CHEMISTRY OF DIGESTION AND NUTRITION. 303 



burnt, it is converted to CO 2 and H 2 O and a certain quantity of heat is liber- 

 ated ; if the same gram of sugar had been taken into the body, it would event- 

 ually have been reduced to the form of CO 2 and H 2 O, and the total quantity 

 of heat liberated would have been the same as in the combustion outside the 

 body, although the destruction of the sugar in the body may not be a direct, 

 but an indirect, oxidation ; that is, the oxygen may first be combined with sugar 

 and other food-stuffs to form a complex molecule which afterward dissociates 

 into simpler compounds similar to those obtained by direct oxidation, or there 

 may be first a dissociation or cleavage followed by oxidation of the dissociation 

 products. In determining the total energy given to the body we need only 

 consider the form in which a substance enters the body and the form in which 

 it is finally eliminated. In the case of proteids the combustion in the body is 

 not so complete as it is outside ; the final products are CO 2 , H 2 O, and urea ; 

 the urea, however, still contains potential energy which may be liberated by 

 combustion, and in determining the energy of proteid available to the body, 

 that which is lost in the urea must be deducted. As a matter of fact, there is 

 some evidence (see origin of urea, p. 276) to show that proteid in the body is 

 completely oxidized to CO 2 , H 2 O, and NH 3 ; but, since the NH 3 in this case 

 recombines with a part of the CO 2 and the H 2 O to form ammonium carbamate, 

 and this in turn is converted into urea, the additional energy liberated in the 

 first combustion is balanced by that absorbed in the synthetic production of the 

 urea. The potential energy of the fats, carbohydrates, and proteids can be 

 determined by combustion outside the body ; the energy liberated is measured 

 in terms of heat by some form of calorimeter, and the quantity of heat so 

 obtained, expressed in calories, is known usually as the " combustion equiva- 

 lent." To be perfectly accurate, each particular form of fat, proteid, etc. 

 should be burnt and its energy be determined, but usually average figures are 

 employed, as the amount of heat given off by the different varieties of any one 

 food-stuff proteids, for example does not vary greatly. According to Stoh- 

 mann, 1 gram of beef deprived of fat = 5641 calories, while 1 gram of veal 

 gives 5663 calories. For muscle extracted with water, Rubner obtained the 

 following figures: 1 gram = 5778 calories. The combustion equivalent of urea 

 (Rubner) is 2523 calories. Since 1 gram of proteid yields about one-third of 

 a gram of urea, we must deduct 841 calories from the combustion equivalent 

 of one gram of proteid to get its available energy to the body: 5778 841 = 

 4937 calories. The combustion equivalents of fats and carbohydrates, as given 

 by Stohmann, are: 1 gram of fat = 931 2 calories; 1 gram of starch = 41 16 

 calories. Weight for weight, fat contains the most energy, and, as we know, 

 in cold weather and in cold climates the proportion of fat in the food is 

 increased. In dietetics, however, the use of fat is limited by the difficulty 

 attending its digestion and absorption as "compared with carbohydrates. Fats 

 and carbohydrates have the same general nutritive value to the body : they 

 serve to supply energy. Since the amount of potential energy contained in 

 each of these substances may be determined accurately by means of its com- 

 bustion equivalent, it would seem probable that they might be mutually 



