Epidemiological evidence suggests that those who consume a traditional Mediterranean diet are protected from cardiovascular disease. Clinical trials have also shown the Mediterranean diet to be beneficial to the vascular system by decreasing oxidation and improving lipid profiles. One component thought to be responsible for this effect is the olive oil that is eaten as part of this diet. Olive oil is comprised mainly of the monounsaturated fatty acid oleic acid, and originally some researchers theorised that the protective factor in the Mediterranean diet was likely to be the high monounsaturated fat content. However, studies investigating the effects of monounsaturated fatty acids on cardiovascular disease have not convincingly demonstrated a beneficial effect. Instead it appears that polyphenolic components of olives that remain in the oil during processing may be responsible for its cardioprotective effects.
Researchers1 have investigated if the polyphenolic content of olive oil is responsible for the beneficial cardiovascular effects in a crossover study involving 200 health male volunteers. The subjects were assigned to receive either 25mL of low (2.7mg/kg), medium (164mg/kg) or high (366mg/kg) polyphenolic content olive oil for 3 weeks. Following a 2 week washout period the subjects switched to another treatment in a random order. The results showed that the polyphenolic content of the olive oil increased the high density lipoprotein (HDL) cholesterol levels in a linear fashion. This increase in HDL in turn resulted in the total cholesterol to HDL cholesterol ratio to decrease. Oxidative stress levels decreased in a linear fashion with increasing phenolic content and LDL oxidation decreased by 1.48 U/L and 3.21U/L for the medium and high polyphenolic content oil, respectively. However the oxidation of LDL actually increased in the low polyphenolic oil group.
These results suggests that it is the polyphenolic content of the oil that is beneficial to the cardiovascular system, rather than the monounsaturated fat content. Olives contain a complex mixture of polyphenols including tyrosol and hydroxytyrosol that may be responsible for the beneficial health properties. These polyphenols remain in the oil during processing of extra virgin olive oil, but are not present in high amounts in refined olive oil. These polyphenolic compounds, and other plant substances can be seen in the darker coloured extra virgin olive oil, but are absent from the lighter coloured refined olive oil. Extra virgin olive oil, as consumed by those who adhere to the traditional Mediterranean diet is the only form of olive oil that should be consumed. In this study, the triglyceride levels decreased in all treatment protocols suggesting that this may be a benefit of the monounsaturated oil, independent of the polyphenolic content.
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