492 NUTRITION. 



bolic activities of the body. The undoubted connection of scurvy with the 

 lack of fresh vegetable food, other conditions helping, may perhaps turn in 

 part on this, for the evidence that the disease is due to the deficiency of 

 potash alone is not conclusive. 



Lastly, water has an effect on metabolism, as shown, among other things, 

 by the fact that when the water of a diet is increased, the urea is increased 

 to an extent beyond that which can be explained by the increase of fluid 

 increasing the facilities of mere excretion. 



THE ENERGY OF THE BODY. 



The Income of Energy. 



441. Broadly speaking, the animal body is a machine for converting 

 potential into actual energy. The potential energy is supplied by food ; 

 this the metabolism of the body converts into the actual energy of heat 

 and mechanical labor. We have in the present section to study what is 

 known of the laws of this conversion, and of the distribution of the energy 

 set free. 



Neglecting all subsidiary and unimportant sources of energy, we may say 

 that the income of animal energy consists in the oxidation of food into its 

 waste products viz., the oxidation of proteids, fats, and carbohydrates into 

 urea, carbonic acid, and water. A principle laid down by the chemist teaches 

 that the potential energy of any body, considered in relation to any chemical 

 change which it may undergo, is the same when the final result is the same, 

 whether that result be gained at one leap or by a series of steps ; that, for 

 instance, the energy set free by the oxidation of 1 grm. of fat into carbonic 

 acid and water is the same, whatever the changes forward or backward which 

 the fat undergoes before it finally reaches the stage of carbonic acid and 

 water; and similarly, that the energy available for the body in 1 grm. of 

 dry proteid is the energy given out by the complete combustion of that 1 

 grm., less the energy given out by the complete combustion of that quantity 

 of urea to which the 1 grm. of proteid gives rise in the body. Taking this 

 as our guide, we can readily calculate the amount of potential energy con- 

 tained in an average twenty-four hours' diet, and thus obtain the average 

 daily income of energy. For the potential energy of most of the substances 

 used as food has been determined by direct calorimetric observations ; and 

 the several determinations, though they vary somewhat, agree sufficiently 

 closely to serve as data for the calculations in question. 



The total combustion of the following substances has given for one 

 gramme of each substance the following results expressed in calories that 

 is, in gramme-degree units of heat : 



Meat, free from fat, 5103 and 5324. Fibrin, 5511. Egg-albumin, 5579. 

 Thus, taking round numbers, we may say that 1 grm. of proteid material 

 contains 5000 or 5500 calories of potential energy, according as we use the 

 lower or higher determinations. 



Fat beef or mutton, 9069, 9365, 9423. Butter, 7267 or 9192. Again, 

 in round numbers, we may say that 1 grm. of fat contains about 9000 

 calories. 



Arrowroot (nearly pure starch), 3912. Starch, 4123. Cellulose, 4146. 

 Dextrose, 3692. Cane sugar, 3866. Here again, taking round numbers, 

 we shall not be far wrong in saying that the potential energy of 1 grm. of 

 carbohydrate material is about 4000 calories. 



The combustion of 1 grm. of urea sets free an amount of energy which 

 has been determined by one observer as 2206, by another as 2465 calories. 



