It is only within the last few years that the importance of the animal fats as sources of the fat-soluble vitamins has been recog nised: the discovery that vitamins (q.v.) may be produced in some non-active vegetable oils by the action of light is among one of the most recent advances of biochemistry. These bodies, of vital physiological importance as accessory growth factors, appear in minute proportions in the unsaponifiable (non-glycer idic) fraction of natural oils probably as a result of their physical solubility in these media; nevertheless the recognition of the part played by these substances attaches a new significance to the role played by the fats in animal metabolism.
Synthesis and Oxidation of Fats in Living Organisms.— The fat of the flesh-eating animals appears to be mainly derived from the fats consumed in the food; it has been demonstrated, however, by the classical researches of Lawes and Gilbert, that animal fats can be derived from carbohydrate food, while it is probable, although the proof is not so clear, that carbohydrates are the source of the fats in herbivorous animals and plants. In the case of vegetable seeds and fruits the formation of fats occurs late in the ripening process ; in the unripe condition carbohydrates (sugars, starches, etc.) are to be found in the fruits and sap, but no fatty acids (or oils) and it is likely that the former are broken up and converted into fatty acids and subsequently into glycer ides during the maturing process. In the case of almonds it has been observed that carbohydrates disappear as fat is formed.
It is a noteworthy fact that, almost without exception, only acids with an even number of carbon atoms in the molecule occur in the natural fats. The preponderance of C18 acids suggests the hypothesis that the fats are derived from the C18 nucleus of the polysaccharides, or, perhaps, from three molecules of glucose.
Secondary oxidations and condensations are necessitated to ex plain the formation of C16 and the higher acids C20, C22 and C24.
To account for the formation of milk fats and fats of the coconut class, which are distinguished by the presence of the lower fatty acids, a second type of synthesis has been postulated, involving a building-up process of the acids, one from another, and origin ating from simple sugars instead of the more complex polysac charides. Yet other theories have been propounded to account
for the formation of the unsaturated link in the middle of the oleic acid chain. It is possible that fats can be formed from protein material.
The utilisation of the stored fat by the plant embryo has been but little studied; it seems that the fat is hydrolysed by lipolytic (fat-splitting) enzymes and broken down, possibly into simpler fatty acids, with the ultimate production of carbohydrates (cf. conversion of fat into sugars in animals under abnormal con ditions of glucosuria).
In the animal body the fats of the food are emulsified and hydrolysed by enzymes in the intestine; the glycerol and fatty acids are absorbed through the intestinal epithelium, recombined to form glycerides characteristic of the animal in question and transported via the blood and chyle to the connective tissues for storage. It may be noted that the fat of an individual animal can vary according to the fatty food supplied; for instance, the milk-fat of cows fed on a diet rich in coconut-oil simulates the latter in its properties; the fat of the Eskimo possesses an abnor mally high iodine value, and resembles blubber oil.
The way in which the fat reserves are circulated to the organs in which combustion of the fat with the liberation of energy occurs, is scarcely understood. It is established that when mobili sation of reserve fat takes place the stream is primarily directed to the liver, where the fatty acids are desaturated, i.e., unsaturated linkages are introduced into the fatty acid molecules. It is uncertain whether unsaturated acids so produced are distributed to the working cells as glycerides; it is possible that compounds of the fatty acids with nitrogen and phosphorus (the phospholi pines, which are found in the liver and other organs) must be built up by the liver prior to transfer by the blood to the various organs. The unsaturated links introduced into the fatty acids by the liver appear to provide weak points for attack by the working cells in the process of combustion to the ultimate products of carbon dioxide and water.