336 ANIMAL METABOLISM 



than the body fats normally are, the latter will become more unsaturated 

 also. This is a matter of considerable economic importance in the 

 feeding of hogs, where a very soft fat is undesirable in the pork. When- 

 ever the body fats are produced by the feeding of carbohydrates, a 

 type of fat characteristic of the animal results. Hogs fed on soybean 

 or peanut meals are "finished" on corn for this reason. 



Fat transport 



The blood stream serves as the vehicle for carrying fats to various 

 organs of the body. The blood normally contains simple fat (triglycer- 

 ides) only for a few hours after a meal. These absorbed glycerides are 

 carried in the form of tiny fat droplets called chylomicrons to the fat- 

 storage tissues {e.g., under the skin). Later, the fat to be oxidized is 

 carried to the liver, apparently in the form of phospholipides. 



A number of conditions are kno^\Ti which bring about a greatly increased 

 amount of fat in the liver. For example, interruption of the normal flow 

 of pancreatic juice in dogs was found experimentally to cause an accumu- 

 lation of over 300 g. of fat in the liver, whereas the liver of a normal dog 

 of similar size contains only 10-15 g. At the same time, the blood phos- 

 pholipide level fell from 60 to about 30 mg. per 100 ml. This "fatty 

 liver" condition is prevented or corrected by feeding choline, which pre- 

 sumably acts by way of forming more phospholipide and thus promoting 

 the transport of fat away from the liver. Methionine also shows lipo- 

 tropic action {i.e., prevents accumulation of fat in the liver) , probably 

 because it can be used in the metabolic synthesis of choline (p. 345) . 



Metabolic oxidation of fat 



Whether or not the fat is stored, eventually it becomes oxidized to 

 carbon dioxide and water with the Uberation of energy. This oxidative 

 catabolism of fat is an aerobic process, which is started chiefly in the 

 liver and finished in the muscles and kidneys. 



The glycerol part of the fat is most probably dehydrogenated and phos- 

 phorylated to form D-glyceraldehyde-3-phosphate (Fig. 13-3) , which may 

 then be metabolized by the carbohydrate pathways already discussed. 

 Thus it may either be converted into glycogen or oxidized to carbon 

 dioxide and water. 



Beta Oxidation. The fatty acids cannot enter the sugar metabolism 

 pathway so simply because of their widely different chemical nature. 

 It now appears quite certain that these long chain acids are chiefly broken 

 down according to Knoop's theory of beta oxidation. Briefly, Knoop's 

 theory states that two carbon pieces, which appear to be molecules of 

 acetic acid or some closely related substance, are broken off from the 



