110 METABOLISM AND NUTRITION 



It has been shown that glycerin, which constitutes about nine per cent of 

 fat and has a heat value per gram equal to about half that of fat, can be burned 

 in the body and can spare both fat and proteid. 



4. Cellulose, which forms so large a part of the vegetable foods, is acted 

 upon by the digestive fluids of the lower animals (snail, Biedermann; carp, 

 Knauthe) ; but in herbivorous mammals it is broken up only by the fermenta- 

 tive action of Bacteria, the end products being carbon dioxide, marsh gas, butyric 

 acid, and acetic acid (Tappeiner). 



It may be regarded as established that cellulose is dissolved to some extent 

 also in the intestine of man. From twenty-five to sixty-three per cent of the 

 cellulose of carrots, celery, cabbage and lettuce is decomposed in the intestine. 

 Cooking appears to favor its solution. The cellulose in " whole-wheat " bread 

 also is dissolved in considerable quantity (Hultgren and Landergren). 



Nitrogen occurs in plants in a number of nonproteid compounds, which with 

 the exception of asparagin (amino-succinic acid) appear to have no real food 

 value. The case of asparagin is not without its practical interest, for this sub- 

 stance is a rather abundant constituent of leguminose seeds, oatmeal and potatoes. 



5. It is perfectly certain that alcohol is burned in the body. Of the total 

 amount absorbed from the stomach only about two per cent is eliminated from 

 the body unchanged; the rest is oxidized to carbon dioxide and water (Atwater 

 and Benedict). 



If alcohol were destroyed in the body without protecting other substances 

 from destruction, the CO 2 -excretion ought of course to be correspondingly in- 

 creased. But this is not the case. Experiments by Zuntz and Berdez and by 

 Geppert show that a dose of alcohol, so long as it is not large enough to intoxi- 

 cate, produces no appreciable increase in the consumption of oxygen, and only 

 an insignificant increase, if any at all, in the excretion of carbon dioxide. By 

 means of experiments in which the total metabolism as well as the heat loss 

 were determined directly, Atwater and Benedict were able to demonstrate also 

 that alcohol can replace the nonnitrogenous foodstuffs to the full extent of its 

 heat value. Replacing a certain quantity of fat by an isodynamic quantity of 

 alcohol produces at first a distinct increase in the destruction of proteid. If 

 the experiments were interrupted at this time, the result naturally would indi- 

 cate that alcohol does not save proteid, but rather intensifies its decomposition. 

 If, however, the experiments be continued, the destruction of proteid falls again 

 and comes back to the original level. The body therefore must become accus- 

 tomed to alcohol, before the latter can exercise its proteid-sparing power (Neu- 

 mann, Clopatt). 



But alcohol cannot play any considerable part in the normal nutrition of 

 man. The quantity which one unaccustomed to its use can drink without symp- 

 toms of intoxication is very small only 16-25 g. With a heat value of 7 Cal. 

 per g., this would amount to 112-175 Cal. that is, estimating the requirements 

 of metabolism at 2,500 Cal., 4.5-7 per cent of the total energy might be supplied 

 in the form of alcohol. Only in very exceptional cases can alcohol be of any 

 practical importance as a foodstuff. In diseases accompanied by reduced powers 

 of digestion, it appears to be of great service as a direct food, quite independently 

 of its effect upon the nervous system. 



5. INFLUENCE OF MUSCULAR WORK ON METABOLISM 



It was apparent from Lavoisier's original experiments on the respiratory 

 exchange that combustion is increased by muscular work, and investigations 

 carried out since that time have established the fact beyond all doubt. 



