296 PRINCIPLES OF GENERAL PHYSIOLOGY 



For the purpose of energy production, the supply of due amounts of carbon 

 and hydrogen alone is, in theory, sufficient. Nitrogen is absolutely necessary 

 merely for the purpose of replacing that which is lost from the structure of the 

 machine in wear and tear. At the same time, there appear to be certain 

 advantages in taking a larger proportion of nitrogen food, at all events, for most 

 people. The protein content of most standard dietaries is certainly, however, 

 unnecessarily high. 



There are some organisms which have very special requirements as to food 

 materials. 



As naturally follows from the facts given in preceding chapters, the presence 

 of inorganic salts in food is essential. 



The method by which atmospheric nitrogen is made available for the food of 

 plants and animals is described in the text. The stages are, briefly, bacteria of 

 the soil, and in the root-nodules of leguminous plants, proteins of plants and 

 animals, ammonia, nitrites, nitrates. The latter are again utilised by plants, 

 part being lost as nitrogen gas, and if not used up, by a reverse process back to 

 ammonia. 



With regard to the " accessory factors," or hormones, it will be clear that, 

 if a certain chemical grouping is required for a special purpose in the organism, 

 such as an internal secretion, and if the organism is unable to synthesise it for 

 itself, then it must be given in the diet. Such a substance is tryptophane for the 

 growing rat. But there is something else needful. Rats will not grow on a diet 

 of pure protein, fat, carbohydrate, and salts, even when containing all the known 

 chemical constituents of food, including tryptophane. There is a substance 

 present in a minute amount of milk, or boiled extracts of fresh vegetables or meat, 

 which is absolutely necessary. Moreover, when these factors are present, aniin.iK 

 are able to preserve their tissue nitrogen on pure amino-acids. 



Certain diseases, such as beri-beri and scurvy, have been shown to be 

 caused by the absence of similar "accessory factors " in diet. 



There is some evidence that even yeast and bacteria are dependent for 

 growth on similar "accessory factors." 



The chemical constitution of proteins, as condensations of amino-acids of 

 different kinds, is described briefly in the text. These substances, when taken 

 as food, are first hydrolysed in the organism, by means of the digestive enzymes, 

 to their constituent amino-acids and related substances. The greater part 

 of these amino-acids, passing into the blood, is de-aminated, mainly, or perhaps 

 exclusively, in the liver ; the resulting ammonia is converted to urea, again 

 mainly, or perhaps exclusively, in the liver ; the hydro xy or ketonic fatty acids 

 produced are burnt up for energy purposes. The small part of the amino-acids 

 not de-aminated is used by the tissues for growth or for replacement of loss 

 by wear and tear. 



There are, then, two more or less distinct forms of protein metabolism, one 

 for energy purposes, "exogenous," in which the nitrogen is lost, the other for 

 replacement of loss or for growth, " endogenoits," in which the nitrogen is 

 retained. 



The question of the minimum nitrogen requirement is discussed in the text. 

 The amount absolutely necessary for a healthy man, doing the ordinary amount 

 of work, has been reduced to 3-5 g. per day, equivalent to 21 g. of protein. 

 The total energy value of the diet, expressed in heat units, must not be less 

 than 4,000 calories, made up with carbohydrate and fat. 



There is evidence that the presence of carbohydrate is essential for the synthesis 

 of protein, both in the animal and in the plant. 



In the wear and tear of the protoplasmic mechanism only a certain part 

 requires replacement, not the whole of a complex molecule. 



The importance of creatine and purine metabolism is pointed out. 



