328 FOOD INGESTION AND ENERGY TRANSFORMATIONS. 



SOME RELATIONSHIPS BETWEEN ENERGY OUTPUT AND 



FOOD INTAKE. 



The evidence presented in the foregoing chapters of this book deals 

 principally with the energy transformations incidental to the ingestion 

 of food. The experimental plan, while undergoing many changes in 

 the decade in which the research has been in progress, nevertheless 

 had, as its fundamental basis, a quantitative measurement of the energy 

 transformations, either directly by means of the calorimeter or indirectly 

 by calculation from data obtained for the respiratory exchange; in many 

 of the experiments the nitrogen excretion was also determined. In sev- 

 eral series of experiments a measurement was made of the increase in 

 the energy output required for the mastication of food or the drinking 

 of such liquids as water, coffee, and beef tea, but aside from the experi- 

 ments in which the effect of mastication was studied, and a few calo- 

 rimeter experiments in which the food materials were taken within the 

 experimental period, the work of prehension and the external muscular 

 work of eating were entirely excluded. 



When possible, pure nutrients were used; if this was not practicable, 

 as was only too frequently the case, a diet was employed in which a 

 special nutrient predominated ; thus a study could be made of the quan- 

 titative transformations following the ingestion of various kinds of 

 food. It has been our plan to discuss in the foregoing chapters the 

 experiments of each division of the research and thus in a way subse- 

 quent discussion is more or less of a repetition. It is perfectly legiti- 

 mate, however, to recapitulate and attempt to correlate the findings for 

 the several classes of food materials. 



The only purely mechanical process studied was that of chewing. 

 Although unfortunately the evidence is not definite in every case, never- 

 theless the general picture is sufficiently clear to state positively that 

 mastication produces a distinct increase in the heat production. It has 

 likewise been pointed out that, though contrary to the belief of some 

 enthusiasts who advocate prolonged mastication for the more perfect 

 digestion and assimilation of food material, the unused portion of mod- 

 ern food materials is, under normal conditions, extraordinarily small. 

 The preliminary preparation of practically all of the food materials of 

 civilized man removes in large part the indigestible portion ; the energy 

 content of almost any mixed diet may therefore be said to be absorbed 

 to the extent of 90 per cent or over. Since pure carbohydrates are 

 almost perfectly absorbed, it is probably safe to assume that with ordi- 

 nary mixed diet approximately 95 per cent of the energy is actually 

 absorbed. 



The error in computing unabsorbed material from an analysis of the 

 feces should again be emphasized. Fecal matter by no means consists 

 wholly of undigested food material, but is made up in large part of 



