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The effects of several flavonoids and other known antioxidants on the thermal autoxidation of refined, bleached and deodorized (RBD)‐palm oil were studied. The lipid peroxidation was indexed by measuring the malonyldialdehyde (MDA) production using the 2‐thiobarbituric acid (TBA) test. The antioxidative action decreased in the order of morin > kaempferol > myricetin > quercetin > vitamin A > α‐tocopherol > apigenin > (+)‐catechin > chrysin > datiscetin > luteolin > naringin > taxifolin > rutin > butylated hydroxytoluene (BHT) > naringenin.The flavonoid aglycones were more potent in their antiperoxidative action than their corresponding glycosides. Structure‐activity revealed that the flavonoid molecule with polyhydroxylated substitutions on rings A and B, a 2–3 double bond, a free 3‐hydroxyl substitution and a 4‐keto moiety, would confer potent antiperoxidative properties upon the compound. The flavonols, namely morin, myricetin, kaempferol and quercetin, would be suitable potential antioxidants for use in the stabilization of RBD‐palm oil and its fractions against thermal autoxidation. The structural activity of the flavonoids on the RBD‐palm oil was similar to those observed for these compounds in animal tissue or enzyme systems.

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Male Wistar rats were fed a high fat diet (HFD) containing 2.5% cholesterol and 16% lard supplemented with polyphenolic natural products namely quercetin, morin or tannic acid (100 mg/rat/day) for 4, 7 and 10 wk. Rats fed HFD without the supplements served as control. The effects of these compounds on blood lipid profiles, enzymes, liver fat and aorta of the rat were studied. In rats fed HFD containing tannic acid, plasma total cholesterol (TC), low density lipoprotein cholesterol (LDLC) and triglyceride (TG) were reduced by 33.3%, 29.6% and 65.1%, respectively, at week 10. High density lipoprotein cholesterol (HDLC) concentration was not altered. Fat deposition was also decreased in the liver of these rats. Morin significantly reduced plasma TG (65.1%) and liver fat only at week 7 while at week 10 it reduced plasma TC and LDLC by 30.9% and 29.3% respectively. The plasma HDLC concentration was increased by 47.3% at week 4 but no effect was seen at weeks 7 and 10. In the rats fed HFD containing quercetin, plasma HDLC was increased by 28.6% at week 7 but at week 10, plasma LDLC was increased by 21.2%. Quercetin did not cause any significant changes on the plasma TC, TG and liver fat at weeks 4, 7 and 10. Plasma alanine aminotransferase, alkaline phosphatase and bilirubin in control and treated groups were not significantly different. However, hepatic lipase activity in rats fed tannic acid was significantly lower. Aortae of all groups of rats showed no abnormalities. The present report indicates that tannic acid and morin are effective in reducing plasma and liver lipids when supplemented with a high fat diet in rats.

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