NUTRITION 279 



is, fat into carbohydrate. It can be shown by the increase of weight that oxygen is actually 

 retained. When a marmot converts the carbohydrate of its food into fat, preparatory to 

 hibernation, the respiratory quotient is high. 



This numerical quantity, the respiratory quotient, has not yet been explained in these pages?. 

 It is simply the ratio of the volume of the carbon dioxide given out to that of the oxygen taken 

 in. Carbohydrate may be looked upon as consisting of carbon plus water, so that, if this were 

 the only material oxidised in the organism, the oxygen would have been entirely used to 

 combine with the carbon and the respiratory quotient would be unity. In fat, on the other 

 hand, besides the carbon there is also hydrogen to be oxidised, as easily seen by the formula, 

 say of palmitin, CjjH^Oe. Part of the oxygen taken in is used for the oxidation of hydrogen, 

 so that there is less carbon dioxide given out than that equivalent to the oxygen taken in and 

 the respiratory quotient is less than unity. The determination of the respiratory quotient 

 enables us to see what is being oxidised, when different diets are given. 



3. Fat from Protein in the Food. Although ammo-acids are de-aminated in 

 the organism, so that pyruvic acid is formed from alanine, and,* from pyruvic 

 acid, as shown in the preceding section, higher fatty acids can be synthesised, 

 it is a remarkable fact that all evidence tends to show that no fat is laid on 

 by the organism, however large a diet of pure protein is taken. The effect is 

 simply to increase the nitrogenous and general metabolism. 



In certain toxic conditions there appears at first sight to be a change of the protoplasm of 

 the cells, especially of the liver, into fat. But careful investigation has shown that there is no 

 actual increase of the total fat of the body ; what happens is that fat from other parts migrates 

 to the liver and there is also some kind of aggregation of the lipoids of the protoplasm, so that, 

 from being invisible as a distinct phase, they become particles or droplets, readily seen under 

 the microscope. 



That fats can be used as sources of energy, in muscular contraction, for 

 example, is shown by the respiratory quotient. We have just seen that, if 

 fat is being consumed, this value falls. When muscular work is performed 

 with a diet consisting almost entirely of carbohydrate, the respiratory quotient is 

 0*9 ; when fat is exclusively taken, it falls to 0'72. This shows that in the latter 

 case a substance containing oxidisable hydrogen as well as carbon, that is, fat, 

 is being consumed. 



The chemical processes taking place in the utilisation of fat are not definitely 

 known. But it is altogether probable that the process of synthesis from lower 

 fatty acids, described above, goes also in the reverse direction and, when arrived 

 at, these acids are quickly oxidised. There is also direct evidence of the 

 breakdown of fats into aceto-acetic and /3-oxy-butyric acids, together with acetone, 

 in diabetes, where they appear on a diet formed exclusively of fat and protein, 

 In the normal organism, an exclusively fatty diet, continued for some days, has 

 been found to give rise to large quantities of these partially oxidised products. 



PYRUVIC ACID 



It will have been noticed how frequently this compound makes its appearance 

 in various metabolic processes. It is interesting to collect together these facts. 

 It is converted into alanine by a reversible reaction. It is a stage in the oxida- 

 tion of glucose, so that all the substances contained in the scheme on page 273 

 above can be obtained from it. Further, Miss Smedley's work has shown how 

 higher fats can be synthesised by starting from pyruvic acid. Fats, carbo- 

 hydrates, and proteins, therefore, come into connection at this meeting place. 



We saw above that the living cell of Aspergillus is able to form several 

 different amino-acids from nitrate. It was pointed out, also, that if the 

 appropriate a-ketonic or hydroxy-acid were available, the corresponding amino- 

 acid could be formed by the liver. But the only such acids known to be 

 present in the organism are pyruvic and lactic, from which alanine alone is 

 formed directly. Miss Smedley's work, however, has shown. how an unsaturated 

 a-ketonic acid with two more carbon atoms can be obtained from pyruvic. 

 From this, by addition of hydrogen, the saturated ketonic acid may be formed, 

 and therefore the amino-acid. But the process has its limitations, since it 

 only gives us a straight chain of carbon atoms, while leucine, for example, has 

 a branched chain. See also Lusk's results, page 277 above. 



