is desirable to limit some- 

 what the amount of fat used. 



Another question which may affect the choice of foods is the 

 degree to which they tax the excretory organs of the Body. We 

 have seen that fuel proteins yield a nitrogenous residue which must 

 be gotten rid of by the excretory organs. There seems to be a 

 rather general belief that this task constitutes a somewhat serious 





