6l8 PHYSIOLOGICAL CHEMISTRY 



fasting. For a discussion of energy value of foods see "Determination 

 of Fuel Value of Foods," below, and the table on page 602. 



Experiment. Ingest a high calorie diet which is very low in nitrogen or 

 actually non-nitrogenous. A satisfactory diet may include sugar, butter, starch, 

 cream, agar-agar and water. (For energy values see below and table, page 

 602.) Ingest such a diet for three days. Collect the urine hi 24-hour periods, 

 preserve and analyze it for total nitrogen, acidity and ammonia. Note the low 

 nitrogen excretion on the third day as compared with the nitrogen output of the 

 third day of fasting. If so desired, you may (at some later date) fast for three 

 days and repeat the above analyses for comparison. 



Determination of Fuel Value o f Food. When organic substances are oxidized 

 or burned in the human body they liberate a certain amount of heat. This calorific 

 energy or heat value varies according to the type of organic matter undergoing 

 oxidation. Thus the proteins, fats and carbohydrates of the diet when they are 

 burned in the body yield different quantities of heat per unit of substance than do 

 organic acids, alcohol, etc. The energy values of pure protein fat and carbohydrate 

 are the following: 



Protein = 4 . i large calories per gram. 



Fat =9-3 large calories per gram. 



Carbohydrate = 4.1 large calories per gram. 



In arriving at the energy value of any given diet it is customary to burn weighed 

 samples of the various foods in an oxygen atmosphere in an apparatus called a 

 bomb calorimeter (see Fig. 192, page 617). By this means we may determine how 

 much heat is liberated when the ingested food is oxidized in the body. A correction 

 must be made for the incompletely oxidized substances of the urine and feces. A 

 large mass of data concerning the heat value of foods has been collected and tabu- 

 lated, and it is therefore possible to arrive at an approximate idea of the energy 

 value of a diet by calculation (see table, page 602). The bomb colorimeter shown 

 in Fig. 192, page 617, is one of the most satisfactory for actual determination of the 

 heat of combustion of organic substances. 



29. Metabolism in Fasting. The metabolism of a fasting man is 

 entirely different from the metabolism of a well-nourished person. 

 The collection and analysis of the urine during a short fast (three 

 to seven days) will demonstrate many important facts. The following 

 table, which contains data from fasting tests made in the author's 

 laboratory, 1 illustrates some of the points in which fasting metabolism 

 differs from normal metabolism: 



Abstinence from food for a few days can in no way operate to the 

 disadvantage, of a normal person. In fact individuals affected with 

 certain types of gastro-intestinal disorders are benefited by fasting. 



1 The chloride, phosphate and acidity determinations were collected during one seven- 

 day fast and the other data collected during a different fast on the same man. (See 

 Howe, Mattill and Hawk: Jour. Amer. Chem. Soc., 33, 568, 1911; and Wilson and Hawk- 

 Jour. Amer. Chem. Soc., 36, 137, 1914.) 



