WORK OF DIGESTION 529 



Range of Metabolic Increase. The variations of metab- 

 olism after the ingestion of a given food have been followed 

 to the end of its effect; and it has been shown that every 

 intake of food is followed by a rise in gas exchange usually 

 ending within twelve hours. The increase in the in-take of 

 oxygen and in the production of heat is least after ingestion 

 of fat, is larger in case of carbohydrates, and greatest in 

 case of protein. Magnus-Levy observed that even after an 

 intake of 200 grams of butter or bacon-fat the consumption 

 of oxygen rarely went beyond the amount in fasting by over 

 ten per cent., but after eating a ration of bread in the first 

 hour an increase of 33 per cent, above the fasting level may 

 be reached in the oxygen consumption. After a meal of 

 meat a marked and longer increase of gas interchange is to 

 be observed. In experiments of Johansson, Landergren, 

 Sonden and Tigerstedt on feeding days as compared with 

 fasting days an average increase of thirty-five per cent, was 

 noted. Buhner determined the caloric requirement in a dog 

 and then on three different days fed the animal on one day 

 exclusively on protein, on another exclusively on fat, and 

 on another exclusively on carbohydrate, the amounts being 

 in each case equivalent; his results showed on the protein 

 day an increase in heat output of 19.7 per cent. ; on the fat 

 day, 6.8 per cent. ; on the carbohydrate day, 10.2 per cent. 31 



The question next arises how this increase in metab- 

 olism is to be interpreted. 



Work of Digestion. In Speck's opinion, as, too, in that 

 of Mering and Zuntz, this rise in metabolism is not due to 

 combustion of the resorbed material but rather to the work 

 of digestion, this including not only the work of the mus- 

 culature of the gastrointestinal tract but also the heightened 

 demands of the collective glandular apparatus connected 

 therewith ; and in addition the increased cardiac and respir- 

 atory activity must also be taken into consideration. There 



81 Cf . A. Jaquet, Ergebn. d. Physiol., 2', 478-486, 1908. 

 34 



