Red palm oil in experimental atherosclerosis.

Abstract

Oil obtained initially in harvesting the fruit of the oil palm is red due to its content of carotenes, tocopherols and tocotrienols. Table 1 presents the average fatty acid, carotene and tocotrienol composition of red palm oil (RPO). In the past, the compounds imparting the red colour to the oil have been removed and sold separately under the name of palmvitee. Palmvitee has been shown to lower cholesterol levels in human subjects1,2 (Table 2). Tocotrienols have also been shown to lower the cholesterol levels of chickens,3 rats4 and pigs.5 Tocopherols have been shown to inhibit atherogenesis in rabbits6,7 and monkeys.8 Antioxidants, in general, are considered to have anti-atherosclerotic properties. Gey et al. examined data from 16 populations in the WHO/MONICA Core study and found no significant correlation between mortality from ischemic heart disease (IHD) and levels of plasma cholesterol or blood pressure.9 However, in 12 of the populations, the levels of vitamin E exhibited a strong inverse correlation with IHD. When all 16 populations were examined there was a modest association between plasma cholesterol levels and IHD mortality, but the inverse correlation between IHD mortality and plasma vitamin E levels was much stronger.9 In epidemiological studies of dietary antioxidant intake and the risk of coronary disease Stampfer et al.10 observed a significant trend towards risk reduction with increasing vitamin E intake in a group of 87 245 nurses, and a similar observation was made in 39 910 male health professionals.11 Carotenoid intake appears to reduce carotid artery wall thickness and plaque accretion.12 The following findings provided a rationale for examining the effects of RPO in experimental atherosclerosis. Palm oil (PO) has been stigmatised as a hypercholesterolaemic fat because of its palmitic acid (16:0) content, despite human studies that show it does not raise serum cholesterol levels.13,14 We have shown that the presence of palmitic acid at the SN2 position of a triglyceride renders that triglyceride more atherogenic.15–17 Cottonseed oil contains 24% palmitic acid but only 2% is at the SN2 position. Randomisation of cottonseed oil increases the amount of 16:0 at the SN2 position to 8.3% and trebles the severity of atherosclerosis.15 Lard and tallow both contain 20–24% palmitic acid. In lard, virtually all of the 16:0 is at the SN2 position and it is 113% more atherogenic than tallow. Randomisation of lard reduces the amount of 16:0 at the SN2 position by about 67% and reduces atherogenicity by 51%. Randomization of tallow increases the amount of 16:0 at the SN2 position by 124% and increases atherogenicity by 10%16 (Table 3). A study of synthetic triglycerides17 showed that 1,3-palmitoyl-2oleoylglycerol was 53% less atherogenic than 1,2-palmitoyl3-oleoylglycerol.17 We have carried out a study in which the atherogenic properties of RPO were compared with those of refined, bleached, deodorized PO (RBD-PO) and randomised PO.18 The fatty acid compositions of the oils are shown in Table 4 and the carotenoid and vitamin E levels of RPO are given in Table 5. The fats were incorporated into semipurified diets containing 0.1% cholesterol (Table 6) and fed to rabbits (10/group) for 90 days. The results are given in Table 7. It is evident that RBD-PO is 21% less atherogenic than randomised palm oil and 15% more atherogenic than RPO. The findings, while indicative, were not striking. The study was repeated using 0.2% cholesterol and feeding was maintained for 65 days. As Table 8 shows, RBD-PO was 25% less atherogenic than randomised PO and 47% more atherogenic than RPO. RPO is rich in both carotenoids and vitamin E and this study did not indicate if one or both of the components of RPO were responsible for our findings. Rice bran oil (RBO) contains about as much vitamin E as PO and RPO but its carotenoid level is very low. We then compared the atherogenic effects of PO, RPO and RBO. We included a fourth group; PO plus the carotenoid and vitamin E contents of RPO. The fatty acid composition of the fats is