1) Antioxidant activity. Hydroxytyrosol is a potent inhibitor of metal-induced oxidation of low density lipoprotein. In addition, metal-independent oxidation is also significantly retarded by hydroxytyrosol. The antioxidant activities of hydroxytyrosol, which has been proven to be more effective than BHT or vitamin E, were further confirmed, by the use of stable free radicals, such as DPPH, Also, hydroxytyrosol is a scavenger of superoxide anions generated by either human polymorphonuclear cells or by the xanthine/xanthine oxidase system. Furthermore, a scavenging effect of hydroxytyrosol was demonstrated with respect to hypochlorous acid, a potent oxidant produced in vivo at the site of inflammation and a major component of chlorine-based bleaches that can often come into contact with food during manufacturing. Antioxidant activities have also been demonstrated versus DNA damage, hydrogen peroxide-induced insult to red blood cells.

2) Interference with enzymes. Hydroxytyrosol is able to modulate several enzymatic activities linked to cardiovascular disease. Among them, inhibition of platelet aggregation and pro-inflammatory enzymes such as 5-lipoxygenase, and stimulation of the inducible form of nitric oxide synthase have been demonstrated in vitro. In in vitro models, hydroxytyrosol is not able to upregulate the activity of the endothelial form of nitric oxide synthase, leaving its role in modulation of vasomotion unresolved.

While the majority of data have been obtained in vitro, several experiments have been performed in laboratory animals. In addition, there are approximately 15 human experiments that compared olive oil with extra virgin olive oil (which, however, contains phenols other than hydroxytyrosol). Finally, hydroxytyrosol and related olive phenols have been tested, as supplements, in humans.

Animal experiments confirm, in vivo, most of the evidence obtained in vitro. In particular, hydroxytyrosol retains its antioxidant activity once ingested (though the human metabolic pathway has been elucidated and shows extensive glucuronidation and subsequent urinary excretion), protects from second hand smoke-induced oxidative damage, inhibits platelet aggregation, increases brain cell resistance to oxidation and mitochondrial membrane potential.Human experiments initially showed that hydroxytyrosol from extra virgin olive oil is absorbed and excreted in the urine. Further experiments confirmed hydroxytyrosol鈥檚 anti-thrombotic potential and its ability to ameliorate osteoarthritis. As a caveat, such experiments have been performed with mixtures of olive phenols in which hydroxytyrosol was the most active ingredient, but not the exclusive one. Synergy with other olive phenols cannot, at present, be excluded.

The safety profile of hydroxytyrosol appears to be excellent: no untoward effects have been demonstrated even at very high doses. Hence, hydroxytyrosol has been granted a GRAS status.In synthesis, hydroxytyrosol is currently one of the most actively investigated natural phenols and is endowed with interesting pharmacological activities, many of which have been demonstrated in vivo. Given its excellent safety profile, future availability as human supplement might be conceivable.