10 



oxygen, and hydrogen, as do also carboh3'drates. In the carbohy- 

 drates, however, the oxygen and hydrogen are always present in 

 the proportions in which they occur in water, namely, two atoms of 

 hydrogen to one of oxygen. 



The group "mineral matter" includes the inorganic bodies present 

 in the form of salts in the juices and tissue of the different feeding 

 stuffs, the principal chemical elements found being sodium, potassium, 

 calcium, chlorin, fluorin, phosphorus, and sulphur. The term ""ash" 

 is often and very properl}" used for this group, since the mineral 

 matter represents the incombustible portion which remains when any 

 g-iven feeding stuff is burned. 



As noted above, the functions of food are (1) to suppl}' material to 

 build and repair the bod}^, and (2) to yield energy. The chemical 

 composition of a feeding stuff serves as a basis for judging of its value 

 for building and repairing body tissue. Its value as a source of energy 

 must, however, be learned in another waj". The most usual wa}^ of 

 measuring energy is in terms of heat, the calorie being taken as a unit. 

 This is the amount of heat which would raise the temperature of 1 

 kilogram of water 1° C, or 1 pound of water 4" F. Instead of this the 

 unit of mechanical energy, the foot-ton (the force which would lift 1 

 ton 1 foot) may be used, but it is not as convenient. One calorie cor- 

 responds very nearly to 1.54 foot- tons. The heat of combustion of 

 foods and feeding stuff's is ordinarily determined with the bomb calo- 

 rimeter or other suitable devices. The fuel value of any food is equal 

 to its heat of combustion less the energy of the excretory products 

 derived from it and may be learned by taking into account the chem- 

 ical composition of the food or feeding stuff', the proportions of the 

 nutrients actually digested and oxidized in the body, and the propor- 

 tion of the whole latent energ-v of each which becomes active and use- 

 ful to the body for warmth and work. However, the fuel value may 

 be and often is calculated from the composition of the food material 

 supplied, since it has been found that 1 gram of protein furnishes 1.1 

 calories, 1 gram fat 9.3 calories, and 1 gram carbohydrates 1.1 calo- 

 ries, or 1 pound protein 1,8(30 calories, 1 pound fat 1,220 calories, and 

 1 pound carbohydrates 1,860 calories." 



The relation between the quantities of nitrogenous and nitrogen-free 

 nutrients in the ration is called the nutritive or nutrient ratio. In cal- 

 culating this ratio 1 pound of fat is taken as equivalent to 2. 25 pounds 

 of carbohj'drates — this being approximately the ratio of their fuel 



« These values, which have been often used in the past, are known to be unsatis- 

 factory, but are retained because better and more generally accepted data, obtained 

 • in experiments with animals, are not available. 



In discussions relating to human food later and more accurate values have been 

 proposed, namely, 1,820 calories per pound for protein and carbohydrates and 4,040 

 calories per pound for fats. 



