PHYSIOLOGICAL UTILIZATION VALUE 527 



full of suggestions for every one trained in natural sci- 

 ence. ' ' 27 The author believes that Rubner 's efforts will be 

 by no means in vain, and will surely be of use in the solution 

 of the problem of growth ; although perhaps it will be many 

 decades before the vast material from observation upon the 

 various forms of life can be collected and placed upon a 

 broad, comparative-physiological basis, so as to enable us 

 hereafter to take up the deduction of generally valid laws. 



ENERGY EXCHANGE AFTER INGESTION OF FOOD 



We may here turn our attention to energy exchange after 

 the intake of nutrient materials. 



If the body is to remain in complete (both in material 

 and in energy) equilibrium after the ingestion of food, 

 metabolism must proceed in accordance with an equation 

 which Robert Tigerstedt 28 formulates as follows: "Com- 

 bustion heat of food = combustion heat of the urine and 

 faeces + heat loss from evaporation of moisture, carbonic 

 acid output, conduction and radiation + heat loss from 

 absorption of heat by ingesta + heat loss from externally 

 useful work not restored to the body as heat. ' ' 



Physiological Utilisation Value. In attempting to get a 

 proper insight into the utilization value of nutrient material 

 it is necessary primarily to consider that the full energy 

 value in calories of a food can be fully realized only if it be 

 completely consumed in the economy. This actually takes 

 place in case of the fats and carbohydrates, their oxidation 

 in the course of metabolism continuing to their transforma- 

 tion into carbonic acid and water. But with the proteins it 

 is different, these substances not being entirely burned into 

 carbonic acid, water, nitrogen and sulphuric acid; their 

 nitrogen appearing in the urine in the form of urea and a 

 number of other organic compounds. Therefore in attempt- 

 ing the determination of the physiological availability of 



27 N. Zuntz, Centralbl. f. d. ges. Biol., 10, No. 28, 1910. 



28 R. Tigerstedt, Hajidb. d. Biochem., 4", 1, 1910. 



