830 PHYSIOLOGY 



of stearic acid, C 18 H 36 2 , it is necessary to split off 16 atoms of oxygen. 

 That this setting free of oxygen actually occurs in the transformation of 

 carbohydrate into fat is shown by the study of the respiratory exchanges of 

 animals which are rapidly laying on a store of fat at the expense of a carbo- 

 hydrate food. Thus the marmot, towards the end of summer, eats large 

 quantities of carbohydrate food and very rapidly lays on a thick layer of 

 subcutaneous fat to last it during the winter. If glucose were entirely 

 oxidised in the body, the amount of oxygen absorbed would be exactly 

 equal to the amount of carbon dioxide evolved. Thus 



C 2 H 12 6 + 60 2 = 6C0 2 + 6H 2 0. 

 In this case the respiratory quotient would be 



6C0 2 _ 

 GO,' 



If however oxygen is being set free by the conversion of part of the 

 carbohydrate into fat, this oxygen will be available for the oxidation of 

 other portions of the carbohydrate. The animal will not need to take in 

 so much oxygen from outside for the production of the same amount of 

 carbon dioxide, and the carbon dioxide output of the animal will therefore 

 be greater than its oxygen intake. Pembrey has shown that under these 

 conditions the respiratory quotient may be as high as 15. We cannot 

 assume however that the process of conversion of glucose into fatty acids 

 takes place by this simple process of deoxidation. The change is probably 

 a more complex one, and occurs in separate stages. Glucose easily breaks 

 up under the action of ferments into two molecules of lactic acid, and 

 lactic acid can be equally easily converted into aldehyde and formic acid, 

 thus : 



C 6 H 12 6 = 2C 3 H 6 3 lactic acid, and 

 CH 3 



CH 3 H 

 CHOH= | + | 



CHO COOH 

 COOH 



Now aldehydes possess a marked tendency to combine with other 

 molecules of itself or other substances, i.e. to undergo polymerisation. 

 Thus from two molecules of aldehyde we get one molecule of aldol, 



CH 3 



CH 3 CHOH 



2| = | 



CHO CH 2 



CHO 



which by a simple transposition of oxygen would give butyric acid, or by 

 oxidation would give ^-oxybutyric acid, a substance which occurs during 

 various abnormal conditions of metabolism. 



The fats occurring in the body, e. y. in milk, include only the fatty acids 



