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Plasma clearance of chylomicrons from butterfat is not dependent on saturation: studies with butterfat fractions and other fats containing triacylglycerols with low or high melting points^1,2,3

Background: Dietary fats influence plasma lipids, and changes in the clearance and metabolism of postprandial lipoproteins can affect atherosclerosis. Butterfat is considered hypercholesterolemic but contains a multitude of constituent fatty acids.

Objectives: We determined triacylglycerol and cholesteryl ester clearances of lymph chylomicrons derived from butterfat, fractions of butterfat, and other dietary fats.

Methods: Radiolabeled lymph chylomicrons resulting from the intestinal absorption of different fats were reinjected into recipient rats to measure plasma clearance. Plasma clearance of [¹⁴C]triacylglycerol was used as an indicator of chylomicron lipolysis whereas clearance of [³H]cholesteryl ester was used as an indicator of chylomicron remnant removal.

Results: [³H]Cholesteryl ester clearance was slower from chylomicrons derived from a solid, high-saturated-butterfat fraction than from whole butterfat, but clearance of chylomicrons from other fractions did not correlate with the fractions' saturated fatty acid contents. Clearance of cholesteryl esters in chylomicrons derived from cocoa butter, palm oil, and butterfat was slower than clearance of cholesteryl esters in chylomicrons derived from safflower oil. Hepatic uptakes of cholesteryl esters were generally lower for chylomicrons from all butterfat fractions, cocoa butter, and palm oil.

Conclusions: In contrast with minor effects on the lipolysis of chylomicron triacylglycerols, chylomicron remnant removal was strongly influenced by the type of dietary fat, with slower cholesteryl ester clearances for saturated fats with higher melting points. However, remnant removal and hepatic uptake of chylomicrons from whole butterfat and fractions of butterfat were not correlated with fat saturation. The mechanisms of this apparent paradox remain unknown but may be attributable to acyl arrangements in the lipid classes of chylomicrons that influence the association with apolipoproteins and receptors and hence remnant removal.

Key Words: Chylomicrons • chylomicron remnants • butterfat • saturated fatty acids • dietary fats • plasma clearance • cholesteryl ester • triacylglycerol • rats

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