NUTRITION 147 



depends, moreover, upon other circumstances, such as the 

 amount of carbohydrate and fat present in the diet. 



Increased excretion of urea occurs soon after the ingestion 

 of nitrogenous food, and this circumstance gave rise to the 

 hypothesis of Voit that there are two varieties of protein, 

 circulating and tissue. The former is not built up into the 

 tissues and is rapidly destroyed, but the latter is built up 

 into the tissues and is slowly removed.* Further, all proteins 

 are not equally efficient in maintaining nitrogenous equili- 

 brium. 



The explanation of these processes is best seen by applying 

 the law of mass action to our modern knowledge of the 

 constitution of the proteins. The proteins are transported 

 in the form of amino acids, and built up into the tissue proteins 

 by the various cells. If one or more of the necessary amino 

 acids are absent" the protein cannot be synthesised, hence a 

 diet deficient in one or more amino acids is inadequate to 

 maintain nitrogenous equilibrium. f Thus gelatine, which is 

 deficient in aromatic amino acids, cannot maintain nitrogenous 

 equilibrium. 



The amino acids present in excess must be removed, and 

 we find that this is done by removal of the amine group and 

 by utilisation of the non-nitrogenous portion for energy 

 requirements. Owing to bacterial changes in the intestine 

 some of the nitrogen is removed before absorption, and the 

 same arguments would apply to that process, but we shall 

 confine our attention to the fate of the amino acids after 

 absorption. 



Some amino acids can be synthesised in the animal body, 

 whilst others must be furnished in the food. Osborne and 

 Mendel have amplified the work of Willcock and Hopkins 

 on zein, a protein deficient in tryptophane and lysine, showing 

 that it does not suffice to maintain the weight of rats, but 

 that if tryptophane is added the animals maintain their 

 weight but will not grow. If, however, lysine is added to 

 zein the animals will not maintain their weight, but if lysine 

 and tryptophane are added to the zein the rats not only 

 maintain their weight but also grow. These authors point 

 out that certain amino acids are necessary for special processes 

 (tryptophane for maintenance, lysine for growth), and that 



* C. von Voit, Hermann's Handbuch der Physiologic, Leipzig, 1881, 

 vol. 6, part I, p. 300, et seq. 



f E. G. Willcock and F. G. Hopkins, Journ. Physiol, 1906, vol. 

 35, P- 88 : 



