Effects of plant sterols and olive oil phenols on serum lipoproteins in humans

The studies described in this thesis investigated whether minor components from vegetable oils can improve health by decreasing cholesterol concentrations or oxidative modification of low-density-lipoprotein (LDL) particles.The plant sterolsβ-sitosterol and sitostanol are known to decrease cholesterol concentrations, but it is not clear whether other chemically related structures have similar effects. We examined the cholesterol-lowering effects of concentrates ofβ-sitosterol and 4,4'-dimethylsterols from rice bran oil and triterpene alcohols from sheanut oil. Plant sterols from rice bran oil lowered serum LDL cholesterol by 9%. This was probably due toβ-sitosterol rather than the 4,4'-dimethylsterols. Triterpene alcohols did not affect serum cholesterol concentration.Oxidative modification of LDL is hypothesised to play a role in the development of atherosclerosis. Extra virgin olive oil contains phenols with antioxidant activity that could prevent oxidative modification of LDL. Three weeks of consumption of phenol-rich olive oil or a single dose of olive oil phenols did not decrease LDL oxidisability, neither in fasting plasma nor postprandial plasma samples. We showed that olive oil phenols reduce LDL oxidisability in vitro , but only in amounts that are much higher than can be reached by olive oil consumption in vivo .The first requirement for an in vivo action of a dietary antioxidant in humans is that it enters the blood circulation. We therefore studied the absorption and urinary excretion of olive oil phenols in humans. We found that apparent absorption of the ingested olive oil phenols was more than 55-66 mol%. Absorption was confirmed by the urinary excretion of at least 5 mol% tyrosol and hydroxytyrosol. A further requirement for a dietary antioxidant to prevent oxidative modification of LDL is that it becomes present in the circulation or in LDL in a form with antioxidant activity. In the body olive oil phenols are extensively metabolised. The antioxidant activity of these phenol metabolites is unknown. To determine the antioxidant activity of olive oil phenols in vivo future studies should focus on the antioxidant activity of the metabolites actually present in plasma rather than on the in vitro antioxidant activity of the phenols as present in the olive oil.In conclusion, although the olive oil phenols are well absorbed, the amount of phenols in olive oil and their consequent attainable plasma concentration in humans is probably too low to reduce LDL oxidisability. Furthermore, our studies provide no evidence that 4,4'-dimethylsterols from rice bran oil or triterpene alcohols from sheanut oil are able to decrease cholesterol concentrations. Thus, there are no indications that the minor components from vegetable oils described in this thesis have important effects on serum lipoproteins.