CHAPTEE XXX 



FOOD 



THE chief chemical compounds or proximate principles in food aro : 



1. Proteins . 



2. Carbohydrates 



3. Fats 



4. Water . 



5. Salts 



organic. 



inorganic. 



In milk and in eggs, which form the exclusive foods of young 

 animals, all varieties of these proximate principles are present in 

 suitable proportions. Hence they are spoken of as perfect foods. 

 Eggs, though a perfect food for the developing bird, contain too little 

 carbohydrate for a mammal. In most vegetable foods carbohydrates 

 are in excess ; while in animal foods, such'as meat, the proteins are 

 predominant. In a suitable diet these should be mixed in proper pro- 

 portions, which must vary for herbivorous and carnivorous animals. 



A healthy and suitable diet must possess the following char- 

 acters : 



1. It must contain the proper amount and proportion of the 

 various proximate principles. 



2. It must be adapted to the climate ; to the age and weight of 

 the individual, and to the amount of work done by him. 



3. The food must contain not only the necessary amount of 

 proximate principles, but these must be present in a digestible form. 

 As an instance of this, many vegetables (peas, beans, lentils) contain 

 even more protein than beef or mutton, but are not so nutritious, as 

 they are less digestible, much passing off in the faeces unused. 



The nutritive value of a diet depends chiefly on the amount of 

 carbon and nitrogen it contains. A man doing a moderate amount of 

 work and taking the usual diet will eliminate, chiefly from the lungs, 

 in the form of carbonic acid, from 250 to 280 grammes of carbon per 

 diem. During the same time he will eliminate, chiefly in the form 

 of urea in the urine, about 15 to 18 grammes of nitrogen. These 

 substances are derived partly from the food and partly from the 

 metabolism of the tissues, various forms of energy mechanical motion 



479 



