Lymphatic absorption of glucose and fatty acids as determined by direct measurement

Treatment of chylous leaks using enteral feeds supplemented with medium chain triglycerides (MCT) is used widely but often ineffective. Few studies have examined the conventional dogma that MCTs and simple sugars are absorbed directly via the portal system. This study tests the hypothesis that significant absorption of MCTs and simple sugars occurs via the lymphatics.

A stable, awake rat model with cannulation of the mesenteric lymph duct, portal and jugular veins, and duodenum was used. Venous catheters were infused with saline at 2.5 mL/hr, while the duodenal tube was infused with emulsified fatty acids at 3 mL/hr (0.3% palmitic acid [C16:0] + 0.35% of test nutrient: either Lauric acid [C12:0; n = 5] octanoic acid [C8:0; n = 6] or 3-0-methylglucose, [n = 5].) After 4 hours of stabilization, the animals were given a differentially radiolabeled pulse of palmitic acid plus test nutrient and the subsequent appearance in lymphatic, portal, and jugular systems was measured. Total lymphatic appearance was measured directly; appearance in the portal and jugular systems was measured using the area under the curve method.

Results are expressed as mean +/- SD. Palmitic acid: lymphatic absorption (LA), 49.2+/-7.4%; portal venous absorption (PA), 19.8+/-15%; total absorption (TA), 69+/-12.4%. Lauric acid: LA, 32.9+/-6.7%; PA, 7.5+/-2.9%; TA, 40.4+/- 6.3%. Octanoic acid: LA, 5.6+/-2.1%; PA, 27.3+/-6.5%; TA, 32.9+/-4.8%. 3-0 methylglucose: LA, 8.6+/-2.2%; PA 69+/-39%; TA, 76.6+/-39.5. Total lymphatic flow: palmitic plus lauric group, 14+/-2 mL; palmitic plus octanoic, 12.5+/-4.4 mL; palmitic plus 3-0 methylglucose, 12.8+/-2.9 mL. (Total volume measure over 4 hours after giving the radiolabeled pulse.)

The portal venous uptake of the medium chain fatty acid (MCFA), octanoic acid, was not significantly greater than the uptake of the long-chain palmitic acid, but the absorption of lauric acid was less. There are different absorption patterns between different medium-chain fatty acids; however, lymphatic absorption of MCFAs and simple sugars does occur. Total measured absorption of MCFAs is less than that of long-chain fatty acids or sugars. Further work is required to demonstrate the utility of MCT supplementation on nutritional status and lymphatic flow rates in chylous leak syndromes, and to examine the factors that control the results of absorption of fatty acids of varying chain lengths, both in normal and pathological states.