NUTRITION 151 



cells. We must bear in mind that there are certain differences 

 in the end products of metabolism. For instance, birds and 

 reptiles form uric acid instead of urea as the main end product 

 of metabolism. In plants many special substances such as 

 alkaloids are formed, but their relation to metabolism is 

 still far from clear. 



DIETETICS 



The bearing of the description of metabolism given above 

 on dietetics is obvious. We see that it is necessary to have 

 sufficient energy value in the food to maintain the energy 

 exchange of the resting body, and also to provide the energy 

 required for any^ additional energy expenditure necessary 

 for the performance of external work. 



The energy can be obtained from protein, carbohydrate or 

 fat, but it has been found advisable to have certain proportions 

 of these various food substances. 



Minimum Protein Requirement. To maintain a healthy 

 condition the body requires a certain daily amount of protein. 

 The least amount that will suffice depends upon the constituent 

 amino acids in the protein of the diet. Many observations 

 have been made on this subject. Chittenden aroused renewed 

 interest in this subject by the statement that nitrogenous 

 equilibrium could be maintained on about 60 grams of 

 protein per day, instead of the higher figures given by previous 

 observers.* Chittenden's conclusions have been established, 

 and it is now recognised that 80 grams of protein of good 

 biological value allows sufficient margin for a normal adult 

 man. 



Having determined the amount of protein in the diet, the 

 energy value of this is deducted from the total energy require- 

 ment, and the remaining energy is obtained from carbohydrate 

 and fat. The relative amounts of carbohydrate and fat 

 depend on the ease of their digestion and on the fact that 

 where large energy expenditures are concerned the fatty 

 foods are much less bulky. It is possible that the minimum 

 amount of fat ought to be sufficient to furnish 20 per cent, of 

 the total energy value, f 



The water and salts of the diet are also to be remembered. 

 The daily loss of water by urine, respiration, etc., must be 

 made good, and the proportions of salts for the normal 

 function of the cells (Chapter V) must be maintained. 



* R. H. Chittenden, Physiological Economy in Nutrition. Heinemann, 

 1905. 



f E. H. Starling, Brit. Med. Journ., 1918, vol. ii, p. 105. 



