Abstract
Epidemiological studies have observed a lower risk of coronary heart disease (CHD) in individuals consuming a diet high in monounsaturated fatty acids (MUFA), however, an understanding of the effects of MUFA on postprandial lipoprotein metabolism is currently lacking. In this thesis, the effects of acute and chronic MUFA intake, as n-9 oleic acid, on postprandial lipoprotein metabolism, especially chylomicron (CM) metabolism was studied. To determine the effects of acute test meals of varying MUFA content on CM metabolism, triacylglycerol (TAG), apolipoprotein (apo) B-48 and retinyl ester (RE) were measured in plasma, triacylglycerol-rich lipoprotein (TRL) and triacylglycerol-poor lipoprotein (TPL) fractions. Fifteen healthy young men were recruited to consume the 12%, 17% and 24% test meals on 3 separate occasions and their metabolism of intestinally derived lipoproteins, was investigated by determining the time to reach peak concentration and area under the curve (AUC) for the RE, apo B-48 and TAG parameters. On increasing the MUFA content of the test meals, the individual volunteer time to peak for the plasma RE and apo B-48 responses were significantly later than that of the TAG response (24% MUFA meal, mean (SD), TAG 266 (126) minutes, apo B-48 293 (75) minutes (P < 0.05) and RE 428 (60) minutes (P < 0.05)). The lipoprotein lipase (LPL) activity, following a bolus injection of heparin, was significantly lower after an overnight fast compared with the activities 9 hours following consumption of the 3 test meals (P < 0.05). The metabolism of CM particles was also compared in 16 Northern (UK and Ireland) (N.E.) and 16 Southern (Greece) European (S.E.) volunteers following the same 3 MUFA test meals. Although there were no significant differences in the fasting TAG and apo B-48 responses, there was a significant difference in the pattern of postprandial curves (P < 0.0001), with a more rapid entry and clearance in the S.E. volunteers. This was possibly caused by significantly higher hepatic lipase (HL) activities in the S.E. volunteers (P < 0.01). The S.E. volunteers showed a different pattern of NEFA suppression following the meal compared with the N.E. volunteers (P < 0.0001). There was a significantly higher fasting glucose dependent insulinotrophic polypeptide (GIP) level and significantly different postprandial insulin and GIP time response curves (P < 0.001) in the S. E. volunteers. A 2 month random cross-over study investigated the effect of increasing the MUFA content of the background diet of individuals in the U.K. Thirteen healthy young men were recruited with a close family history of CHD. No significant differences were found in the fasting lipoprotein levels, but there was a greater decline in total cholesterol (TC) and low density lipoprotein cholesterol (LDL-C) following the MUFA-enriched compared with the control diet. Similar postprandial responses were observed following the 2 diets, with a tendency for the lipoprotein and hormonal responses to begin to mimic those of the S.E. responses, with an earlier entry of TAG and apo B-48 and higher peak insulin and GIP responses. Lower LPL and HL activities were observed following the MUFA-enriched compared with the control diet.