The phenolic compounds of olive oil: structure, biological activity
and beneﬁcial effects on human health
Elisa Tripoli, Marco Giammanco*, Garden Tabacchi, Danila Di Majo,
Santo Giammanco and Maurizio La Guardia
Institute of Physiology and Human Nutrition, Faculty of Pharmacy, University of Palermo,
Via Augusto Elia 3, 90 127, Palermo, Italy
The Mediterranean diet is rich in vegetables, cereals, fruit, ﬁsh, milk, wine and olive oil and has
salutary biological functions. Epidemiological studies have shown a lower incidence of
atherosclerosis, cardiovascular diseases and certain kinds of cancer in the Mediterranean area.
Olive oil is the main source of fat, and the Mediterranean diet’s healthy effects can in particular be
attributed not only to the high relationship between unsaturated and saturated fatty acids in olive
oil but also to the antioxidant property of its phenolic compounds. The main phenolic compounds,
hydroxytyrosol and oleuropein, which give extra-virgin olive oil its bitter, pungent taste, have
powerful antioxidant activity both in vivo and in vitro. The present review focuses on recent
works analysing the relationship between the structure of olive oil polyphenolic compounds and
their antioxidant activity. These compounds’ possible beneﬁcial effects are due to their
antioxidant activity, which is related to the development of atherosclerosis and cancer, and to
anti-inﬂammatory and antimicrobial activity.
Olive oil: Antioxidants: Cardiovascular diseases: Phenolic compounds: Oleuropein
Olive oil, a product of the mechanical extraction from the
fruit of Olea europeae L. (Oleaceae family), is composed of
a glycerol fraction, constituting approximately 90– 99 %,
and of a non-glycerol or unsaponiﬁable fraction (0·4–5 %).
Oleic acid, a MUFA (18 : 1n-9), represents 70– 80 % of the
fatty acids present in olive oil. Epidemiological studies have
shown a lower incidence of atherosclerosis, cardiovascular
diseases and certain kinds of cancer in the Mediterranean
area than in other areas. The results of these studies have
been in part attributed to the characteristic kind of diet of the
local population. The traditional Mediterranean diet
contains, unlike the Northern European and American
diet, a considerable proportion of vegetables, cereals, fruit,
ﬁsh, milk, wine and olive oil. The substantial difference
between the two kinds of diet – despite the similarity
between the classic risk factors for cardiovascular
pathologies, such as high plasma cholesterol levels – has
been associated with a lower risk of their development
(Keys, 1995; Trichopoulou, 1995; Willet et al. 1995;
Lipworth et al. 1997; Visioli & Galli, 1998a; Trichopoulou
et al. 1999; Visioli et al. 2000b).
It is known that an increased consumption of MUFA
instead of PUFA reduces the risk of atherosclerosis because
it makes the circulating lipoprotein less sensitive to
peroxidation (Reaven et al. 1991; Bonanome et al. 1992;
Moreno & Mitjavilab, 2003).
Also, the inclusion in the diet (approximately 15 % of
total energy) of oleic acid reduces plasma levels of the
complex LDL-cholesterol and increases HDL-cholesterol.
However, the protective role of the Mediterranean diet is
much higher than that of the single foods that characterise it,
and the protective role played by many of these foods has
still to be deﬁned. Recent studies have demonstrated that
other constituents of certain characteristic Mediterranean
diet foods have beneﬁcial biological effects on health. It has
been established that olive oil has beneﬁcial effects as
regards breast and colon cancer (Owen et al. 2000b),
diabetes accompanied by hypertriacylglycerolaemia,
inﬂammatory, and autoimmune diseases such as rheumatoid
arthritis (Alarcon de la Lastra et al. 2001).
We will therefore consider the unsaponiﬁable fraction of
extra-virgin olive oil, which is rich in tocopherols, aromatic
hydrocarbon compounds and sterols. In particular, we will
study the biological functions of its polyphenolic
The phenolic compounds
The beneﬁcial effects of the Mediterranean diet can be
attributed not only to the high relationship between
Abbreviations: HMG, 3-hydroxy3-methylglutaryl; ROS, reactive oxygen species.
* Corresponding author: Professor M. Giammanco, fax þ39 091 6236407, email email@example.com
Nutrition Research Reviews (2005), 18, 98–112
qThe Authors 2005
unsaturated and saturated fatty acids of olive oil, but also to
the antioxidant property of its phenolic compounds. The
pulp of olives contains these compounds, which are
hydrophilic, but they are also found in the oil. The class
of phenols includes numerous substances, such as simple
phenolic compounds like vanillic, gallic, coumaric and
caffeic acids, tyrosol and hydroxytyrosol and more complex
compounds like the secoiridoids (oleuropein and
ligstroside), and the lignans (1-acetoxypinoresinol and
The main antioxidants of virgin olive oil are carotenoids and
phenolic compounds, which are both lipophilic and
hydrophilic. The lipophilics include tocopherols, while the
hydrophilics include ﬂavonoids, phenolic alcohols and
acids, secoiridoids and their metabolites. The polyphenols
include phenolic alcohols and acids, secoiridoids and their
metabolites and the lignans; however, since some of these
(tyrosol) do not possess two hydroxyl groups, it would be
incorrect to put them in this class (Visioli et al. 2002).
The ﬂavonoids include the glycosides of ﬂavonol
(luteolin-7-glucoside and rutin), anthocians, cyanidin and
the glucosides of delphinidin.
The polyphenols can be distinguished as simple or
complex. In the ﬁrst class, 3,4-dihydroxyphenyl-ethanol, or
hydroxytyrosol, and p-hydroxyphenyl-ethanol, or tyrosol,
are the most abundant phenolic alcohols in olives (Fig. 1 (B)).
Other phenolic acids, with the chemical structure C6 –C1
(benzoic acids) and C6 –C3 (cinnamic acid), are also present
in olives (Garrido Ferna
´ndez et al. 1997).
Historically, these compounds (caffeic, vanillic, syringic,
protocatechuic, p-coumaric and o-coumaric, 4-hydroxyben-
zoic acids) represent the ﬁrst group of simple phenols
observed in virgin olive oil (Montedoro, 1972; Vasquez
The presence of simple phenolic acids as secondary
components in olive oil has been widely reported (Solinas &
Cichelli, 1981; Tsimidou et al. 1996). The presence of gallic
acid has also been documented (a substance also present in
tea) (Mannino et al. 1993).
The secoiridoids oleuropein, demethyloleuropein, ligstro-
side and nu
¨zhenide, the main complex phenols in virgin
olive oil, are secondary glycosidic compounds similar to
coumarins; secoiridoids are characterised by the presence of
elenolic acid in its glucosidic or aglyconic form, in their
molecular structure (Bianco & Uccella, 2000) (Fig. 1).
The secoiridoids, which are glycosidated compounds, are
produced from the secondary metabolism of terpenes as
precursors of several indole alkaloids (Soler-Rivas et al.
Oleuropein is the ester between 2-(30,40-dihydroxyphe-
nyl)ethanol (hydroxytyrosol) and the oleosidic skeleton
common to the glycosidic secoiridoids of the Oleaceae
(Fig. 1 (A)).
Hydroxytyrosol can be present as a simple or esteriﬁed
phenol with elenoic acid, forming oleuropein and its
aglycone, or as part of the molecule of verbascoside (Amiot
et al. 1986; Servili et al. 1999b); it can also be present in
several glycosidic forms, depending on the hydroxyl group
to which the glucoside is bound (Bianco et al. 1998a,b;
Ryan et al. 2001).
While tocopherols, phenolic acids, phenolic alcohols and
ﬂavonoids are present in many fruits and vegetables
belonging to several botanical families, secoiridoids are
present exclusively in plants of the family of Olearaceae.
Oleuropein, demethyoleuropein and verbascoside are
present in all the constituent parts of the fruit, but more
abundantly in the pulp (Soler-Rivas et al. 2000) (Fig. 2 (A)
and (B)). Nu
¨zhenide has been only found in the seed (Servili
et al. 1999a) (Fig. 2 (C)).
Hydroxytyrosol is one of the main phenolic compounds
in olives, virgin oil and waste water obtained during the
production of olive oil. In fresh virgin oil, hydroxytyrosol
mostly occurs esteriﬁed, while in time the non-esteriﬁed
form prevails owing to hydrolytic reactions (Angerosa et al.
1995; Cinquanta et al. 1997) (Fig. 1 (A)).
Another group of substances present in the phenolic
fraction has been isolated by MS and NMR from
Fig. 1. (A) Chemical structures of oleuropein, ligstroside, 10-
hydroxyligstroside and 10-hydroxyoleuropein. Hydroxytyrosol and
tyrosol derive from the hydrolysis of oleuropein. (B) Chemical
structures of hydroxytyrosol and tyrosol. (C) Chemical structures of
elenolic acid and elenolic acid glucocoside.
Phenolic compounds of olive oil 99
extra-virgin olive oil, i.e. lignans, (þ)-1-acetoxypinoresinol
and (þ)-pinoresinol (Owen et al. 2000c).
The substance (þ)-pinoresinol is a common compound of
the lignan fraction of several plants, such as the seeds of the
species Forsythia (Oleaceae family) (Davin et al. 1992) and
Sesamum indicum (sesame) (Kato et al. 1998), while (þ)-1-
acetoxypinoresinol, (þ)-1-hydroxypinoresinol and their
glycosides have been found in the bark of the Olea
europeae L. (olive) (Tsukamoto et al. 1984, 1985). How
lignans are transformed into the main component of the
phenolic fraction of olive oil is not known.
They are not present in the pericarp of the olive drupe or
in the leaves and sprigs that can be present in the residual
vegetable after pressing the olives. It has been recently
shown that (þ)-pinoresinol is an important component of
the phenolic fraction of the olive kernel (Owen et al. 2000c)
Content of phenolic compounds in olive oil
It is necessary to point out that reﬁned oils do not have a
signiﬁcant content of polyphenols. The data on the
concentrations of the phenolic compounds, which are
responsible for the sensory and antioxidant properties of
high-quality olive oils, are not always in agreement. The lack
of a suitable analytic methodology is the main cause of
inaccuracies in the quantitative evaluation of the phenolic
compounds of olive oil. Currently, the commonest methods
for evaluating olive oil polyphenol content are the Folin –
Ciocalteau colorimetric test and liquid chromatography
(Montedoro et al. 1992). The former method gives imprecise
results because of the reagent’s low speciﬁcity towards
phenolic compounds; also, such methods do not yield
quantitative information about single phenolic compounds.
On the contrary, HPLC is very sensitive and speciﬁc but
requires time to perform the analysis (approximately 1 h). It
does not supply information regarding phenolic molecules.
Standards are therefore not available (Visioli et al. 2002).
Mosca et al. (2000) described an enzymic test for the
quantitative determination of the phenolic compounds of
olive oil. This method is rapid and easy to perform; it is
more sensitive and speciﬁc for phenolic compounds than the
Folin–Ciocalteau method, but it supplies only quantitative
information and does not detect the important ‘minor
constituents’, i.e. cinnamic and vanillic acids.
Finally, a fast and sensitive method for estimating olive oil
phenolic compounds is the combination of MS with
atmospheric pressure chemical ionisation. This methodology
(Caruso et al. 2000) analyses a crude methanolic extract of
olive oil, avoiding a complex analytical workup, and also
allows quantiﬁcation of the oleuropein aglycone (Table 1).
In spite of these limits, it is possible to establish some
fundamental principles. The quality of the olives and the oil
is affected by the amount of oleuropein and its hydrolytic
products (Limiroli et al. 1995). In turn, the phenolic
compound content of the oil depends on the place of
cultivation, the climate, the variety, and the olives’ level of
maturation at the time of harvesting (Cinquanta et al. 1997;
Visioli & Galli, 1998b; Brenes et al. 1999). Their level
usually diminishes with over-ripening of olives (Monte-
leone et al. 1998; Gutierrez et al. 1999), even if there are
some exceptions to this rule. For example, olives cultivated
in warmer climates, in spite of their faster maturation,
produce oils richer in phenols (Visioli et al. 1998); also, as
we will show later, the phenolic content of olive oil is
inﬂuenced by the production process.
Oleuropein is the main polyphenol found in olive oil, both
in this form and as the aglycone. In nature, it accumulates in
the fruit of the olive tree during the growth phase up to 14 %
of net weight (Amiot et al. 1986); on the contrary, as the
olive turns greener, the amount reduces. Finally, when the
olive turns dark brown owing to the presence of
anthocyanins, the reduction in oleuropein concentration
becomes more evident. It has been shown that the
oleuropein content is higher in the ﬁrst stages of fruit
maturation and in green cultivars than in black olives.
During the reduction in the levels of oleuropein and other
oleosides, such as the quantitatively less important ligstro-
side, it is possible to observe an increase of other
compounds – some more complex, like ﬂavonoids and
verbascosides, and others simpler, like single phenols. The
reduction in the oleuropein level is also accompanied by an
increase in the levels of its glycosylated secondary products,
which reach maximum levels in black olives (Amiot et al.
1986, 1989; Bianco et al. 1993; Soler-Rivas et al. 2000). In
nature, the concentration of hydroxytyrosol and tyrosol
increases as the fruits ripen, in parallel with the hydrolysis
of compounds of higher molecular weight, while the total
amount of phenolic compounds and a-tocopherol decreases
Fig. 2. Chemical structures of demethyloleuropein (A), (B)
verbascoside and (C) nu
¨zhenide. OGlu, O-glucose.
E. Tripoli et al.100
as the fruits ripen (Climato et al. 1990; Angerosa et al. 1995;
Limiroli et al. 1996; Esti et al. 1998; Brenes et al. 1999;
Gutierrez et al. 1999).
Lignans, (þ)-1-acetoxypinoresinol and (þ)-pinoresinol
are not present in seed oils and are virtually absent from
reﬁned virgin oils but are present in extra-virgin olive oil up
to a concentration of 100 mg/kg. As occurs in simple
phenols and secoiridoids, a considerable variation in lignans
concentrations between olive oils of various origins also
occurs in this case, the reasons probably being related to
Table 1. Methods for the evaluation of the olive oil polyphenols content
Polyphenolic compound Method employed Phenol content Reference
Total phenols Enzymic
566·0 –0·8 ppm
(mg caffeic acid/kg oil)
Mosca et al. (2000)
Oleuropein and its isomers, ligstroside and
oleuropein aglycones, deacetoxyligstroside
and deacetoxyoleuropein aglycones,
APCI–MS Caruso et al. (2000)
Hydroxytyrosol, tyrosol, vanillic, caffeic, syringic,
p-coumaric, ferulic, cinnamic and elenolic acids
HPLC Low concentration (total
phenols 50– 200 mg/kg);
medium concentration (total
phenols 200 –500 mg/kg);
high concentration (total
phenols 500– 1000 mg/kg)
Montedoro et al. (1992)
ppm, Parts per million; APCI, atmospheric pressure chemical ionisation.
Fig. 3. Chemical structures of the lignans. (A) (þ)-1-Acetoxypinoresinol; (B) (þ)-1-pinoresinol; (C) (þ)-1-hydroxypinoresinol.
Phenolic compounds of olive oil 101
differences between the production zones, in the climate, in
the varieties of olives and in the oil production techniques.
Any alteration in the concentration of the various
chemicals changes olive oil’s particular taste. Phenolic
compounds, and in particular oleuropein, give the oil a bitter
taste (Visioli & Galli, 2001).
Effect of oil extraction processes on the content of phenolic
As has been shown, the concentration of phenolic
compounds in olive oil is the result of a complex interaction
of various factors; for example, the cultivar, the level of
maturation and the climate (Cinquanta et al. 1997; Esti et al.
1998; Monteleone et al. 1998; Visioli & Galli, 1998b;
Visioli et al. 1998; Brenes et al. 1999; Gutierrez et al. 1999).
It is also affected by the extraction process. Nowadays,
various methods are used to extract olive oil: the traditional
discontinuous cycle of pressure; continuous centrifugation;
systems of percolation –centrifugation. The crushing of the
olives, the pressure applied to the paste, the extraction, the
separation of vegetation water and the puriﬁcation process
are all steps common to the three systems of manufacture.
Through these three processes, oil, sansa (the solid refuse)
and vegetable water are obtained. In the traditional cycle, a
grindstone (or stone hammer) is used to mill and press the
olives. In continuous cycles, metallic crushers that use
hammer, disc and roller are used to mill the olives, and a
decanter with a centrifuge, horizontally placed, is used for
centrifugation of the paste. A vertical centrifuge is used to
separate the oily paste into oil and water (Ranalli et al.
1999). Extra-virgin olive oil is obtained from the ﬁrst
physical cold pressure of the olive paste and is rich in
phenolic compounds (Visioli et al. 1998). Virgin olive oil,
obtained through percolation (ﬁrst extraction), has a higher
content in phenols, o-diphenols, hydroxytyrosol and tyrosol
aglycones, and tocopherols than oils obtained through
centrifugation (second extraction) (Ranalli et al. 1997,
1998, 1999). The type of rolling-mill used for the pressure
and the centrifugation has an important effect. The hammer
is more effective in the extraction of phenolic compounds of
the olives and should be used for the extraction of oil from
olives that have a low content of phenolic compounds, in
order to avoid the production of oils with a bitter foretaste.
The stone rolling-mill produces oils with a stability towards
oxidation similar to that obtained with the hammering-mill,
and can be used in order to prepare oil from olives that
generally yield oil characterised by a bitter taste (Alloggio
& Caponio, 1997).
Oils obtained through centrifugation have a lower
phenolic content, probably because this process involves
the use of large amounts of hot water that remove a
considerable proportion of the phenols that is then
eliminated in the watery phase (Lo Scalzo et al. 1993;
Visioli & Galli, 1998a). This vegetable water is regarded as
a toxic residue and a pollutant for plants, because the
phenolic compounds, hydroxytyrosol, tyrosol and other
polyphenols (Capasso et al. 1992), have phytotoxic activity
(Capasso et al. 1995). However, this vegetable water could
be used as a good source of phenolic antioxidants (Limiroli
et al. 1996) or as a bactericidal solution to protect other
crops from parasites and from diseases caused by parasites
(Capasso et al. 1995).
Absorption and pharmacokinetics of polyphenols
It is essential to establish whether olive oil phenolic
compounds are absorbed in the intestine and how they are
distributed in the organism, to verify if they have the same
effects both in vivo and in vitro. To this purpose, many
studies in vitro have been carried out, but the results are not
satisfactory. An intestinal perfusion technique in situ has
been developed to estimate oleuropein absorption, both in
iso-osmotic and in hypotonic luminal conditions (Edge-
combe et al. 2000). This technique makes it possible to
exclude the inﬂuence of the hepatic and renal metabolism
and other factors that usually complicate the quantitative
evaluation of absorption (Stretch et al. 1999). In iso-osmotic
conditions, oleuropein is absorbed, with an apparent
permeability coefﬁcient (Papp) of 1·47 (SE 0·13) £
cm/s. The mechanism of absorption is not clear;
transcellular transport (carrier Na-dependent glucose
transporter 1) or paracellular movement may be involved.
In hypotonic conditions, the permeability of oleuropein is
signiﬁcantly higher (5·92 (SE 0·49) £10
P,0·001). This is probably due to an increase in
paracellular movement facilitated by the opening of the
paracellular junctions in response to hypotonicity. In an iso-
osmotic solution, oleuropein is absorbed at a constant rate of
20·023 per min (r
0·962). Its stability is dependent on pH,
since absorption occurs at pH 7. Absorption of oleuropein in
such circumstances occurs mainly by way of a transcellular
pathway. Since oleuropein is to some extent polar, it is
unlikely that it diffuses rapidly through the lipid bilayer of
the epithelial cell membrane; a carrier therefore has to be
used (Edgecombe et al. 2000). As it is a glycoside,
oleuropein can probably use a glucose carrier. Three carriers
in the epithelial cells of the small intestine have been
identiﬁed. Two of these (Glut2 and Glut5) carry glucose by
facilitated diffusion, while the third is Na-dependent
glucose transporter 1, which actively carries the glucose
across a concentration gradient (Takata, 1995). Both Glut5
and Na-dependent glucose transporter 1 are on the apical
side of intestinal epithelial cells; however, Glut5 is speciﬁc
for the transport of the fructose, and it is therefore unlikely
that it is involved in the absorption of oleuropein (Burant
et al. 1992; Kane et al. 1997).
Glut2 has been localised on the basolateral side of
epithelial cells and it probably mediates the passage of
glucose and similar substrates from epithelial cells into
the circulation (Kayano et al. 1990; Nomoto et al. 1998). In
a study on the absorption and pharmacokinetics of
hydroxytyrosol performed in the rat, it was found that the
absorption of a single dose of hydroxytyrosol was very rapid:
the maximum plasma concentration was obtained in 5 –
10 min, while after 60 min the concentration was much
reduced. However, the concentration of hydroxytyrosol in rat
plasma was smaller than the amount administered. This
discordance is presumably due to the fact that the experiment
did not take into account the presence of hydroxytyrosol
metabolites (Bai et al. 1998). Studies on the transport kinetics
of radiolabelled hydroxytyrosol (
C) performed using
E. Tripoli et al.102
differentiated cells Caco-2 have demonstrated that the
transport occurs by passive diffusion (Manna et al. 2000).
The metabolic fate of hydroxytyrosol and tyrosol in vivo
has also been evaluated by administration to rats, both by
mouth and intravenously, of the radiolabelled polyphenols.
Also in this case, hydroxytyrosol appeared in the
plasma, at maximum levels, as soon as 10 min after oral
administration. Hydroxytyrosol is quickly eliminated from
the plasma and excreted in the urine, as a free compound,
and bound to glucuronic acid; to a smaller extent (5 %) it is
also eliminated in the faeces (D’Angelo et al. 2001; Tuck
et al. 2001). Conjugation with glucuronic acid is generally
regarded as the common ﬁnal metabolic step of the intact
phenolic compounds (Bourne & Rice-Evans, 1998). Other
studies carried out in vivo in human subjects evaluated the
intestinal absorption and urinary excretion of tyrosol and
hydroxytyrosol. It was observed that the amount of
absorption of these phenols was dose-dependent and that
their urinary excretion mostly occurred by conjugation with
glucuronic acid (Visioli et al. 2000c). Urinary excretion of
both free phenolic compounds was much higher in the ﬁrst
4 h and was correlated with the intake: high doses of
phenolic compounds increased their rate of conjugation
with glucuronide (Visioli et al. 2000c; Miro
´Casas et al.
2001). In the particular case of hydroxytyrosol, excretion
from the human organism occurred in a short time. The
estimated hydroxytyrosol elimination half-life was 2·43 h.
Free forms of these phenolic compounds were not detected
in plasma samples (Miro
´Casas et al. 2003).
The entire quantity of tyrosol or hydroxytyrosol
administered was obviously not found in the urine. It
remains to be established the quantity not absorbed and that
accumulated in organs or erythrocytes, as well as the
quantity eliminated after 24 h. Other antioxidants in olive oil
could also compete with its intestinal absorption (Tuck et al.
2001). The future development of suitable techniques will
have to clarify this point.
In the rat, hydroxytyrosol is converted enzymically into
four oxidised and/or methylated derivatives. These metab-
olites have been identiﬁed as homovanillic alcohol and acid,
3,4-dihydroxyphenylacetic acid, 3,4-dihydroxyphenylace-
taldehyde and its sulfate conjugate. Also, a signiﬁcant
fraction of total radioactivity is associated with the sulfate-
conjugated derivatives that represent the main urinary
products of excretion (D’Angelo et al. 2001).
On the basis of the results reported, the pathway of
hydroxytyrosol metabolism has been proposed with the
participation of catechol-O-methyltransferase (an enzyme
involved in the catabolism of the catecholamines), alcohol
dehydrogenase, aldehyde dehydrogenase and phenolsulfo-
transferase (Tuck & Hayball, 2002) (Fig. 4).
After administration of virgin olive oil to healthy
volunteers, a signiﬁcant increase was observed in homo-
vanillic alcohol and acid urinary excretion over 24 h. This
suggests that also in man these compounds undergo the
action of catechol-O-methyltransferase. Also, the increase in
homovanillic acid excretion indicates that in man the
ethanolic derived compound of hydroxytyrosol and/or
homovanillic alcohol is oxidised (Caruso et al. 2001) (Fig. 4).
One should be cautious before extrapolating these results
and associating them with the typical Mediterranean diet.
The daily intake of olive oil is on average less than 50 ml, an
amount that Visioli et al. (2000c) gave to the subjects in
their study in a single dose, and the phenolic content of the
oils they used was higher than that of typical virgin olive oil.
However, it cannot be excluded that continuous exposure to
the phenols in olive oil can in the long run cause phenomena
of accumulation, since the absorption of simple phenols (at
least at the doses used) appears to be dose-dependent and
not saturable (Tuck et al. 2001).
The antioxidant activity of the polyphenolic compounds
The ‘reactive oxygen species’ (ROS), which are continu-
ously formed as a result of normal metabolic processes, can
oxidise and damage cellular macromolecules, possibly
leading to the development of degenerative diseases (for
example, atherosclerosis, cancer, diabetes, rheumatoid
arthritis and inﬂammatory diseases). Exogenous antiox-
idants are important because they have a twofold function,
preventing food oxidation – and in particular lipid
oxidation – and at the same time increasing the amount of
antioxidant agents present in the organism, protecting
against degenerative diseases. The most important dietary
antioxidants are certain vitamins (ascorbic acid, tocopher-
ols, carotenes) and phenolic compounds, which are present
in various foods of vegetable origin characteristic of the
Mediterranean diet, such as olive oil (Berra et al. 1995).
Phenolic compounds can act as antioxidants in various
ways. In oxidative systems using transition metals such as
Cu and Fe, they can chelate metallic ions, which can prevent
their involvement in Fenton reactions that can generate high
concentrations of hydroxyl radicals (Halliwell & Gutterige,
1990; Halliwell et al. 1995). However, the most important
antioxidant activity is related to the free radical-scavenging
ability, by breaking the chain of reactions triggered by free
radicals. The antioxidant properties of the o-diphenols are
associated with their ability to form intramolecular
hydrogen bonds between the hydroxyl group and the
phenoxylic radicals (Visioli & Galli, 1998b)(Fig. 5). As
similar studies on the ﬂavonoids have already shown, the
degree of antioxidant activity is correlated with the number
of hydroxyl groups (Rice-Evans et al. 1996; Cao et al.
1997). The number of – OH groups and their positions on
the ring are important for both ﬂavonoids and phenols. From
the study of the resonance structures formed during the
oxidation processes, it can be observed that the ortho- and
para-substitutes of the radicals are more stable than the
meta-substitute (Finotti & Di Majo, 2003). In particular,
ortho-diphenolic substitution gives high antioxidant ability,
while a single hydroxyl substitution, as in tyrosol, does not
confer any activity, since tyrosol does not protect LDL from
chemically induced oxidation.
Although olive oil contains a relatively low concentration
of a-tocopherol, it is known to be highly resistant to
oxidative degradation. This is due, in part, to the relatively
low content of PUFA and also to the high concentration of
polyphenolic antioxidants, particularly in extra-virgin olive
oil. The antioxidant activity of olive oil phenolic
compounds, and in particular of oleuropein and its by-
product hydroxytyrosol, has been studied in many
experimental models: with the use of transition metals; the
Phenolic compounds of olive oil 103
chemically induced oxidation of LDL; ROS formation, for
example the radicals superoxide and trichlormethylper-
oxylic, and hypochlorous acid (Aeschbach et al. 1994;
Salami et al. 1995; Visioli et al. 1995a, 1998; Aruoma et al.
1998). By estimating the antioxidant activity of these
polyphenolic compounds on the basis of their ability to
inhibit the formation of peroxides, it has been shown that
hydroxytyrosol and caffeic and protocatechuic acids have a
higher protective activity (Papadopoulos & Boskou, 1991).
The antioxidant activity of oleuropein and hydroxytyrosol
has also been demonstrated in cellular models and animals
(Manna et al. 1997; Speroni et al. 1998).
Some polyphenols can contribute to the regeneration of
vitamin E, as has been demonstrated by treating human
lipoproteins in vitro with peroxides.
In a recent study, the antioxidant activity of a-tocopherol
and phenolic extracts from olives and olive oil was compared
over time. It was demonstrated that in the ﬁrst 15 min the
scavenger activity of a-tocopherol was higher but soon
terminated. The extract from stoned olives and oil contained
compounds that continued to reduce the concentration of
these radicals more slowly; when on the other hand the
reaction time was delayed to 60 min, all the extracts of the
olives were much more active than a-tocopherol. On day 6
the extracts of the olives and the oil continued to be more
effective than a-tocopherol (Keceli & Gordon, 2001).
The biological activity of phenolic compounds of olive
oil is not limited to their antioxidant ability but extends to
their interaction with important enzymic systems. In
particular, it has been found out that olive oil phenols:
inhibit platelet aggregation;
reduce pro-inﬂammatory molecule formation such as
and leucotriene B
inhibit the use of oxygen in human neutrophils;
increase NO production by the macrophages of rats
Fig. 4. Postulated enzymic pathways for the metabolites of hydroxytyrosol in vivo.
E. Tripoli et al.104
exposed to endotoxin – they therefore act by up
regulating the immune system.
Other biological actions of phenolic compounds have
been discovered that can be important for their effects on
human health. For example, caffeic acid could have
cytoprotective effects on endothelial cells, correlated not
only with its action as an antioxidant agent but also with its
ability to block the increase of the concentration of
in response to lipoprotein oxidation
(Vieira et al. 1998). The ability of polyphenolic compounds
to react with metal ions could make them pro-oxidant. It has
in fact been widely observed that caffeic acid, a simple
polyphenol with an ortho-diphenolic structure, can have
pro-oxidant activity on LDL oxidation induced by Cu
(Yamanaka et al. 1997). However, this pro-oxidant activity
has been found only in the propagation phase of oxidation,
and not in the initiation phase, in which caffeic acid inhibits
lipoprotein oxidation, as has been found in previous studies
(Laranjinha et al. 1994; Nardini et al. 1995).
The effects of the antioxidant activity of olive oil
polyphenols on the integrity and function of the cells have
been studied in erythrocytes and intestinal cells (Caco-2).
The capacity of polyphenols to prevent damage in these
cells was veriﬁed when they were exposed to oxidative
stress, as in treatment with H
. Human erythrocytes were
chosen because they are the cells most exposed to oxidative
risk, since their speciﬁc role is to carry oxygen. The main
target of H
is Hb, which is oxidised to methaemoglobin.
Exposure of erythrocytes to H
also causes lipid
peroxidation, and alterations in proteins, for example the
formation of carbonyl dimers. As a consequence of this
oxidative damage, the shape of the erythrocytes changes,
causing haemolysis. The spontaneous oxidation of Hb
produces superoxide anion radicals that cause the dismuta-
tion of H
. In the presence of reduced metal ions,
especially Fe, these compounds form the highly reactive
hydroxyl radical that can damage the cellular membrane,
with consequent haemolysis (Sadrzadeh et al. 1984; van
Dyke & Saltman, 1996). Some studies on isolated
erythrocyte membranes have demonstrated that the ATP-
dependent ion transport (such as amino acid transport) is
considerably compromised by oxidative damage (Rohn et al.
1993). Under physiological conditions, ROS are quickly
removed by both enzymic and non-enzymic systems;
however, if ROS production is excessive, or if antioxidant
defence is impaired, serious oxidative damage can occur, to
both the plasma membrane and the cytosol, which ﬁnally
leads to haemolysis. Erythrocytes pre-treated with phenols
extracted from extra-virgin olive oil show signiﬁcantly less
lipid oxidation and haemolysis after treatment with H
In erythrocytes pre-treated with H
and incubated in
the presence of [
H]methionine or [
H]leucine, there is a
marked reduction in the absorption of both the amino acids
compared with control erythrocytes.
3,4-Dihydroxyphenyl-ethanol, or hydroxytyrosol, pre-
vents the alteration of amino acid transport by H
erythrocytes (Manna et al. 1999). Similarly in intestinal
tumour cells (Caco-2) treated with H
, pre-treatment with
olive oil polyphenols exerts a strong antioxidant effect.
induces a clear increase in the intracellular
concentration of malonyldialdehyde and the paracellular
transport of inulin, respectively indicating the occurrence of
lipid peroxidation and changes in cellular permeability. Pre-
incubation of the Caco-2 cells with hydroxytyrosol totally
prevents the alterations induced by H
(Manna et al.
Polyphenolic compounds in the prevention
Plasma LDL is atherogenic only after oxidative modiﬁcation
(Brown & Goldstein, 1983; Parthasarathy, 1991); some
studies have shown that oxidative stress provokes the onset
of atherosclerosis by inducing lipid peroxidation (Halliwell,
1997). From this point of view, antioxidants that can prevent
lipid peroxidation can have an important role in preventing
oxidative modiﬁcation of LDL. Human LDL contain a
variety of antioxidants capable of inhibiting peroxidation,
such as a-tocopherol, ubiquinol-10, b-carotene, lycopene
and other hydroxy-carotenoids. a-Tocopherol is the most
abundant antioxidant in LDL (Princen et al. 1992; Abbey
et al. 1993; Reaven et al. 1993; Jialal et al. 1995); however, it
has been demonstrated that other antioxidants are also able to
protect LDL from oxidation (Cominacini et al. 1991;
Esterbauer et al. 1992). On the basis of previous
epidemiological studies pointing out the direct correlation
between the Mediterranean diet and a lower incidence of
cardiovascular diseases (Hertog et al. 1993), various studies
performed in vitro and in vivo (Table 2) have shown that the
polyphenolic compounds of extra-virgin olive oil play an
important role in the prevention of atherosclerotic damage
through their inhibition of LDL oxidation (Visioli et al.
1995a; Rice-Evans et al. 1996; Cao et al. 1997; Masella et al.
Fig. 5. Lipoperoxidation. LOO
, lipoperoxyl radical.
Phenolic compounds of olive oil 105
1999). In a sample of LDL, the vitamin E oxidation induced
was prevented by the addition of hydroxytyrosol
or the secondary compounds of oleuropein; this effect was
linearly correlated with the hydroxytyrosol concentration. In
LDL, the addition of polyphenolic compounds caused
signiﬁcant reduction in lipid peroxide formation. In LDL not
treated with polyphenolic compounds, these lipid peroxides
are formed at the same time as the reduction of vitamin
E levels. This vitamin E depletion by LDL occurs before
massive lipid peroxidation. Phenolic compounds thus delay
the beginning of the oxidative process, preserving the
endogenous antioxidant pool (Visioli et al. 1995a, 2000a).
The antioxidant effect of the various polyphenolic
compounds of olive oil has recently been compared.
The results show that protocatechuic and 3,4-dihydroxy-
phenylethanol-elenolic acids have an antioxidant activity
comparable with that of caffeic acid, oleuropein and 3,4-
dihydroxyphenyl-ethanol in hydroxytyrosol (Masella et al.
1999). Some studies of the antioxidant effect of polyphenolic
compounds on plasma LDL have been performed, in an
attempt to simulate as well as possible the situation in vivo.
Plasma was incubated with various olive oil phenols; LDL
was subsequently isolated and subjected to the action of free
radicals, in order to test the relative resistance to oxidation.
The results indicate that hydroxytyrosol and oleuropein are
more effective than monohydroxyphenols (tyrosol and
ligstroside aglycone), conﬁrming previous results (Rice-
Evans et al. 1996; Cao et al. 1997). However, the
concentration of antioxidants added to whole plasma to
inhibit LDL oxidation was substantially higher than in
previous studies, where the antioxidants were directly added
to isolated LDL (Leenen et al. 2002). These data conﬁrm
other studies performed in vivo on animals fed with phenol-
rich olive oils; in these animals, the lipoproteins were much
more resistant to oxidation than in other control animals fed
with equal amounts of oleic acid (Scaccini et al. 1992), care
being taken to maintain constant levels of vitamin E
(Wiseman et al. 1996).
Another important risk factor for the onset of
atherosclerosis is a high blood concentration of cholesterol.
The regulation of plasma cholesterol is related to the activity
of 3-hydroxy3-methylglutaryl (HMG)-CoA reductase, the
ﬁrst enzyme involved in the synthesis of cholesterol. The use
of substances inhibiting HMG-CoA reductase (statins) is
very effective in blood cholesterol reduction. Some studies
have focused attention on the effect of the polyphenolic
compounds contained in virgin olive oil on cholesterol
metabolism, and recently it has been demonstrated that the
activity of HMG-CoA reductase (Table 2) is signiﬁcantly
diminished in the liver microsomes of rats fed with the
polyphenolic compounds. The inhibition of HMG-CoA
reductase by polyphenolic compounds may thus represent a
beneﬁcial effect through olive oil ingestion and play an
important role in the prevention of cardiovascular diseases.
However, further studies are necessary in order to test the
concentration of polyphenolic compounds capable of
eliciting a therapeutic response (Benkhalti et al. 2002).
Polyphenolic compounds in the prevention of cancer
Many vegetable foods contain substances possessing
anticancer properties (Huang et al. 1994; Johnson et al.
1994; Pezzuto, 1997), most of them active as antioxidants
(Aruoma, 1994). Since ROS have been implicated in the
genesis of tumours, the study of the antitumoral activity of
olive oil phenolic compounds is very interesting.
) are highly reactive compounds
capable of inducing peroxidation in lipids, oxidising
methionine and damaging the DNA by deamination and
nitration (Yermilov et al. 1995). Peroxynitrites are formed
by reaction between NO and O
(superoxide radical). The
deamination of guanine and adenine causes breaks in the
DNA chain, with consequent mutations (de Rojas-Walker
et al. 1995); DNA oxidation is also potentially mutagenic
(Newcomb & Loeb, 1998). In vitro, the presence of
hydroxytyrosol reduces the biochemical effects of peroxy-
nitrites, such as the deamination of adenine and guanine in
some cell lines (Deiana et al. 1999).
The antioxidant activity of virgin olive oil extracts, shown
in vitro by their ability to inhibit the effect of oxygen
radicals on salicylic acid, is apparent at concentrations much
lower than those of the single antioxidant compounds tested
individually; this is probably due to the presence of other
polyphenolic compounds, some of which are still unknown
(Owen et al. 2000a). In addition to this action, extracts of
Table 2. Biological properties of olive oil phenolics
Polyphenolic compound Mechanism of action Salutary effect on human health
Oleuropein, hydroxytyrosol, caffeic acid,
protocatechuic acid and 3,4-dihydroxy-
Inhibition of LDL oxidation, both in vitro and in vivo;
inhibition of HMG-CoA reductase; inhibition of
and consequently platelet
Prevention of cardiovascular diseases
Secoiridoids (hydroxytyrosol and tyrosol)
Inhibitory action on activity of xanthine oxidase and
reduction of superoxide formation; lignans act as
anti-oestrogens and increase sex hormone-
Prevention of tumoral diseases
Hydroxytyrosol and other polyphenolics Inhibitory action on cyclo-oxygenase and
lipo-oxygenase; reduce pro-inﬂammatory
molecule formation such as thromboxane B
and leucotriene B
Oleuropein; verbascoside (hydroxytyrosol
Inhibition of viral and bacterial growth and activity Antimicrobial and antiviral activity
E. Tripoli et al.106
virgin olive oil show an inhibitory action on the activity of
xanthine oxidase (Table 2), with a consequent reduction in
superoxide formation. This action cannot be demonstrated
for simple polyphenolic compounds (tyrosol and hydro-
xytyrosol) but it is due to secoiridoids and lignans (Owen
et al. 2000a). An adequate intake of olive oil therefore has a
double action: it gives protection from the effects of oxygen
radicals and reduces the activity of xanthine oxidase, an
enzyme potentially involved in carcinogenesis (Tanaka et al.
Among the substances possessing anticancer activity, the
lignans are of special interest. It has been demonstrated that
they inhibit the development of various kind of tumours:
cutaneous, mammary, colonic, and pulmonary (Hirano et al.
1990; Kardono et al. 1990). In animals, the administration
of ﬂax seeds (a notable source of lignans) prevents the onset
of mammary carcinoma (Serraino & Thompson, 1991,
1992; Thompson et al. 1996). The antitumoral effect of the
lignans is based both on their antioxidant activity (Prasad,
1997; Owen et al. 2000b) and on their antiviral activity
(Schroder et al. 1990). Also, the structural similarity to
oestradiol and the synthetic anti-oestrogen tamoxifen
suggests that the lignans can act, in part, as anti-oestrogens
(Table 2). This is because they are able to inhibit
the synthesis of oestradiol in the placenta (Adlercreutz
et al. 1993) and adipose tissue (Wang et al. 1994), as well as
the proliferation of breast cancer cells induced by
oestrogens (Mousavi & Adlercreutz, 1992), and to increase
sex hormone-binding globulin (a plasma protein carrier of
sexual steroids) levels, with a consequent reduction in the
biologically active levels of free oestrogens (Adlercreutz
et al. 1992).
Some of these effects are particularly important in the
pathogenesis of mammary carcinoma in obese women. In
obesity, the plasma levels of sex-hormone-binding globulin
are reduced, with consequent higher plasma levels of free
oestrogens. The mammary cells, which are typically
hormone-sensitive, are constantly exposed to the action of
high amounts of oestrogens (Schapira et al. 1991; Colditz,
1993; Maggino et al. 1993; Kissebah & Krakower, 1994;
Hankinson et al. 1995). Also, inhibition by lignans of
oestrogen synthesis in adipose tissue is fundamental in the
prevention of breast cancer in obese woman, since adipose
tissue is not only an energy-store tissue but also carries out
an important endocrine function. It picks up and metabolises
steroid hormones, converting androstenedione into oestrone
(E1) and testosterone into 17-b-oestradiol (E2) (De Pergola
et al. 1996).
The anticancer effect of the lignans is therefore probably
due to their action on the metabolism of oestrogens.
Phenolic components as compounds with
Lipid radicals are also produced during reactions involved in
the metabolism of arachidonic acid, during the synthesis of
the eicosanoids by the action of the lipo-oxygenase and
cyclo-oxygenase (Table 2). During these reactions, the
radicals that are generated are partially inactivated by
glutathione peroxidase (Eling et al. 1986; Mirochnitchenko
et al. 2000). Some studies hypothesise an inhibitory activity
on cyclo-oxygenase (Petroni et al. 1995; de La Puerta et al.
2000) and lipo-oxygenase by olive oil phenolic compounds
(Kohyama et al. 1997; De La Puerta et al. 1999; Martinez-
Dominguez et al. 2001). Considering the functions of the
prostaglandins and leucotrienes, the results of these studies
have important implications for the genesis of the
inﬂammatory response and for atherosclerosis. In one of
these studies, the effects of hydroxytyrosol and of the
polyphenols extracted from waste waters were examined in
vitro in parameters of platelet activity. It was found that the
hydroxytyrosol and polyphenols extracted from waste
waters inhibited in vitro platelet aggregation induced by
collagen and thromboxane B
production. The effectiveness
of hydroxytyrosol in inhibition of the aggregation induced
by collagen is similar to that of aspirin, a drug that is well
known for its powerful activity in platelet anti-aggregation
and cyclo-oxygenase inhibition (Petroni et al. 1995).
Polyphenols as compounds with antimicrobial activity
The bacteriostatic and bactericidal activities (Table 2) of
oleuropein and the hydrolysis products, hydroxytyrosol and
tyrosol, have been studied in vitro in comparison with many
pathogenic micro-organisms: bacteria, fungi, viruses and
protozoa (Hirschman, 1972; Federici & Bongi, 1983;
Bisignano et al. 1999). Oleuropein and the hydrolysis
products are able to inhibit the development and production
of enterotoxin B by Staphylococcus aureus, the develop-
ment of Salmonella enteritidis and the germination and
consequent development of spores of Bacillus cereus
(Walter et al. 1973; Tassou et al. 1991; Tranter et al.
1993; Tassou & Nychas, 1994, 1995). Oleuropein and other
phenolic compounds ( p-hydroxybenzoic, vanillic and
p-coumaric acids) completely inhibit the development of
Klebsiella pneumoniae,Escherichia coli and B. cereus
(Aziz et al. 1998). Verbascoside shows antibacterial activity
against Staphylococcus aureus,E. coli and other bacteria of
clinical interest; it also shows antiviral activity against the
syncytial virus, which affects the human respiratory system
(Calis et al. 1988; Pardo et al. 1993; Chen et al. 1998;
Kernan et al. 1998). Hydroxytyrosol is highly toxic to
Pseudomonas syringae pv savastanoi and Corynebacterium
michiganense, which are both phytopathogenic, and tyrosol
may act as a mycotoxin (Venkatasubbaiah & Chilton, 1990;
Capasso et al. 1995). Both phenols therefore protect the
drupe from attack by pathogenic agents.
It is not clear why the polyphenolic compounds of olive
oil have such a wide antimicrobial activity. They may cause
surface activity that damages the membranes of bacterial
cells (Juven et al. 1972). However, oleuropein, in spite of
tyrosol, is ineffective against some bacterial chains
(Moraxella catarrhalis and Haemophilus inﬂuenzae): in
fact the presence in its chemical structure of the glycosidic
group is responsible for the steric hindrance, which blocks
the passage through the cell membrane. This simply does
not make sense. Whatever the case, the antibacterial activity
of olive oil’s phenolic compounds is due to the presence of
the ortho-diphenolic system (catechol) (Bisignano et al.
These data indicate that the active compounds of olive oil,
in addition to their use as food additives, could also be used
Phenolic compounds of olive oil 107
as a potential antimicrobial agent in the treatment of some
infections. Oleuropein can also interfere with the synthesis
of amino acids necessary for viral activity, and in this way it
prevents the diffusion, development and attack on the cell
membrane, it inhibits reproduction and, in the case of
retroviruses, it inhibits the production of reverse transcrip-
tase and protease. Finally, oleuropein stimulates phagocy-
tosis as a response of the immune system against pathogenic
micro-organisms (Hirschman, 1972).
While a bactericidal effect has been observed on a wide
range of bacteria, no effect has been observed on yeasts
(Beuchat & Golden, 1989). However, oleuropein has some
inﬂuence, though only slight, on the delay of the
development and sporulation of Aspergillus parasiticus;
also, the production of aﬂatoxin is notably reduced
(Gourama & Bullerman, 1987).
The positive correlation between the Mediterranean diet and
the low incidence of cardiovascular diseases and certain
kinds of cancer (breast, prostate, intestine and skin cancer)
leads us to conclude that a diet rich in grain, legumes, fresh
fruit, vegetables, wine in moderate amounts and olive oil has
beneﬁcial effects on human health.
On the one hand, these effects are due to the high
MUFA:saturated fatty acid ratio; on the other hand, some
components of the Mediterranean diet, such as ﬁbre,
vitamins, ﬂavonoids and polyphenolic compounds, play an
important role in the prevention of these diseases (Visioli,
The normal consumption of extra-virgin olive oil, which
is rich in polyphenolic compounds, antioxidant substances
that combat the free radicals, could contribute, in
appropriate amounts (three to ﬁve spoonfuls per d, in a
balanced diet), together with other biologically active
compounds, to reduce the risk of development of these
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industries in natural antioxidants is constantly growing; the
waste waters produced by the processing of olive oil could
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