CHAPTER XIII. 

 FOOD AND DIET. 



THE substances used as food by man and animals contain protein, fat, 

 carbohydrate, salts, and water; and in order to construct a suitable 

 diet for man, it is necessary to know first what amount of these 

 substances in the food best meets the needs of the body, and secondly, 

 the composition of the different food-stuffs. 



The mere composition of the food-stuffs, however, is an uncertain 

 guide to their true nutritive value, since this depends upon the ease 

 with which they can be digested and assimilated ; it is important, 

 therefore, that the food should be palatable and digestible. 



The food is derived either directly or indirectly from vegetable 

 substances which are synthesised by plants from inorganic materials, the 

 energy of the sun's rays being used in the process. The kinetic energy 

 of the sun's rays is thus transformed into the potential energy of the 

 organic food-stuffs, and when these are consumed their potential energy 

 is again converted into kinetic energy as heat and muscular work. 



DIET. 



From this point of view, the body may be regarded as a machine 

 which converts potential energy into the kinetic energy of muscular 

 work and heat, the daily loss of kinetic energy being replaced by the 

 potential energy of food. The living tissues also undergo a constant 

 wear and tear, the tissue which is broken down being replaced by the 

 building up of fresh tissue from the digested food stuffs. A suitable 

 diet must thus fulfil two functions. On the one hand, it serves as a 

 source of energy, and, on the other, it contains the constituents 

 necessary to replace the breaking down of the tissues. 



(1) Diet as a Source of Energy. Calculating work in terms of 

 heat, it is found that the daily loss of energy in man in the form of 

 heat and work is usually about 3000 large calories. It is less in those 

 who lead a sedentary life, and may be increased by severe exercise to 



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