Olive Tree: a Heavenly Gift from Hercules

When many people hear of the olive tree, they think of the olives and the oil used frequently in many cuisines around the world. However, the leaves are a popular remedy among many cultures, and scientific investigations have provided evidence to warrant their use. There is only one biological species, possibly more than six cultivated varieties. The truly wild olive is var. oleaster, though the leaves of all are similar. Olive leaves can be used to treat a vast array of problems including; diabetes, gastric problems, blood pressure, against bacteria and fungal growth, and for neuroprotection. Revered in the ancient times as holy, and still used in Orthodox churches today, the olive leaf has been an important cultural symbol in the past and may prove to be a useful medicine for the future.

Popular History and contemporary use
The olive tree is one of the most iconic species to the Mediterranean due to its historical, economic, cultural and ecological value. It is a symbol in sacred literature and has been cited in the Koran and Bible, as well as in ancient literature. The ancient Greeks considered it to have divine power embedded in it as it was said in Greek mythology that the tree was a gift from the goddess of wisdom Athena, to the Athenians and therefore had the emblematic presence of the goddess (Stoneman, 1991). Another myth states that it was Hercules that brought down the first olive tree from the heavens, and Homer refers to olive oil as “liquid gold” (Stoneman, 1991). There were festivals where the olive oil was used, and it was used to confer messages to the deities and ancestors and for offerings to the Gods. In the Olympic Games the ultimate prize was the wreath of an olive branch, and olive oil as a prize (Gaff, 2004).

In ancient times the wood was also used for architecture and for tools, and was of great economical importance as well. When the Spartans invaded Athens the first thing they did was burn down the olive groves of Athens to weaken their opponents (Tames and Tames, 2009). In fact the olive tree was so important to the Athenians that when their crop failed they voted to honor a foreign merchant who sold the 56,000 litres of oil (Kaniewski et al., 2012). The olive branch is also symbol of peace and can be found on the flag of Cyprus.

Traditional Medicinal Uses
Decoctions of dried leaves and fruit are traditionally used orally to treat diarrhea, respiratory and urinary tract infections, stomach and intestinal diseases, and as mouth cleanser. In Greece hot water extract of olive leaves is taken orally to treat high blood pressure. It is also used to improve blood circulation through the heart (Plants of the World Online, 2018). Furthermore folk knowledge says it helps to facilitate the passage of gall stones, to lower blood sugar levels as well as for cystitis. According to herbalists its action is as follows: hypoglycaemic, hypotensive, diuretic, antispasmodic (mild), astringent diuretic, febrifuge, vulnerary, vasodilator, and cholagogue (Bartram, 1998).

Biological characteristics and identification

There are estimated to be around 800 million olive trees in the world. The Mediterranean olive tree is an evergreen which is very tolerant to drought. Shrubby forms are common however the tree can reach up to 15 metres in height, and it is slow to mature but can live for hundreds of years (Kaniewski et al., 2012). In may the tree blossoms small white flowers, and also the tree bears olives. The fruit has a hard endocarp (olive stone) and is surrounded by a fleshy mesocarp that is edible. The flowers are borne in axillary clusters and have a four-lobed calyx as well as a four-lobed corolla. On the top surface of the leaf the colour is a dark green, and on the bottom surface the leaf is whitish grey. Leaves are narrowly elliptic to linear-oblanceolate. Currently there are 6 known species of the Olea europaea tree (Plants of the World Online, 2018/ bartrams)

Phytochemistry
Olive leaves are a cheap and rich source of bioactive constituents which is partially why they are so commonly used as popular medicine. Some of the main active constituents include the biophenol oleuropein and also hydroxytyrosol, oleine and oleasterol. The triterpenoids oleanolic acid 67%, uvaol 12% and erythrodial 6% are also important components of the olive leaf (Besnard et al., 2013). Olive leaves also contain the flavonoids apigenin and luteolin. Furthermore tocopherols, waxes, triglycerIdes, hydrocarbons and esters are present in the leaves which contribute to this ancient trees medicinal properties (18). Factors that affect the chemical make up of a plant include the climate, soil and water conditions, herbivory, processing, plant age and geographical region of the tree.

Pharmacology/ Pre-Clinical Data

Diabetes
Olive leaves have traditionally been used to combat diabetes, and many studies have been conducted to test this. It has been hypothesised that antioxidants could be used to reduce blood glucose levels, and so an experiment using oleuropein was conducted on diabetic rabits. Treatment lasted for up to 16 weeks and doses were at 20mg/Kg body weight. It was observed that blood glucose levels were restored to near to the normal control rabbits providing evidence of oleuropeins ability to lower blood sugar and antioxidant effect. (Al-Azzawie and Alhamdani, 2006).

Other studies have supported this notion, as oleuropein and hydroxytyrosol were administered to rats that suffered depletion in antoxidant enzymatic activities, and had increased lipid peroxidation, as well as high cholestorol. Results showed that serum glucose and cholestorol levels were restored to a healthy level and enzymatic activities were restored. Again these activities were thought to be due to tackling oxidative stress as this has been highly associated with pathological complications of diabetes (Jemai, El Feki and Sayadi, 2009)

The antidiabetic activity in olive leaves is thought to be partially due to secoiridoid glycosides. However there is no evidence that antioxidants are relevant for diabetes. They have been shown in animal studies to be an agonist for TGR5 (a member of G-protein coupled receptor family), and it signifficantly lowered serum glucose levels in mice (Sato et al., 2007). It was suggested that this was also partially due to both oleanolic acid and oleuropein. In another study the mechanisms through which olive leaves antidiabetic activity works was investigated with olive leaf extract which included oleuropein. The extract caused the secretion of insulin to increase, there was antioxidant activity and necrotic and apoptotic cell death was improved (de Bock et al., 2013).

A study has shown that olive leaf extracts at doses of 100,200 and 500 mg/Kg of rat body weight is more effective than the reference antidiabetes drug Glibenclamide given at a dose of 600 μg/Kg for 14 days. What was particularly interesting about this study was that that olive leaf extracts increased the serum insulin levels in diabetic rats but not in normal rats. In addition it also lowered urea, uric acid, triglycerides, total cholestorol and creatinine levels (Eidi, Eidi and Darzi, 2009). Importantly, the dose range used in this study is very high and, consequently, they cannot be used to demostrate any specific therapeutic benefits. In addition, most of the studies conducted are nearly all short term and are conducted in animals, so results may not hold true long term in people.

Activity Against High Blood Pressure
Cardiovascular diseases are the leading cause of death worldwide, and hypertension is the cause of many heart diseases. It can lead to stroke of the arteries and peripheral arterial diseases, and chronic kidney diseases if not treated.

Interestingly a study showed that olive leaf extract showed a decrease in blood pressure in mice when their blood pressure had been elevated by rennin and noradrenaline, however it did not lower normal blood pressure (Somova et al., 2003). There is also some limited clinical evidence for olive leaves efficacy in helping high blood pressure, described under the clinical evidence section

Cytotoxicity activities of Olea europaea
Olive leaves have been shown to have some effect with a variety of cancers in cell cultures. Erythrodial uvaol and oleanolic acid and maslinic acid are triterpenes investigated against the MCF-7 breast cancer cell line. The results showed that there was a signifficant cytotoxic effect in a dose dependent manner and they protected against oxidative DNA damage at 10 μM (Allouche et al., 2011). Biophenols oleuropein (OLP) and hydroxytyrosol (HT) were also investigated in treatment of tumor growth in breast cancer in mice. Results showed that the treatment reduced the tumor volume and weight at doses of 150 and 225 mg/kg/day (Milanizadeh et al., 2014).

Extracts from the leaves of a Lebanese olive tree were also tested on leukemia cell lines, and were shown to induce apoptosis (programmed cell death) dose-dependently (Fares et al., 2011). Similarly oleanolic acid has also been shown to exhibit inhibitory effect in liver cancer through apoptosis and cell cycle arrest (Wang et al., 2013).

Maslinic acid from olive leaves and fruit has been shown to interact with the proteins that are needed to maintain cellular structure and function in cancer cells, and has been shown to block the G1 cell cycle phase therefore having an antiproliferative effect in colon cancer (Rufino-Palomares et al., 2013). Oleuropein has also been shown to limit the growth and induce apoptosis (programmed cell death) in colon cancer cells (Cárdeno et al., 2013). Again it must be noted that these experiments do not show clinical evidence in treating cancer, and these studies show cytotoxicity (showing cell damage) not anticancer effects, so the evidence is limited.

Other Activities
Olive leaves have been shown to have antioxidant activity. The flavanoids rutin, luteolin and catechin were isolated from the leaves of the olive tree were shown to have high activity against ABTS radicals (Somova, Shode and Mipando, 2004).

Studies show olive leaf extract to be effective in treating gastric lesions in mice. The protective effect was suggested to be due to antioxidants present (12). Furthermore the aqueous extract was also shown to be effective against aspirin- induced gastric ulcers (GUAN et al., 2010)

Maslinic acid from olive leaves was investigated for neuroprotective activity (Qian et al., 2011). Neuronal survival was promoted in cultures of neurons from the cerebral cortex during glutamate toxicity and so olive leaf extract may be a lead for future neuroprotective drugs (15).

Olive leaves have been described as having “remarkable” antimicrobial effects, particularly when water is used to extract the active components. Olive leaf extract has also been shown to have antifungal and antiparasitic effects (Keskin, Ceyhan and Uğur, 2012). The leaves have been used as a popular remedy for treating numerous illnesses from fungal, bacterial and viral origin.

Clinical Evidence
A clinical trial was conducted to understand the antihypertensive effects of olive leaf extract in comparison with the commonly used pharmaceutical drug Captopril. The study investigated the three triterpenoids uvaol, oleanolic acid and ursolic acid from an olive leaf extract over 8 weeks. The results were positive as all patients showed a signifficant decrease in their systolic as well as diastolic blood pressure after treatment and in fact olive leaf showed better results than Captopril (Susalit et al., 2011).

Further studies show that there was a signifficant decrease in blood pressure of patients when compared to placebo, with no side effects and the mechanism of action is thought to be via vasodilation which is when blood vessels dilate to lower blood pressure (Scheffler et al., 2008).

There has also been a small clinical trial where 46 patients received capsules of olive leaves extract or placebos over the period of 12 weeks. Results showed that those who received the pills had a 15% improvement in insulin sensitivity when compared to placebo (de Bock et al., 2013).

Toxicology
Olive leaf in general is safe, however as it can lower blood pressure it can cause dizziness in people who already have low blood pressure. It can also irritate the stomach if the dose is too high or the tea is too strong. If this happens then add extra water to the tea. Heartburn, acid reflux and diarrhea are also risks (Bartram, 1998). Avoid olive leaf extracts if you are pregnant or breastfeeding as there have not been enough studies conducted to prove that the extract is safe under these conditions. Furthermore do not take olive leaf with medicines for blood pressure as olive leaves lower blood pressure. This is true also if you are taking medicine for diabetes. If you are taking any medicines then speak to your doctor before using olive leaves for medicine. There is risk of interaction with all herbal remedies if used with pharmaceuticals, particularly if they have similar or opposing mechanisms of action.

Final Words:
Olive leaf appears to be a useful herbal medicine, particularly for treating diabetes and high blood pressure. Some experiments have shown it to be more efficacious than some commonly used pharmaceuticals, though one must always be wary of the quality of products on sale. This is particularly true for products available on the internet which have little or no regulation, and a way to ensure good quality is to purchase products with the THR label. High quality extracts ensure that the active compounds needed for healing are present in maximal amounts, and that there are no adulterations. Alternatively one can pick olive leaves , as they are a common household plant now in Britain and make olive leaf tea. Herbal medicines work best in combination with a healthy lifestyle, good nutrition and exercise as they work as supplementary aids in holistic health. Olive leaves have been used for a variety of minor, self limiting conditions and to improve well-being but there is not enough evidence to use them for major diseases. However scientific studies show that this ancient medicine can be helpful for a modern and healthy life.

Disclaimer
In this essay we do not to advise or recommend herbs for medicinal or health use. This information is intended for educational purposes only and should not be considered as a recommendation or an endorsement of any particular medical or health treatment. The use of any such product should be based on the appropriate advice of a health care professional or based on the information available in the patient information leaflets (i.e. for THR products).

About the author
My name is Rebecca Lazarou, and I study and work within the field of natural health and wellbeing. I am an aspiring herbalist currently on the MSc. Medicinal Natural Products and Phytochemistry at UCL. I currently work as a hypnotherapist and masseuse, to support holistic health.

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