Effects of dietary fat saturation and triglyceride and cholesterol load on lipid absorption in the rat.

Lipid absorption was studied in non-fasting thoracic lymph duct-cannulated rats receiving continuous intraduodenal infusions of oils varying in triglyceride fatty acid saturation (P/S = 4.6 or 0.2), triglyceride load (66 or 165 mumoles/h) and cholesterol load (0 and 3.5 mumoles/h at the low oil infusion rate, and 0 and 11 mumoles/h at the high infusion rate). Triglyceride absorption averaged 95% for rats receiving the low and high infusion rates of the polyunsaturated oil (groups defined as 10P and 25P, respectively), and 77% for those receiving the saturated oil (10S and 25S). Estimated average relative lymph lipoprotein size was significantly larger during infusion of the high vs. low triglyceride load, and of the P vs. S oil (25P greater than 25S greater than 10P greater than 10S). Lymph cholesterol flux during infusion of oil only averaged 3.2 mumoles/h for all groups and was not significantly influenced by dietary fat saturation or triglyceride load. The initial rapid rise in lymph cholesterol after addition of cholesterol (mass + tracer) to the oils was due primarily to endogenous cholesterol. Thus, absorption of dietary cholesterol resulted in an initial displacement of endogenous cholesterol into the lymph. However, in samples collected 18--24 h after addition of cholesterol to the oil, 87--107% of the increased lymph cholesterol flux was exogenous (labeled). This increased flux was not significantly influenced by fat saturation of the infused oil and averaged 5.6 mumoles/h for rats in the low oil/lower cholesterol infusion groups, and 9.5 mumoles/h for those in the higher infusion groups. The percent esterified cholesterol in lymph of rats in the low oil infusion groups was significantly higher in the S vs. P animals during both infusion of oil only and of oil + cholesterol. The observed effects of fat saturation on lymph triglyceride flux, cholesterol esterification and lipoprotein size may have important consequences for subsequent metabolism of absorptive lipoproteins, and for their ultimate effects on plasma lipid levels.