NUTRITION 247 



NECESSARY CONSTITUENTS 



From the three different purposes to which food is applied, it will be obvious 

 that substances necessary for one object may not be so for another. There are, 

 however, some food-stuffs which are indispensable for all purposes, such as oxygen, 

 hydrogen, nitrogen, and carbon. 



Oxygen. This, although not commonly regarded as a food, is actually the most 

 important of all. Life, except in rare cases of a special nature, is impossible 

 without it for more than a very short time. As already pointed out (page 29) the 

 energy of the animal body is derived from the oxidation of food. It is important 

 to note that there is, in the animal, no formation of substances which endow the 

 organism with more energy than that supplied to it in the food. Energy given 

 out in one reaction may, however, be used to raise energy potential in another 

 reaction, as we shall see exemplified in the case of muscle. In the green plant, 

 on the contrary, energy derived from the sun is made use of to raise the energy of 

 carbon dioxide and water to that of carbohydrate. 



Water and Salts. Although these substances afford no energy, their supply 

 is essential for the numerous purposes made plain in the preceding chapters of 

 this book. A continued supply is needed, since the kidney must excrete water in 

 order to dissolve the waste products, and salts from the blood pass throug*h the 

 glomerular filter along with the water. Consideration of the osmotic pressure 

 of these salts as they exist in the blood, about 3'5 atmospheres, shows that a large 

 amount of work would be required to separate them from the water in which they 

 are dissolved. 



The value of Carbon and Hydrogen as giving energy by oxidation is obvious. 

 Their heats of combustion are sufficient to show this. They are, of course, always 

 in various forms of combination in food-stuffs, so that the whole of their energy is 

 not available. It might appear that, for purposes of giving energy, hydrogen 

 alone might serve, but it is unnecessary to state that it would be useless as gas 

 and no chemical compounds except those with carbon are available. Similar 

 remarks apply to carbon itself. These two elements are then always taken in 

 combination and in fact partially oxidised, since the hydrocarbons are too inert 

 chemically to admit of reaction under the conditions compatible with the existence 

 of the protoplasmic system. The special value of carbon, with respect to the 

 great variety of compounds which its peculiarities enable it to form, has been 

 pointed out on page 41 above. 



The position of nitrogen is somewhat different. As a direct source of energy 

 its value is small. But there is, as we shall see later, a certain value in protein 

 food, even as a source of energy, notwithstanding the fact that its nitrogen is 

 almost immediately excreted unoxidised. It appears as if the amino-acids, 

 produced by the action of enzymes on this protein food, after de-amination by the 

 liver, leave certain residues which are, for some reason or other, more readily 

 oxidised and utilised as sources of energy, perhaps because the two processes are 

 parts of the same reaction, or, in other words, because of the " nascent " state of 

 the ketonic or hydroxy-fatty acids formed. 



It is clear that, for the growth or repair of structures containing nitrogen, this 

 element must be supplied. The same may be said of sulphur and phosphorus, 

 which are always found as constituents of cells. 



Notwithstanding what has been said as to the value of nitrogen food, it is astonishing how 

 little is absolutely necessary for the mere maintenance of life even in the higher animals. 

 M'Collum (1911, 1, p. 212) found that pigs may be fed on a diet free from nitrogen for more 

 than three weeks, without losing weight. Nitrogen is always excreted, none the less. In 

 M'Collum's pigs, the nitrogen excreted per day amounted to O'.Sl g. per pig of 84 Ibs. weight. 

 This then, in the case referred to, is the minimum amount which must be given, theoretically, 

 if the loss of nitrogen is to be prevented. The amount of nitrogen given off from wear and 

 tear is sometimes known as the endogenous protein metabolism. The value of 0'31 g. just 

 given should be contrasted with that of 12 to 15 g. excreted on ordinary diet. In these 

 particular animals, it appears that the minimum quantity required for repair is only about 

 2 per cent, of the whole protein metabolism on an ordinary diet. The question of the nitrogen 

 minimum will come up for discussion subsequently. 



Finally, it is obvious that products of secretion, containing particular elements, 



