CHEMISTRY OF DIGESTION AND NUTRITION. 365 



H..O, and NH 3 ; but, since the XH,in this case is recombined to form an ammo- 

 nium compound, and this in turn is converted into urea, the additional energy lib- 

 erated in the first combustion is balanced by that absorbed in the synthetic produc- 

 tion of the urea. Thepotential energy ofthe fats, carbohydrates, and proteids can 

 be determined by combustion outside the body; the energy liberated is meas- 

 ured 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 should deduct 841 calories from the combustion equiva- 

 lent of one gram of proteid to get its available energy to the body : 5778 — 

 841=4937 calories. Practically, however, this value is found to be too high. 

 Direct determinations upon the body in a calorimeter gave to Rubner the fol- 

 lowing values, which seem to be generally adopted by workers in this field: 

 1 gram of proteid=4100 calories, 1 gram of fat=9300 calories, 1 gram of carbo- 

 hvdrate=4100 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 fatis limited by thedifficulty 

 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 

 interchangeable in dietetics in the ratio of their combustion equivalents. 

 Such, in fact, is the case. The ratio of interchange is known as the " i so- 

 dynamic equivalent," and it is given usually as 1 : 2,4 or 2.2 ; that is, fats 

 may replace over twice their weight of carbohydrate in the diet. It follows 

 from the general principles just stated that if we wished to know the amount 

 of heat produced in the body in a given time, say twenty-lour hours, we might 

 ascertain it in one of two ways: In the first place, the animal might be placed 

 in a calorimeter and the heat given off in twenty-four hours be measured 

 directly. This method, which is that of direct calorimetry, is described more 

 completely in the section treating of Animal Ileal. Secondly, one might 

 feed the animal upon a diet containing known quantities of proteid, fats, and 

 carbohydrates, and by collecting the total N and C excreta determine how much 

 of each of these had been destroyed in the body. Knowing the combustion 

 equivalent of each, the total quantity of heat liberated in the body could be 

 ascertained. This latter method is known as indirect calorimetry. The two 

 methods, if applied simultaneously to the same animal, should give identical 

 results. It is very interesting to know that an experiment of this character 



