28 GASEOUS METABOLISM OF INFANTS. 



require careful consideration in studies of infant metabolism. With 

 normal metabolism and normal digestion, the loss of unoxidized material 

 in the feces and urine may be considered as essentially constant. The 

 feces represent not merely the undigested residues of food but the 

 epithelial debris, the residues of digested juices, and bacteria; with 

 normal digestion, these remain essentially a constant percentage of the 

 total amount digested. 



It is not the loss of energy through the urine and feces, however, that 

 is of special interest in studying infant metabolism, but the losses 

 resulting from the katabolism of materials in the body, namely, the loss 

 through the combustion of organic substances by virtue of the oxygen 

 taken into the system and there combined with carbon and hydrogen 

 to form carbon dioxide and water. The carbon dioxide thus formed 

 has been considered so close a measure of the total amount of material 

 burned that for years it served as the index of the total combustion of 

 the materials in the body. It may be computed from the data given in 

 table 14 that if the combustion in the body were exclusively of carbo- 

 hydrate, the production of 1 liter of carbon dioxide would be equivalent 

 to the liberation of approximately 5.05 calories of heat. If, on the 

 other hand, the carbon dioxide resulted exclusively from the combus- 

 tion of fat, it can be computed that 1 liter of carbon dioxide corresponds 

 to 6.7 calories of heat. 



While with an infant on a normal diet the proportions of protein, 

 fat, and carbohydrate in the oxidized material may remain relatively 

 constant, it is certainly true that in pathological cases these proportions 

 may be greatly disturbed, and the calorific value of the carbon dioxide 

 may vary considerably with different conditions. Inasmuch as infor- 

 mation regarding the diet and treatment of infants is more especially 

 required in pathological conditions, it is evident that a better method 

 for estimating the total heat output than by the carbon-dioxide excre- 

 tion is to be desired. If the relationship between the heat-output and 

 the oxygen consumption is closely examined, it will be seen that the 

 ratio is much more constant than is the ratio between the carbon 

 dioxide and the heat. Thus, for every gram of lactose burned, 1.066 

 grams of oxygen are consumed, each gram of oxygen thus used being 

 equivalent to 3.51 calories of heat. Likewise, for every gram of fat 

 burned, 2.85 grams of oxygen are consumed, each gram of oxygen 

 resulting in the production of 3.35 calories of heat. It will be seen 

 from these figures that while the calorific equivalent of carbon dioxide 

 varies some 25 per cent according to whether carbohydrate or fat is 

 burned in the body, the calorific equivalent of oxygen varies only 

 about 6 per cent. It is clear, therefore, that the best measurement of 

 the caloric output of the body is the amount of oxygen consumed 

 rather than the amount of carbon dioxide produced. 



