Fatty acid compositional analysis established chylomicron triacylglycerols (TAGs) closely followed the test fat FAC patterns for each dietary rotation in this study. Plasma TAG fatty acids mimic dietary fats and form a valid tool for compliancy measures . The dietary fats in this study incorporated palm olein in varying proportions to achieve P/S ratios ranging from 0.27 (palm olein only), 1.0 Step 1 or AHA recommendation (palm olein with soyabean oil) which has a higher content of n-6 PUFA and 1.32 targeting a higher MUFA (palm olein + rapeseed oil) content which is the therapeutic lifestyle change (TLC) diet advocated by the ATP III guidelines. The overall fatty acid composition ratio of SFA:PUFA:MUFA achieved a proportion of 3.5:1:3.8 for the POL diet, 1:1.1:1.5 for the AHA diet and 1:1.3:3 for the PCAN diet. Palm oil in varying proportions allowed for the MUFA content to be kept constantly > 40% for all diets with only the proportion of palmitic acid as a SFA source and linoleic acid as a PUFA source differing between diets.
This study found that varying the P/S ratio did not significantly affect plasma TC and TAG levels postprandially. Plasma HDL-C concentrations were significantly affected by the P/S ratio of the diets tested. HDL-C concentrations increased with the POL diet (P/S = 0.27) but were lowered with the AHA (P/S = 1.0) and PCAN (P/S = 1.3) diets. A consequence of decreasing dietary fat saturation in humans is the concomitant decrease in HDL-C concentrations in both adult and pediatric populations [31–34]. In long-term studies it has been hypothesized that the lowering of HDL-C with increasing unsaturation was a consequence of isoenergetic substitution with carbohydrates [35–37]. We however kept carbohydrate content of all test diets constant, and the only dietary parameters that were interchanged were the SFA and n-6 PUFA content. Percentage increase in AUC plasma HDL-C for the POL diet compared to the PCAN diet was 31.4% whilst the increase for the AHA diet compared to the PCAN diet was only 8.4%. The increase in HDL-C caused by the POL diet during the postprandial period compared to the higher P/S diets may indicate an ability of the POL diet to promote reverse cholesterol transport (RCT). However, study limitations prevented the inclusion and evaluation of parameters such as HDL particle size or the enzymes involved in RCT such as cholesteryl ester transfer protein, lipoprotein lipase and lecithin:cholesteryl acyl transfer protein .
We observed a monophasic lipemic response irrespective of P/S ratios with peaking taking place between 3.5-5.5 h and the associated lipemia was not significantly different between test meals. However, the magnitude and duration of lipemia was greatest with the POL diet but lesser with oils of increasing P/S ratio. The monophasic pattern of postprandial TAG behaviors in this study is in variance with other studies which report peaking either once, twice or three times during the postprandial period [39–41]. A biphasic response has also been associated with increasing MUFA content after a single meal challenge . In a study comparing palm oil, lard and puff pastry margarine, Jensen et al. reported a biphasic response curve with an initial peak 1-2 h and a second peak 4-7 h after the meal . But the diet used was almost fat-free ~ 1 g of fat (total energy- 104 kcal; carbohydrates providing 83% energy; protein providing 10% energy). This was in stark contrast to the nutritional content of the postprandial meal supplied in this study (total energy 1010 kcal, 101 g carbohydrate, 53 g fat and 32 g protein) .
In agreement with our study, Pedersen et al. (1999) in comparing rapeseed oil, sunflower oil and palm oil as sources of MUFA, PUFA and SFA respectively, did not find significant differences between fat classes in relation to fat clearance and lipoprotein response . Weintraub et al. (1988) also did not report any significant difference in TAG levels between SFA and n-6 PUFA diets . However others have noted slower postprandial fat clearance of long-chain SFAs compared to n-6 PUFAs [19, 21, 43] whilst n-3 PUFA had the ability to markedly attenuate lipemia compared to SFA and n-6 PUFA diets . Preferential hydrolysis by lipoprotein lipase for larger TRL particles has been reported in rat studies [44, 45]. Therefore the lower lipemia caused by the AHA and PCAN diets compared to the POL diet may perhaps be explained by the particle size of TRLs which is increased by unsaturated fatty acids compared to SFAs [18, 20].
The major finding of this postprandial human study was the post-meal effect of increasing HDL-C levels occurred with a decreasing P/S ratio which was achieved with a higher palmitic acid content. This finding is in agreement with other studies using palmitic-rich fats (palm oil) which suggest an association of palmitic acid with greater HDL-C levels and lower TC/HDL-C [46–48]. Increased HDL-C levels are cardioprotective with an anti-atherogenic benefit associated with its primary role in reverse cholesterol transport. A 1% increase in HDL-C results in a 1 to 2% reduction in major cardiovascular events . Raising HDL-C is now a treatment goal for atherogenic dyslipidemia in CVD risk management in addition to reducing LDL-C [50, 51]. This area of research is intense and remains complex as noted from the negative results associated with nicotinic acid or fibrate in combination therapy with statin . The evidence from alternative efforts such as promoting exercise, moderate alcohol use, weight loss and smoking cessation as a means to promote HDL-C is scarce. Dietary factors that affect HDL-C remain to be identified .