DIETETICS 151 



as the turning of a wheel, or the lifting of a weight which can 

 be measured. 



In testing diets, it suffices to make sure that nitrogenous 

 equilibrium is maintained, and then to estimate the gain or loss 

 in weight and the output of energy in external work. 



The Relative Value of Foods. Dried proteins contain 

 about 15 per cent, nitrogen, 54 per cent, carbon, 7 per cent, 

 hydrogen, 22 per cent, oxygen, a little sulphur, and frequently 

 some phosphorus. A large proportion of their carbon and 

 hydrogen is available for combustion. Fats contain 75 per 

 cent, of carbon, and a considerable quantity of hydrogen avail- 

 able for combustion ; carbohydrates, 40 per cent, of carbon, 

 with hydrogen and oxygen in the proportions in which they 

 occur in water. If 1 gramme of protein is oxidized to the con- 

 dition of urea, carbonic acid, and water, sufficient heat is liber- 

 ated to raise the temperature of 4,100 grammes of water 

 1 degree centigrade. Its calorific value is therefore ex- 

 pressed as 4,100 calories, the unit of measurement a calorie 

 being the amount of heat needed to raise 1 gramme of water 

 1. The calorific value of 1 gramme of fat is 9,300 calories ; 

 of 1 gramme of starch, 4,100 calories. Thus, the energy poten- 

 tial in protein and in starch is the same ; that in fat more 

 than twice as great as that in either of the other foods. 



A Normal Diet. Nitrogenous equilibrium and body-weight 

 can be maintained and work done on diets which vary widely 

 in percentage composition. This is a question which we shall 

 consider at greater length later on. In the meantime, for the 

 sake of illustration, it is necessary to formulate a diet which is 

 fairly representative of the selection of foods made by a man 

 of average weight say 70 kilogrammes (145 pounds) who 

 desires to do a moderate day's work in comfort. It has been 

 found to amount to about 100 grammes of protein, 100 grammes 

 of fat, 240 grammes of carbohydrate, all measured dry and as 

 pure foods. If the several elements of such a diet be multiplied 

 by the figures which represent their calorific value, it will be 

 found that the man is supplied with 2,324,000 calories. The 

 illustration that we have chosen is the diet of a professional 

 man who is not engaged in hard physical work. The pure 

 foods would be found to the amounts stated in 17 ounces 

 lean meat, 4 ounces butter, and 17 ounces bread. The day's 



