A significant body of scientific evidence now indicates that extracts from the olive tree, including the leaves, have in their health-promoting repertoire the potential to resist or overcome attack by an impressively wide range of infectious organisms as well as to generally boost the immune system. This article reviews the available scientific and clinical evidence.

Fever-lowering properties
     Interest in the potential benefits of extracts from the olive tree has stemmed from two main historical sources of
independent origins. The first of these, in the mid-19th century, involved reports of fever-lowering properties, including the ability of olive leaf extracts to prevent or cure the symptoms of malaria. In 1854, Hanbury published an article in the Pharmaceutical Journal of Provincial Transactions relating that a “decoction of the leaves” of the olive tree had been found to be extremely effective in reducing fevers due to a severe, and otherwise often fatal, disease that had swept the island of Mytelene in 1843¹. The olive leaf extract was reported subsequently to be more effective in its fever-lowering properties than quinine.
     Hanbury recalled that similar observations had been made in France and Spain many years previously (between 1811 and 1828). It appears that, in the early 19th century, Spanish physicians sometimes prescribed olive leaves as a “febrifuge”, and consequently, during the Spanish war of 1808—1813, the French Officiers de Sante often used them to treat cases of “intermittent fever”². Hanbury went on the describe how Pallas, following observations of clinical benefits^3,4 made an analysis of the leaves and young bark of the olive tree and found them to contain, among other compounds, a bitter crystallisable substance which he designated as “Vauqueline”³. Pallas ascribed most of the “febrifuge” properties of the olive tree to Vauqueline.

Antimicrobial properties – manufacturing problems
     The second historical source indicating that components of the olive tree had biologically important properties came from the European olive fermentation industry. Up until the 1970s, the industry had suffered problems in the fermentation of olives, a process involving lactic acid pickling, because of strong resistance of the fresh fruits to the action of lactic acid bacteria.^5,6,7,8
     In 1960, Panizzi et al⁹ had isolated a bitter glucoside, oleuropein, from olive leaves, with the empirical formula C₂₅H₃₂O₁₃. The substance, later determined to be a phenolic compound belonging to the iridoid group,¹⁰ was also present in the olive itself. Oleuropein, as with Pallas’ “Vauqueline”, was considered to be the source of the olive tree’s powerful disease-resistant properties. It was subsequently found that removal of oleuropein from olives enabled fermentation to take place successfully.^11,6
     The olive oil manufacturing industry had also long been well aware of the rich antibacterial properties of the olive tree. The manufacturing process involves milling of olive paste and continuous washing with water, known as malaxation. The waste waters from this process were generally discarded; however, it was found that if the waters found their way into the soil, they displaced beneficial bacterial flora and adversely affected the natural biodegradation process.^12,13,14,15

The chemical components
     Over a period of more than 30 years since Panizzi et al’s⁹ isolation of oleuropein, extracts from various parts of the olive tree have been extensively investigated. Oleuropein appears to be present throughout the olive tree, including leaves, buds, fruit, wood, bark and roots.^16,3,17,18 Olive leaves contain around 60—90 mg per gram (dry weight) oleuropein,¹⁹ plus
significant levels of a glucosidic ester of elenolic acid and hydroxytyrosol (3,4-dihydrophenylethanol). However, it turns out that oleuropein and the products of its hydrolysis, oleuropein aglycone, elenolic acid, beta-3,4-dihydroxyphenyethyl alcohol and methyl-o-methyl elenolate,20 are the major molecules of interest biologically.

Antibacterial actions – in vitro studies
     A variety of antibacterial actions of oleuropein and its associated compounds have been demonstrated in vitro. Fleming et al⁸ isolated six major phenolic compounds from green olives; one particular compound, possibly a hydrolysis product of oleuropein, was much more inhibitory than oleuropein itself to the lactic acid bacterium Leuconostoc mesenteroides FBB 42. Later on, the oleuropein aglycone and elenolic acid were found to strongly inhibit the growth of three further species of lactic acid bacteria – Lactobacillus plantarum, Pediococcus cerevisiae, and Lactobacillus brevis.²⁰ Since the aglycone is composed of elenolic acid bound to b-3,4-dihydroxyphenylethyl alcohol, and the latter compound was not inhibitory, the investigators concluded that elenolic acid was the inhibitory part of the aglycone molecule. Oleuropein itself was not inhibitory to these bacteria, but did inhibit three species of non-lactic acid bacteria – Staphylococcus aureus, Bacillis subtilis and Pseudomonas solanecearum. In addition, an acid hydrolysate of an extract of oleuropein (containing hydrolysis products of oleuropein not specifically identified) inhibited the growth of a further eight species of bacteria.
     Some more recent in vitro studies have shown that oleuropein and/or its hydrolysis products also inhibit the germination and sporulation of Bacillus megaterium¹⁵  and inhibit outgrowth of germinating spores of Bacillus cereus T.²¹

Antiviral actions
     In addition to its antibacterial actions, elenolic acid has been shown to be a potent inhibitor of a wide spectrum of viruses. In search of new antiviral compounds, Renis²² tested the effects of the calcium salt of elenolic acid (which had proved to be the most active olive-derived compound against bacteria) on a range of viruses in vitro, and found that calcium elenolate destroyed all the viruses it was tested against. These included herpes, vaccinia, pseudorabies, influenza A (PR8), Newcastle disease, parainfluenza 3, Coxsackie A21, encephalomyocarditis, polio 1, 2 and 3, vesicular stomatitis, Sindbis and reovirus 3 (Deering) viruses. Calcium elenolate also inhibits the RNA-dependent DNA polymerase I enzymes (reverse transcriptases) of murine leukaemia viruses (MuLV(M) and Rauscher),²³ and the DNA polymerase II and III enzymes of Eschericha coli²⁴ in vitro. In addition to its in vitro effects, Soret²⁵ showed that calcium elenolate effectively reduced viral titres in vivo when given before and/or after inoculation of hamsters with myxovirus parainfluenza type 3 (HA-1 virus, strain C-243). Treatment with calcium elenolate, but not placebo, prevented spread of viral infection to the lungs.

Cardiovascular effects in animals
     Not only are Olea europea-derived compounds active against infectious organisms; they also appear to have some interesting effects on the cardiovascular system that are unrelated to their antioxidant properties (see later), including blood-pressure- lowering and anti-arrhythmic actions, and effects on coronary blood flow in certain situations.
     In anaesthetised cats, 20—40 mg/kg oleuropein caused a clear-cut, dose-dependent drop in blood pressure lasting more than 1 hour.²⁶ In dogs with experimentally induced hypertension, 10—30 mg/kg oleuropein caused a sharp, long-lasting drop in both systolic and diastolic blood pressure in three out of four animals, and a lesser, shorter-lived decrease in blood pressure in the fourth dog. The same investigators found that oleuropein caused an increase in blood flow through the coronary vessels of isolated rabbit heart preparations, but no change in coronary flow in anaesthetised cats at doses of 10—30 mg/kg. However, in a model of experimentally disturbed coronary circulation, oleuropein (30 mg/kg intravenously) largely abolished the characteristic ECG (electrocardiogram) changes caused by Pituitrin (which diminishes coronary blood flow) in conscious rabbits, when given 1 minute after the Pituitrin injection. Lastly, Petkov and Manolov²⁶ found that oleuropein eliminated cardiac arrhythmia in dogs with induced hypertension for 1.5—2 hours, normalised cardiac rhythm in rabbits with barium chloride-induced arrhythmia for about 1 hour, and prevented or reduced the duration of disturbed cardiac rhythm in rats with calcium chloride-induced arrhythmia. The pharmacological mechanisms underlying any of these effects on the heart and vasculature are unknown.

Antioxidant effects – in vitro studies
     Oxidation of low density lipoproteins (LDL) contributes to the development of atherosclerosis,^27,28 the process underlying peripheral vascular disease, coronary heart disease, stroke and multi-infarct dementia. Dietary composition significantly affects plasma LDL-cholesterol levels and the incidence of coronary heart disease.²⁹ Notably, the traditional Mediterranean diet, rich in fresh fruits and vegetables, legumes, grains and vegetable (mainly olive) oil, is associated with a lower incidence of coronary heart disease. Consumption of olive oil and dressed olives (both rich in oleuropein) has also been reported to lower the incidence of cardiovascular disease.^30,31 This dietary effect was initially thought to be due to the intake of a relatively low level of saturated fat and higher levels of monounsaturated and polyunsaturated fatty acids.^32–36 However, it now appears that natural antioxidants present in the diet may also play a part in the prevention atherosclerosis.^37–39
     Phenolic compounds derived from the leaves, fruits and oil of the olive tree (Olea europaea L) have long been known to have anti-oxidative properties.^40–44 More recently, Le Tutour and Guedon¹⁹ demonstrated that oleuropein, hydroxytyrosol, and in particular, extracts of Olea europaea leaf (containing 19% oleuropein, 1.8% flavonoid glycosides, and 3,4-dihydroxy- phenethyl esters) were more potent antioxidants than vitamin E or another established antioxidant, BHT, in a model chemical system (inhibition of oxidation of methyl linoleate in heptanol or propanol-water, initiated by 2,2’-azo-bis-isobutyronitrile (AIBN)). Another recent in vitro study³² showed that oleuropein (at a concentration of 10–5 M) significantly inhibited copper sulphate-induced oxidation of low density lipoprotein (LDL) extracted from normal human plasma.

Safety studies in animals
     Several studies in animals have provided information about the in vivo safety and toxicity of compounds present in extracts from Olea europaea. Elliott et al⁴⁵ determined the LD50 (the dose that is lethal for at least 50% of a designated population of laboratory animals) for calcium elenolate to be 120 mg/kg in mice when given intraperitoneally, and 160 mg/kg in rats via the intraperitoneal route and 1,700 mg/kg via the oral route. Petkov and Manolov²⁶ gave single intraperitoneal doses of oleuropein to mice ranging from 100 to 1000 mg/kg (in solutions of 1, 5 and 10%), but observed no toxic effects and no deaths during the 7-days post-treatment period, and so were unable to determine oleuropein’s LD50 in this study.
     In repeated-dose (“subacute”) studies, Elliott et al⁴⁵ found calcium elenolate to be well tolerated in rats given daily oral doses of 0, 30, 100 or 300 mg/kg for 1 month. The only drug-related change observed was a yellowing of the nonglandular fore-stomach in 40% of the rats receiving the highest dose (300 mg/kg). In 7-month-old beagle dogs given daily oral doses of 0, 3, 10 or 30 mg/kg calcium elenolate for 1 month, all but the highest dose were well tolerated – three out of the four dogs receiving 30 mg/kg showed a mild gastric irritation with sporadic vomiting. Tissue analysis revealed a few small gastric erosions in these animals.
     In their investigations of the cardiovascular effects of oleuropein in animals, described earlier (see Cardiovascular effects, above), Petkov and Manolov²⁶ observed that 3—50 mg/kg oleuropein given intraperitoneally caused a slight stimulation of the respiratory rate in anaesthetised cats. Also, in doses of 10—30 mg/kg, it caused a brief depressed state with decreased motor activity in two out of four conscious dogs with induced hypertension, and was badly tolerated in a third dog, causing excitation, scratching, and vigorous jolting movements, red, watery eyes, and hyperaemic (warm, reddened) abdominal skin.
     Lastly, Ruiz-Gutierrez et al,⁴⁶ investigating the effects of oleuropein on lipids and fatty acids in heart tissue, did not report any adverse behavioural or other effects (for example, on food consumption, body weight, heart weight or heart total lipid content) in rats given intraperitoneal injections of 25 or 50 mg/kg daily for 3 weeks. Oleuropein did significantly reduce the linoleic acid content and the ratio of unsaturated to saturated fatty acids in heart polar lipids, depleted heart levels of vitamin E, and itself became incorporated in heart tissue, but the significance of these findings is unclear. However, heart tissue that had been pre-treated with oleuropein in vitro was not susceptible to peroxidation.

Olive leaf extract – a new formulation
     The weight of evidence from the in vitro and in vivo studies strongly favours beneficial effects of olive tree extracts in the fight against infectious diseases as well as cardiovascular disease, and, on the whole, calcium elenolate and oleuropein at therapeutic doses appear to be safe and well tolerated in animals. Why, then, has no drug company snapped up this promising avenue of research to capitalise on the likely benefits in humans? In fact, a US drug company, The Upjohn Co of Kalamazoo, Michigan, was responsible for much of the work on the antiviral properties of calcium elenolate in the 1960s and 1970s. However, they came across a problem that reduced to insignificant the practical usefulness of the compound in humans. Calcium elenolate has a strong affinity for plasma proteins, and when administered to humans, the drug quickly bound to these molecules, effectively taking it out of action within minutes. The researchers at Upjohn Co were unable to overcome this problem, and so, in the mid-1970s, abandoned the development of calcium elenolate as an antiviral agent.
     Independent researchers, however, continued to investigate the potential of olive leaf extracts and finally made a breakthrough in 1994. By making certain structural changes to the active molecule (now a closely-guarded and patented secret process), they found they could significantly reduce if not eliminate the binding of calcium elenolate to serum protein. The result was Eden Extract™, a pure olive leaf extract obtained by a hydro-ethanolic process, manufactured by East Park Research, Inc., of Henderson, Nevada, USA, who also owns the patent to the product.

Clinical evidence of efficacy
     From the above review, the preclinical evidence for the anti-infective and cardiovascular effects of olive tree extracts is fairly extensive and convincing. By contrast, however, the clinical evidence is relatively scarce. This is not to say that what clinical evidence there is is not compelling. But, because development of the olive leaf extract as a possible pharmaceutical product was abandoned in the 1970s, and has continued via private research as a food supplement, extensive clinical studies have not been carried out. As a food supplement, the manufacturer cannot make any claims about the effects of the product (but relies on independent publicity gained through consumers’ and health practitioners’ use of the product), but conversely is not required to conduct lengthy and costly clinical trials to prove its efficacy in any medical condition. The product may be sold legally for human dietary consumption based on its natural origins, conventional extraction process, proven safety in animals at the recommended doses for humans, and its documented historical safe use in humans in Europe for more than a century.

Clinical studies
     A limited number of open (uncontrolled) clinical studies have been or are being conducted with Eden Extract™ or an earlier version of the product, Viliv, although results from these studies have not yet been published by the respective investigators. In 1993, a preliminary study was carried out by investigators at the NFN Company, Los Angeles, California, USA.⁴⁷ Six subjects with herpes simplex II (and possibly I) infection, previously diagnosed by a physician, were treated with 2—4 oz of Viliv (a wine-based tincture containing concentrated olive leaf extract) orally every 6 hours for 6 weeks. Three subjects reported complete remission of lesions and associated pain/discomfort after 36—48 hours, and a fourth reported relief of pain after a further 48 hours. The other two subjects reported relief of pain/discomfort over the course of the study. There was a trend towards reduced blood levels of antibodies after 2—3 weeks of treatment, but the number of samples was too few to give a definitive conclusion.
     A clinical study involving the use of Eden Extract™ is reported to be underway at The “R” Clinic, Budapest, Hungary,⁴⁸ which employs innovative medical alternatives to help provide improved healthcare for Hungarian citizens. The medical director, Dr. Robert Lyons, along with 40 physicians from the US, has already treated 500 patients with Eden Extract™. Patients initially took two capsules (each containing 500 mg of concentrated olive leaf extract) three times daily, in accordance with the manufacturer’s recommendations, and the dose was reduced to one capsule four times daily if their disease symptoms improved.
     According to US medical journalist Morton Walker,⁴⁸ who has corresponded with Dr. Lyons in regard to this study, 157 out of 164 patients with respiratory diseases or lung conditions (tonsillitis, pharyngitis, tracheitis, pneumonia, bronchitis) recovered fully and six improved (one patient was unaccounted for in the article); 60 out of 67 patients with dental problems (pulpitis, leukoplakia, stomatitis) fully recovered, five improved and two remained unchanged; 150 out of 209 patients with viral or bacterial skin infections fully recovered and 59 improved; all 17 patients with gastric ulcer and Helicobacter pylori infection improved, though none recovered fully; and 40 out of 43 patients with impaired immunity showed improved immune status (details of how this was assessed were not given) while three remained unchanged. It is unclear how long patients were continued on treatment, but some appear to have responded within a matter of a few days or weeks.
     A further clinical study, investigating the efficacy of olive leaf extract in the treatment of malaria, is reported to be underway in Taiwan under the direction of Dr. Bernard Friedlander, a chiropractor from San Mateo, California, USA.⁴⁹ Results from this study, however, are not yet available.

Clinical anecdotes and individual cases
     Other than from the above-mentioned clinical studies, indications of clinical efficacy of Eden Extract™ come from consumers’ letters sent directly to the manufacturer (East Park Research, Inc., Henderson, Nevada, USA) or indirectly via health practitioners (including physicians, chiropractors and nutritionists); and case reports or clinical anecdotes provided by a number of US health practitioners who have prescribed Eden Extract™ to their patients and observed beneficial effects.
     General practitioner Dr. James Privitera, M.D., of Covina, California, appears to have had the most extensive clinical experience with use of the olive leaf extract, which has been available in the US since 1995. He has reportedly observed the following benefits: relief of arthritic inflammations; reduction of insulin dosages in diabetics; elimination of the symptoms of chronic fatigue syndrome; increased energy/stamina; improved blood flow in cardiovascular disorders; lessening of haemorrhoid pain; attenuation of toothaches; elimination of fungal infections such as onychomycosis and tinea pedis; prevention or cure of numerous viral infections; relief of many of the symptoms of Candida albicans and other yeast infections; and elimination of a variety of parasites including protozoa and helminth worms.⁴⁸
     Other case reports or anecdotes mention the following benefits with Eden Extract™: probable prevention and successful treatment of herpes genitalis (herpes simplex II);^48,49 improved symptoms of rheumatoid arthritis, prostate cancer and some other cancers, and skin conditions; improvement in chronic fatigue syndrome; improvement of sore throats, coughs, colds, and chronic sinusitis;⁴⁹ improvement of tinea (pityriasis) versicolor, psoriasis, persistent respiratory infection, and chronic scalp infection;⁵⁰ relief from the pain of shingles (herpes zoster infection); elimination of the “yeast syndrome”/ Candida albicans infection; and restoration of immune function in a severely immune-depressed patient with multiple long-term allergies and opportunistic infections.⁴⁸

Side-effects in humans
     The only side-effect that appears to have been reported with clinical use is a so-called “die-off” effect, which has been likened to the Herxheimer reaction sometimes encountered during the treatment of yeast infections.^51,52 This reaction is believed to occur when a large quantity of infectious organisms in the body are killed off in a relatively short period of time. Large amounts of toxic substances are released into the body tissues and blood stream from the dying organisms together with cellular debris, and the person’s immune system rapidly reacts to these substances to remove them from the body as quickly as possible. As a result, the person may temporarily experience a number of allergic- or flu-like symptoms such as headache, fever, fatigue, muscle/joint aches, and diarrhoea.⁵³ The symptoms of this “die-off”, or detoxification, reaction last for between 4 and 7 days. Some patients may experience only a mild headache, and many experience no such effects at all. The effects of the “die-off” reaction are not thought to be harmful, but the manufacturer advises that if symptoms do occur, the patient should temporarily stop taking the capsules or cut back on the daily amount he/she is taking, so that the body has a chance to eliminate the toxic waste products accumulating in the system.

Summary and conclusions
     Extracts from the European olive tree have a long history of association with fever-lowering and antimicrobial properties, and these are now convincingly supported by laboratory studies of antibacterial and antiviral actions conducted over the last 30 years or more. The association of olive oil and other oils containing high levels of mono- and polyunsaturated fatty acids and low levels of saturated fats with a reduced risk of coronary heart disease is also well-established. Evidence from laboratory studies of further possible cardiovascular benefits, such as blood pressure-lowering, anti-arrhythmic, coronary blood flow-reducing and antioxidant actions, adds a further exciting dimension to the possible health-promoting benefits of these extracts, and deserves deeper exploration.
     Most of the laboratory evidence has involved the major phenolic compound of olive tree extracts, oleuropein, and its hydrolysis product elenolic acid, and these agents have been shown to be safe and well-tolerated by the oral, as well as intraperitoneal, route in a variety of animals at the levels present in doses of olive leaf extract recommended for human dietary supplementation. Eden Extract™ incorporates structural changes to the elenolic acid molecule that overcome the bioavailability problems in humans encountered with earlier such preparations (due to rapid binding to serum proteins). This product has been available to the US public as a food supplement since 1995 and has recently become available in the UK.
     Formal clinical studies of possible health benefits of extracts from the olive tree in humans are scarce; however, case reports and clinical anecdotes received by the manufacturer from consumers and health practitioners in the US indicate that the product may well have effective antibacterial and antiviral properties in humans, as well as hitherto unrecognised benefits to the cardiovascular and immune systems. Other health-promoting properties, such as antifungal, anti-inflammatory and anticancer actions, are also suggested by these unofficial reports. However, such reports cannot be presented as proof of clinical efficacy, since the placebo effect is likely to be a significant factor in any non-controlled study and in individual cases.
     Published findings from the clinical studies reported to be underway should provide important supporting evidence for olive leaf extract’s clinical potential. Organised, well-designed studies targeting particular human ailments would provide further convincing proof of the range and depth of health-promoting effects of this potentially far-reaching product. From its historical origins, which have been said to date back as far as biblical times and to ancient Egypt, the olive tree has come a long way in gaining recognition for its remarkable properties. It would be a great shame if such a possible source of power against human ailments remained unrecognised and untapped because of a lack of investment in clinically definitive studies in the final stages of its development.

References
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48.    Walker M. Olive leaf extract. The new oral treatment to counteract most types of pathological organisms. Explore! Volume 7, Number 4, 1996. Explore? Publications, PO Box 1508, Mt Vernon, WA 98273, USA.
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53.    Information provided by the UK distributor of Eden Extract™, Tigon Limited, Loughborough, Leicestershire.

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