The Digestion and Absorption of Triglycerides

Abstract

Studies both in vitro and in vivo have established that the hydrolysis of triglycerides in the intestinal tract can yield diglycerides, monoglycerides, free fatty acids, and free glycerol. Thus far it has not been possible to establish which of these materials are absorbed as such into the intestinal wall. That a solution to t.his problem has not been forthcoming was undoubtedly attributable to the lack of satisfactory techniques for testing the various hypotheses that have been proposed. Thus, information has been gained by the use of unusual compounds, such as dimethyl stearic acid (1) or conjugated linoleic acid (2), but there is the possibility that the results from such experiments are not applicable to triglycerides containing the usual fatty acids. Even when normal fatty acids are used, the results are not necessarily conclusive. Particular examples of this are the recent studies of Savary, Constantin, and Desnuelle (3) and Mattson and Volpenhein (4), in which the digestion and absorption of triglycerides containing both saturated and unsaturated fatty acids were followed. These experiments demonstrated that only a portion of the fatty acids that are esterified with the /3 position of the dietary triglycerides undergo hydrolysis during the processes of digestion and absorption. As we pointed out at that time, the absorption of palmitic acid is poor when the acid is fed in the free form, improved when the acid occupies the a! position on a triglyceride, and greatest when the acid is esterified at the fl position of the triglyceride. Thus, no final conclusions could be drawn from these experiments as to the nature of the materials that are absorbed. The shortcomings of our earlier study have been overcome by preparing a series of glycerides containing labeled oleic acid as the sole fatty acid and labeled glycerol. The hydrolytic and synthetic products of the processes of digestion and absorption were determined when glycerol and oleic acid were fed either in the free form or as mono-, di-, or triolein; in the last instance, the fatty acids at the o( and p positions were studied separately. From these results, together with the studies reported by others, it is possible to elucidate t.he composition of the lipids that are absorbed, the way they arise by enzymatic hydrolysis, and how they are subsequently resynthesized into the triglycerides that are found in the lymph.