Absorption and metabolism of orally fed arachidonic acid and linoleic acid in the rat.

[3H]arachidonic [( 3H]20:4) and [14C]linoleic acid [14C]18:2) were fed to rats in Intralipid or cream. Later (30-240 min) the stomach, small intestine, plasma, and liver were analyzed for radioactivity in different lipid classes. [3H]20:4 and [14C]18:2 were emptied from the stomach and absorbed by the intestine at similar rates. The [3H]20:4:[14C]18:2 ratio of the lipids in the small intestinal wall increased, however, with time. This was due to a higher retention of [3H]20:4 than [14C]18:2 in intestinal phospholipids. In contrast, more of the [14C]18:2 was in triacylglycerol of the small intestine and plasma. The highest 3H:14C ratios were found in phosphatidylethanolamine and phosphatidylinositol. The 3H:14C ratio of intestinal phosphatidylcholine varied with the type of fat vehicle used, being highest in the Intralipid experiments. After feeding Intralipid (30-60 min), significantly more of the plasma [3H]20:4 than plasma [14C]18:2 was in diacylglycerol, the 3H:14C ratio of which was much higher than that of plasma free fatty acids. [3H]20:4 and [14C]18:2 of chyle triacylglycerol are thus metabolized differently.