Their Digestion And Assimilation

Fats exist in large quantities in the body in three situations, viz., bone marrow, adipose tissue, and milk. Body fat is a mixture of the three fats, stearin, palmitin, and olein, and its melting-point is 25° C. For this reason the contents of the fat cells of the adipose tissue are fluid during life, and this is due to the low solidification point of olein, viz. 5° C, that of palmitin being 45° C. and of stearin from 53 to 65° C. Fats are compounds of fatty acids and glycerine, and are, in the body, under the influence of enzymes, split up into the substances out of which they are built up. The fatty acids involved are oleic, stearic, and palmitic; the two last belonging to the series of normal saturated fatty acids, CnH2nO2.

Effect Of Cooking

The fats contained in food do not appear to be much affected by heat, although upon cooking they are decidedly more granular than before. Probably this is occasioned by the evaporation of some contained water making the fat more brittle, and distinctly more digestible. Hence the preference of people with a weak digestion for cold boiled bacon.

Digestion

Except for the melting of solid fat, and the solution of the surrounding protein envelope in the stomach, there is no digestive action on fat worth mentioning until the bowel is reached. It is a point of some clinical interest to note that, amongst the digestive secretions, gastric juice alone is capable of digesting raw connective tissue, so that even in uncooked meat the fatty globules are set free in the stomach. No doubt a minute quantity of fat is broken up into its constituent fatty acid and glycerine in the stomach, because lipase is found there, although its action is somewhat inhibited by the hydrochloric acid. Doubtless much of this lipolytic action is produced by regurgitation of the contents of the duodenum mixed with the pancreatic juice.

On their reaching the small intestine, however, lipase (the lipolytic or fat-splitting enzyme) of the pancreatic fluid causes neutral fats to take up a molecule of water and split up into glycerine and fatty acid, while at the same time, under further influence of the pancreatic fluid, a large part of the fats is formed into a fine permanent emulsion. Doubtless lipase, or steapsin, as it was called formerly, does not exist as such, but in the form of a zymogen which is activated by the bile. The fatty acids liberated are partly saponified by the alkali of the pancreatic and intestinal juices. As in the case of the proteins and carbohydrates, it is believed that bacteria are also in operation producing similar results, and carry the process still further in the stage of fatty acids. Fats have a decidedly restraining influence on the gastric secretion, and Boldyrieff has discovered that under appropriate conditions, chiefly when acid secretion has been repressed to a minimum by the ingestion of fats, regurgitation of the duodenal contents may take place into the stomach. In these circumstances it is possible to obtain samples of the pancreatic fluid by Einhorn's bucket.

The absorption of fats is much aided by the bile in which the saponified fats are soluble, the bile contributing alkali for saponification from the sodium of its disintegrated salts. In addition it facilitates the passage of the fatty particles through the intestinal wall. Fleig has demonstrated that soluble soaps also act on the mucous membrane of the bowel to produce sapokrinin, a substance which stimulates the pancreas in much the same way as secretin. The soaps pass partly into the lacteals and partly through the capillaries into the blood, although the direct absorption of fat by the intestinal capillaries is doubtful. The fatty acids and glycerine are absorbed by the epithelial cells of the villus and then enter the lacteals, being by that time re-formed into neutral fats. Altogether about 95 per cent. of the fat in the diet is absorbed, although when consumed in large pieces not more than 84 per cent. may be utilised. The lower the melting-point of fat the greater quantity of it is absorbed. Thus only 2.3 per cent. of olive oil is recoverable in the faeces, whereas 9.2 per cent. of mutton fat, and as much as 90 per cent. of stearin remain unabsorbed.

Metabolism

As with the carbohydrates, there is much conjecture as to the final disposal of fat by the tissues. Pavy declares that carbohydrates are converted into fats in the villi, and this is not at all incredible, as in plants they are mutually interchangeable. In experiments on rabbits he could see the villi full of small particles after feeding with oats, and this he thinks could hardly be due to the 5 per cent. of contained fat. Although fat is quite easily oxidised by living cells, the tissues are not inclined to treat it quite so kindly as they do the protein, i.e., by embracing it; for whenever it is presented in excess of heat requirements it is deposited in the subperitoneal, subcutaneous, and inter-muscular spaces as adipose tissue, with a composition very similar to that of the fat consumed. Thus the adiposity produced by mutton fat is more lasting and firmer than that produced by cod-liver oil, and therefore when fat is presented in an easily assimilable form, e.g., in hot milk, it is much more valuable for phthisical patients than fat of a less soluble order.

The storing up of fat is to prevent physical bankruptcy. When it is required it is again broken down, doubtless by the tissue-enzyme lipase, into fatty acids and glycerine, and so utilised for the production of energy. Just how this takes place is quite unknown, although various hypotheses have been constructed to account for it. Carbohydrates can be theoretically conceived as being converted into fat by the loss of carbon dioxide and water, provided a large number of carbohydrate molecules be assumed to be transformed to produce only one molecule of fat. It has, of course, been proved experimentally that carbohydrate in the body is converted into fat, and this has been stated to take place even when the supplies are not excessive. Feeding experiments on pigs, dogs, cows and sheep with food freed as much as possible from fat and albumin, but very rich in carbohydrate, are attended by an increase of the adipose tissue, much more fat being deposited than could have been accounted for by the fat and albumin in the food. The carbon excreted in the expired air and the urine was infinitely less than that contained in the carbohydrate food, so that large stores were being retained in the body. Carbohydrates possess a much greater amount of oxygen than fats, and it may be that in the combustion of the former part of the sugar carbon completely saturated with oxygen is excreted as carbon dioxide, and the remaining carbon compounds, being now poorer in oxygen, unite to form fatty acids. It is calculated that 100 grams of grape sugar ought to yield 37 grams of fat.

Moore mentions that Bainbridge and Leathes by partial interference with the blood supply have been able to make the liver cells take on the appearance of fatty degeneration, the cells becoming loaded with obvious fat globules. The appearance presented is as if the fat had enormously increased in quantity, but analysis reveals no increase. Normal liver tissue can contain from 5 to 10 per cent, of fat without displaying the slightest appearance of fatty material, and this must therefore be an integral part of the bioplasm. It must, in other words, be linked on to the protein by the process of adsorption, because a much smaller quantity of fat in the free form would present itself as a thick emulsion. Union of a similar loose character must take place in the blood serum, for from a perfectly clear serum showing no oil globules under the microscope from .5 to 1.0 per cent. may be extracted by alcohol and ether, an amount sufficient to show itself as an emulsion. After a heavy fatty meal the serum looks quite milky, but this disappears in an hour or two, being all taken into union by the liver bioplasm and other tissue cells. This is undoubtedly the explanation of fat transference from tissue to tissue without any appearance of emulsification, and in the same way fats are oxidised without any globules of fat being in evidence.

- In any case, like the carbohydrates, the end-products of the fats are carbon dioxide and water, which are excreted by the skin, kidneys, and lungs.