Olive leaf extract

1.1. Sources

Olive (Olea europaea of the family Oleaceae) leaves appear to have medicinal use for the treatment of diabetes,^[1] blood pressure, and artherosclerosis. ^[2] Some less frequent uses for olive leaf include use as a diuretic, hypotensive, emollient, febrifuge, tonic for urinary and bladder infections and as a treatment for headaches.^[3] Traditional usage seems to be localized to regions around the mediterranean area (Spain, Italy, France, Greece, Israel, Morrocco, Tunisia, and Turkey.^[3]) Olive leaf is also used in Africa by the Sotho, Xhosa, and Zulu tribes where it holds particularly high importance.^[4]

The main bioactive appears to be oleuropein, which is a highly pungent compound claimed to be the cause of olive oil's distinct taste.^[5] Black olives tend to have their oleuropein content decline towards maturation,^[6] with some species having no detectable oleuropein at full blackening.^[7] This is thought to be related to an increased level of esterase activity and metabolism into other compounds.^[8]

Olive leaves, derived from the plants that bear the olive fruits (and from the fruits, cooking oils), have some medicinal history mostly associated with anti-diabetic and cardioprotective activity. The extracts of olive leaves share a large degree of similarity with the phenolics in olive oil itself.

1.2. Composition

Olive leaf extracts tend to contain:

• Oleuropein, commonly seen as the main bioactive^[9][10] and in studies is commonly standardized to 18-22% of the supplement^{[9][11][2][12]} while consisting of around 60-90mg/g (6-9%) of the leaf by dry weight^[13][5]

• Tyrosol,, hydroxytyrosol and dihydroxytyrosol;^[9] olives appear to have a small salidroside component (main bioactive of Rhodiola rosea)^[5]

• Elenolic acid (sometimes at levels that are unquantifiable^[14]) and its glucoside oleoside^[10] and the latter's 11-methyl ester^[10]

• Oleacein (fully known as 2-(3,4-dihydroxyphenyl)ethyl 4-formyl-3-(2-oxoethyl)-4 E-hexenoate)^[10][3]

• Oleanolic acid,^[10] maslinic acid,^[15] and some ursolic acid (this study noting African sourced leaves only)^[3] in about a 1:1 ratio or oleanolic:ursolic sometimes called oleuafricein at 0.27% of the leaves dry weight^[3] with total oleanolic acid ranging from 0.71-2.47%;^[3] these pentacyclic terpenoids are common to anti-diabetic plants^[16]

• Ligstroside^[10]

• Excelcioside^[10]

• Lucidumoside C^[17]

• Loleuropeindiale and oleuropeindiale^[18]

• Quercetin at 0.04%^[12] and Rutin^[9]

• Kaempferol^[9] and aromadendrin (Dihydrokaempferol)^[19]

• Apigenin^[9] with its 7-O-glucoside at 0.07%^[12]

• Luteolin^[9] with its 7-O-glucoside at 0.04%^[12]

• Vanillin and vanillic acid^[20]

• Chlorophyll^[21]

• Tannin structures (0.52%^[12])

• Verbascoside^[9][22] as the most prominent hydroxycinnamic acid derivative^[5]

• Caffeic acid (0.02%^[12]) noted to be higher in green and cherry olives relative to black^[18] with hydroxycaffeic acid present^[18]

Olive leaf supplements are primarily a concentrated source of tyrosol, hydroxytyrosol, qne elenolic acid. They also consist of conjugates binding tyrosol and elenolic acid (ligstroside) and hydroxytyrosol and elenolic acid (oleuropein) with some other phenolics.

With studies on the fruits of olive finding:

• Pinoresinol^{[23][24][25][26]} (0.016-0.037% dry weight), acetylpinoresinol (0.13% or less), and hydroxypinoresinol (0.10-0.29%) (lignans)^[27] with a theorized maximum level of 670mg/kg total lignans in the oil^[27] although usually measured in the range of 100mg/kg.^[28] These are thought to underlie the differences between virgin and refined olive oil (absent in the latter)^[28]

• Oleuropein (0.005-0.012%, sometimes 0.87%, of the oil^[29] while being up to 140mg/kg of the dry weight of the fruit prior to oil pressing^[30]) with demethyloleuropein being the main phenolic in mature black olives^[7]

• Hydroxytyrosol (with extra virgin olive oil containing 14.42+/-3.01mg/kg^[31])

• Anthocyanins^[32] such as Cyanidin (3-glucoside and 3-rutinoside^[5])

• A lipase, highly active at pH 5.0^[33]

• MicroRNAs (fruit)^[34]

All structures of phenolics can be assessed via this article, the above are merely the more important structures in olive leaf supplementation.^[5]

Oil products tend to have a higher lignan content (pinoresinol) and still contain the polyphenolics including oleuropein. The amount of Oleuropein present determines the pungency of the olive product.

2.1. PPARs

Oleuropein has been shown to be an in vitro inhibitor of PPARγ in adipocytes, as while it suppressed proliferation of adipocytes in the concentration range of 10-400 μM (in a manner prevented by the antioxidant enzyme catalase), it reduced lipid accumulation between 200-400 μM per se due to a 30-50% inhibitory effect of 200 μM oleurpein on basal and rosiglitazone-induced PPARγ activity.^[35] There was no influence of oleuropein on PPARα or PPARβ/δ at concentrations up to 400 μM, and 10 μM oleuropein was ineffective.^[35]

While oleuropein is technically an inhibitor of PPARγ, the concentration required for this action may be higher than can be acheived through oral ingestion of the compound.

3.1. Absorption

Hydroxytyrosol appears to be passively absorbed through the intestinal wall in a concentration and time dependent manner^[36] and is dose-dependently absorbed in humans (as detected by graded oral doses and urinary measurements).^[37] Olive oil phenolics (Tyrosols, ligstroside, and oleuropein) are absorbed to a degree of about 55-60% in general,^[38] although distictions between phenolics were not made in this study.

Oleuropein is poorly absorbed in vitro,^[39][40] although it is known to be fermented by bacteria^[41] which has been noted with intestinal bacteria.^[39] Fermentation of oleuropein in the colon results in free hydroxytyrosol and elenolic acid (and fermentation of ligstroside results in tyrosol and elenolic acid). Some metabolism into tyrosol (and homovanillyl alcohol) has been noted to occur in the small intestine.^[42] 

One human study has noted that oral oleuropein supplementation only resulted in increased urinary hydroxytyrosol.^[38]

Tyrosol and hydroxytyrosol are readily absorbed in the small intestines. The dialdehyde forms of these molecules (ligstroside and oleuropein, respectively) are poorly absorbed but can be metabolized into free tyrosol and hydroxytyrosol for absorption.

3.2. Metabolism

Tyrosol and Hydroxytyrosol are excreted in the urine mostly as glucuronide conjugates,^[37] although low levels of free tyrosols can be detected.^[43][44]

Tyrosol molecules are excreted in the urine after phase II conjugation.

4.1. Mechanisms

10mg/kg olive polyphenolics (extracted from pomace; 30% hydroxytyrosol and 20% other tyrosol molecules) injected for 10 days into mice has been noted to increase NGF and BDNF levels in the hippocampus and olfactory bulb while suppressing levels of these growth factors in the frontal cortex and striatum (latter NGF only).^[45] The receptors for these growth factors (TrkA and TrkB) showed the same trends but only the increase seen in the hippocampus and olfactory bulb reached statistical significance.^[45] These modulatory effects have been noted with antioxidant effects in general^[46] (and noted with green tea catechins^[47]), and may extend to rhodiola rosea as well (due to a high tyrosol content).

There appears to be an interaction of olive leaf with brain neurotrophic factors, although the practical relevance of this is not certain.

4.2. Neuroprotection

Oleuropein appears to exert antioxidative effects in PC12 cells and reduce the rates of apoptosis induced by 6-hydroxydopamine at 20-25 µg/mL (olive leaf at 400-600 µg/mL).^[48] Similar concentrations of olive leaf extract have shown protective effects against hyperglycemia.^[49]

May be neuroprotective secondary to the antioxidative effects. Although it is more potent than other dietary supplements (due to the large antioxidant effects of hydroxytyrosol), this does not appear to occur through a unique mechanism.

There may be some crossover between olive leaf and rhodiola rosea, both of which are fairly neuroprotective via similar molecules.

4.3. Pain

A study using 300-500 mg/kg olive leaf extract in diabetic rats found that ingestion of olive leaf attenuated neuropathic pain as assessed by a tail flick test, where latency decreased to 51.38% of baseline but was preserved to 84.78% and 85.97% (300 mg/kg and 500 mg/kg; 100 mg/kg ineffective).^[49] These changes were associated with reductions in blood glucose in the two higher doses but not 100 mg/kg over 4 weeks.^[49]

May reduce diabetic neuropathy secondary to reducing blood glucose levels in diabetic rats.

5.1. Cardiac Tissue

In rats fed an obesogenic diet with olive leaf (20.7+/-0.3 mg/kg oleuropein and 4.3+/-0.1 mg/kg hydroxytyrosol) for 4 weeks, the adverse changes seen in LV diastolic stiffness and fibrosis appear to be abolished (no significant effect on echocardiography variables).^[14]

May have cardioprotective effects against cardiovascular damage. The relative potency of these effects is not known.

5.2. Blood Pressure

Oleanolic acid (triterpenoid) has shown ACE inhibitory potential in vitro while other irioid compounds in olive leaf showed no such effect in their glycoside form but inhibited the ACE enzyme as aglycones.^[10] Oleacin has demonstrated a potency of 26 μM (IC₅₀ value),^[10] but the degree of absorption of oleacin is not certain.

Some compounds present in olive leaf possess ACE inhibiting properties, which may be a mechanism of reducing blood pressure

In animal studies, rats that are prone to develop hypotension have noted reductions in blood pressure with olive leaf extracts and isolated triterpenoids from olive leaf (oleanolic acid and ursolic acid).^[3] The leaf extract has shown benefit in L-NAME (nitric oxide inhibitor) induced hypertensive rats (100 mg/kg olive leaf)^[50] and normotensive rats under anaesthesia (180 mg/kg olive leaf).^[51] Conversely, a high fat and high carbohydrate fed obsesogenic model of high blood pressure failed to show reductions in blood pressyre with 20.7+/-0.3 mg/kg oleuropein and 4.3+/-0.1 mg/kg hydroxytyrosol equivalents (3% of feed) over 4 weeks.^[14] This study that failed to find a reduction in blood pressure did find increased vascular reactivity (increased aortic responsiveness to acetylcholine and sodium nitroprusside), which was thought to be secondary to the antioxidant effects preserving the bioactivity of nitric oxide (seen with pycnogenol supplements and common to most antioxidants).^[14]

Olive leaf supplementation (51.1 mg oleuropein) for 6 weeks has failed to influence blood pressure in otherwise healthy but overweight individuals.^[9]

A preliminary study in 50 twin pairs with mildly elevated blood pressure given 1000 mg of olive leaf daily noted reductions in systolic blood presssure by 8% (500 mg daily was ineffective).^[11] Moreover, 500 mg of olive leaf extract twice daily (1000 mg total; 19.9% oleuropein) for 8 weeks in persons with stage 1 hypertension was able to reduce systolic (−11.5+/-8.5) and diastolic (−4.8+/-5.5) blood pressure to a comparable degree as the active control of 25mg Captopril (titrated up to 50 mg if needed).^[2] Some studies that compared low phenolic olive oil against high phenolic olive oil also noted slight blood pressure reductions that were associated with the consumption of olive phenolics in hypertensive persons.^[52] Similar studies in normotensive persons failed to find a hypotensive effect.^[52][53][54]

There appears to be a hypotensive (blood pressure reducing) effect that only occurs in hypertensive persons, although it is not 100% clear under what conditions it works. At least one rat study has noted that olive leaf extract failed to reduce blood pressure in diet-induced obesity, and the etiology of the human hypertension studies is not known.

5.3. Cholesterol and Triglycerides

Animal studies using rats prone to artherosclerosis and high cholesterol have noted protective effects of olive leaf extract.^[3]

In overweight men without any significant metabolic abnormality, oral ingestion of olive leaf extract (51.1 mg oleuropein; 9.7 mg hydroxytyrosol) for 6 weeks failed to significantly modify any tested lipid parameter.^[9] One other study has noted a significant reduction in triglycerides (as well as cholesterol; no influence on HDL-C) in hypertensives.^[2]

Mixed influences on circulating triglyceride and lipoprotein levels; there may be a small increase in circulating HDL-C levels but this does not appear to be to a clinically relevant degree.

5.4. Artherosclerosis

In regards to LDL-C oxidation, it has been noted that the procedure for isolated LDL cholesterol for ex vivo oxidation testing reduces the protective effects of hydroxytyrosol^[55] which is thought to explain the lack of results seen in studies using hydroxytyrosol ex vivo.^[55][56] For studies that avoid this and measure oxidized LDL in vivo, a collection of studies that compare variants of olive oil that differ only by their phenolic concentration (mostly hydroxytyrosol) noted a dose-dependent reduction in LDL oxidation rates.^[57] Trials following this methodology noted a 5.2% reduction (3 mg) and 28.2% reduction (20 mg), while controls experienced a 3.2% increase.^[52] A 3% (4 mg) and 6.5% (9 mg) reduction when controls were assessed at 2.6%,^[53] and an 8.9% increase (6 mg) and 15.2% decrease (15 mg) when controls experienced a 20.9% increase (study assessed oxidation following a meal).^[58] A reduction of 8% with 9 mg relative to 1 mg,^[59]and a 5% (3 mg) and 35% (12 mg) reduction when no change occurred in controls.^[60] Finally, a 12% (2 mg) and 34% (4 mg) reduction was found where controls decreased by 18% (performing similarly to 2 mg polyphenolics).^[54]

When looking at supplements, oral ingestion of olive leaf conferring 51.1 mg oleuropein and 9.7 mg hydroxytyrosol failed to significantly alter LDL oxidation rates. This study used in vivo measurements and thus the methodology does not appear at fault.^[9]

The polyphenolic content of olive oil appears to be the reason that virgin olive oil is more cardioprotective than processed olive oil, and tends to refer to the ability of polyphenolics in olive oil to reduce LDL oxidation rates. Ingestion of these polyphenolics in low concentrations appears to greatly reduce LDL oxidation rates in humans, and appears to occur during moderate olive oil consumption (virgin products only; not refined olive oil).

Oleuropein appears to have antiartherogenic activity mostly via the reduction in LDL. oxidation^[61] This results in less LDL aggregation on arterial walls) and has been noted to reduce monocytoid cell adhesion to the endothelium.^[62] Following oral consumption of 35 mL of olive oil, a reduced level of ICAM-1 and OLR-1 was noted to be related to the serum hydroxytyrosol concentrations (which appeared to suppress receptor levels of CD40, ADRB2, and IL8RA) with every 10% increase in urinary tyrosols being met with a 2.8- and 2.6-fold downregulation of ICAM-1 and OLR-1 (respectively).^[63]

The tyrosols appear to interact at the level of the endothelium, and may reduce oxidation not directly but secondary to suppressing activity of an inflammatory cascade.

6.1. Mechanisms

Oleanolic acid has been found to be an agonist of the TGR5 receptor with an EC₅₀ value of 1.42µM (comparable to lithocholic acid at 0.89µM) without influencing FXR.^[64] TGR5 is a G-protein coupled receptor (GPRC) for bile acids^[65] that, upon activation, leads to bioactivity of thyroid hormone and an increase in the metabolic rate.^[66][67] This may explain why one study giving 100, 250, or 500 mcg of olive leaf water extract to male rats for 14 days (125-150g in weight, and thus around 0.6-3.3mg/kg) was associated with a sharp decrease in TSH hormone levels (25% of control, no dose-dependence) with increases in T3 (dose dependent at 50%, 91%, and 150%). The extract did not have a significant influence on circulating T4 levels.^[68]

Other studies using extra virgin olive oil relative to refined olive oil (difference being phenolics) noted that despite no changes in weight occurring between the two groups the extra virgin group had higher serum adrenaline and UCP1 levels in adipose tissue.^[69] A later test noted that increases in both adrenaline and noradrenaline occur in a dose-dependent manner between 1.41-4.23mg phenolics, and that this was not due to the hydroxytyrosol content but instead thought to be due to the oleuropein.^[69] This was later confirmed with 0.1% of the diet as oleuropein and appeared to have more efficacy in rats fed higher protein intakes (although there was no significant differences in body weight after 28 days).^[70] A possible explanation for the lack of weight loss despite increased circulating catecholamines is a downregulation of their receptor (adrenergic β2) seen in humans following consumption of low dose olive oil phenolics.^[63] This was detected in endothelial cells rather than fat cells, however.

Potentially has mechanisms to increase the metabolic rate secondary to increasing active levels of thyroid hormone and catecholamines. Interventions using standard doses of olive leaf polyphenolics to see these effects failed to note changes in body weight over short periods of time, however.

6.2. Interventions

Studies comparing refined olive oil against virgin or extra virgin olive oil (in which the difference is olive polyphenolics), despite finding changes in lipid parameters, have routinely failed to detect a reduction in fat mass associated with polyphenolics.^{[54][59][53][52]} These trials tended to last 3 weeks.

Supplementation of olive leaf (51.1mg oleuropein; 9.7mg hydroxytyrosol) for 6 weeks in overweight men has failed to significantly modify total weight or body fat percentage.^[9] Another trial assessing the effects of olive leaf on blood pressure (which found benefit) failed to find a weight reducing effect.^[2]

Neither extra virgin olive oil, refined oil, nor supplemental olive leaf extracts have been shown to have fat-reducing effects.

7.1. Cold and Flu Interactions

Calcium elenolate (a base form of elenolic acid) can be isolated from olive leaf extract after mild acid hydrolysis.^[71] It has been shown to have virucidal activity against influenza A virus (PR8) in vitro.^[72] However, given that elenolic acid is at levels hard to detect in olive leaf extract,^[14] the relevance of this result for the whole extract is uncertain.

7.2. Bacterial Interactions

Olive leaf extract seems to inhibit or kill many pathogenic bacteria in vitro, but the details depend on the type of extract tested. In one set of experiments, methanolic extract of olive leaves with a yeild of 5.89% MICs of 125-250 µg/L against Staphylococcus aureus, Staphylococcus epidermidis, and Streptococcus pyogenes, with essentially no effect (MICs 500-2000 µg/L) against Salmonella enterica, Serovar Typhi, Acinetobacter calcoaceticus, and Pseudomonas aeruginosa.^[73] An Australian commercial preparation of olive leaf extract with a guaranteed minimum oleuropein content was 4.4 mg/mL was found to be most active against Campylobacter jejuni, Helicobacter pylori and Staphylococcus aureus (including meticillin-resistant S. aureus, or MRSA), with MICs in the range of 0.3–12.5% (v/v), but with weak or no effect against 79 other organisms, indicating a lack of broad-spectrum activity.^[74] Water extract from olive leaves was found to be bactericidal when bacteria were exposed to the extract for 3 hours, with MBCs of 0.13% (w/v) for Pseudomonas aeruginosa, 0.3% (w/v) for Klebsiella pneumoniae , 0.3% (w/v) for Escherichia coli, and 0.6% (w/v) Staphylococcus aureus; Bacillus subtilis was only inhibited upon exposure to 20% (w/v) olive leaf extract for 24 hours, not killed.^[75]

Individual phenolic compounds seem to have antibacteral activities on their own, but tend to work better when combined. One study found that oleuropein and caffeic acid showed inhibitory activity against some bacteria individually, but a combination of compounds found in olive leaf extract (oleuropein, rutin, vanillin, and caffeic acid) worked synergistically.^[76]

Olive leaf extract has antibacterial effects against many (but not all) pathogenic bacteria in vitro, but whether these effects carry over to supplementation has not been tested. It seems that a combination of components found in olive leaf extract works better at inhibiting bacterial growth than any single individual component alone.

7.3. Virological Interactions

An early in vitro study of calcium elenolate, a base form of elenolic acid which can be isolated from olive leaf extract after mild acid hydrolysis,^[71] found that this compund has antiviral properties in vitro against a host of viruses, including coxackievirus A21, parainfluenza 3 virus, herpesvirus (MRS), pseudorabies virus, vesicular stomatitis virus, encephalomyocarditis virus, Newcastle virus (GB), influenza A virus (PR8), and Sindbis virus.^[72] Calcium elenolate has also shown in vivo antiviral activity against parainfluenza 3 virus in hamsters when administered either minutes after infection, where it showed virucidal effects, or therapeutic effects when administered 8 hours after infection.^[77]

In vitro assays have also found that olive leaf extract can inhibit rotavirus (a major cause of diarrhea in children) with an IC₅₀ of approximately 300 μg/mL,^[78] as well as completely abolish infectivity of haemorrhagic septicaemia rhabdovirus (which infects farmed and sea fish) at 54 μg/mL,^[79] and can inhibit HIV-1 replication with an EC₅₀ of 0.2 μg/mL.^[80]

Olive leaf extract shows antiviral activity in vitro against a range of viruses, and in vivo in one animal study involving parainfluenza 3 virus. Its antiviral properties have not yet been tested in man.

Hydroxytyrosol appears to be the most effective anti-oxidant phenolic in olive leaf extract in vitro^[81][82][31] and for both olive oil and olives themselves it consists of 50% of the total phenolics.^[54][83] This number is lower in the leaf extracts due to a higher oleuropein content.

Most antioxidative effects of olive leaf supplements are attributed to the hydroxytyrosol content and the oleuropein content (via conferring hydroxytyrosol after digestion)

9.1. Absorption

Olive leaf (ethanolic) extract has been shown to inhibit the α-Amylase enzyme with IC₅₀ values of 4 mg/mL (salivary amylase) and 0.2 mg/mL (pancreatic amylase) which were attributed to the luteolin glycosides (7-O-β and 4'-O-β glucoside) with IC₅₀ values of 0.3-0.5 mg/mL^[1] (which is less than that reported for the luteolin aglycone at 0.01 mg/mL^[1] or 0.05-0.5 mg/mL^[84]). The water extract was less effective, with IC₅₀ values between 67-70.2 mg/mL.^[1] Oleuropein failed to inhibit the α-amylase enzyme, but its aglycone form appeared to have a potent inhibitory effect (0.03 mg/mL).^[1]

In rats, 20 mg/kg of olive leaf extract was able to reduce postprandial glucose but to a lesser degree than the active controls of 1 mg/kg oleanolic acid and 0.1 mg/kg luteolin. In volunteers who consumed 300 g cooked rice, there was no effect in healthy volunteers but those with borderline high glucose experienced less glucose spikes at 30-90 minutes after ingestion.^[1] These effects have also been noted in rats given a starch test (100 mg/kg olive leaf) which affected both normal and diabetic rats.^[85]

Appears to either reduce the absorption of or attenuate the rate of carbohydrate absorption, secondary to inhibitory effects on carbohydrate digestive enzymes. The degree of potency appears to be relevant to supplemental consumption, but the effects are modest.

9.2. Mechanisms

Olive leaf is known to protect the pancrease from autoimmune damage, with a study conducted in rats chemically inducing type I diabetes noting that olive leaf ingestion (100 mg/kg; 19.8% oleuropein) was able to prevent the diabetogenic consequence of low-dose streptozotocin injections and cyclophosphamide.^[86] The mechanisms were thought to be from interfering with the immune system's interactions with the pancreas (as nitric oxide levels increased in the spleen and periphery but not the pancreas, where they decreased. No interference was noted on Treg cell count, but lower levels of IFN-γ, IL-17 and TNF-α were detected).^[86] In toxin-induced diabetic rat models, olive leaf (200 mg/kg) appears to have a potency similar to that of Metformin (this study also noting comparable efficacy with Murraya koenigii, the curry tree)^[87] and 500 mg/kg of olive leaf over 14 days (but not 100-250 mg/kg) has been found to outperform 600 mcg/kg glibenclamide for benficially influencing diabetic rat glucose and insulin.^[88]

Studies conducted in otherwise normal and healthy research animals fail to find significant influence on glucose parameters such as fasting glucose or fasting insulin with 500 mg/kg.^[88] However, one (human) study has noted a 28% improvement in pancreatic β-cell responsiveness following oral ingestion of an olive leaf extract (51.1 mg oleuropein; 9.7 mg hydroxytyrosol) for 6 weeks in otherwise healthy persons.^[9] 

Olive leaf appears to be more preventative in animal models where diabetes is induced via a toxin, and this may occur via protection of pancreatic function. There may be a general enhancement of pancreatic β-cell function (and insulin secretion in response to carbohydrate) that persists independent of health state, but the reduction in blood glucose has not been noted yet in healthy persons.

Oleanolic acid has been found to be an agonist of the TGR5 receptor with an EC₅₀ value of 1.42 µM (comparable to lithocholic acid at 0.89 µM) without influencing FXR;^[64] TGR5 is a G-protein coupled receptor (GPRC) for bile acids^[65] that, upon activation, leads to bioactivity of thyroid hormone and an increase in the metabolic rate.^[66][67] This study then demonstrated that 100 mg/kg oleanolic acid was able to enhance insulin sensitivity while decreasing plasma glucose (40%) and insulin (47%), but failed to establish that it was via TGR5.^[64]

TGR5 activation may play a hypoglycemic role, but this is currently not established.

9.3. Interventions

Supplementation of olive leaf extract (51.1 mg oleuropein; 9.7 mg hydroxytyrosol) in overweight persons for 6 weeks was associated with a 15% increase in insulin sensitivity, less glucose AUC in response to a tolerance test (6%), and less insulin secretion (14%).^[9] 

One trial using 500 mg olive leaf for 14 weeks in type II diabetics noted lower HbA1c levels (8% relative to control's 8.9%), fasting insulin (11.3+/-4.5 versus 13.7+/-4.1) with no influence on postprandial insulin; glucose was not measured.^[85]

Pleminary evidence in diabetics and healthy subjects indicates that olive leaf extract exerts protective effects on glucose metabolism.

10.1. IGF

One intervention in overweight men using olive leaf extract (51.1 mg oleuropein; 9.7 mg hydroxytyrosol) for 6 weeks noted an increase in the binding proteins IGFBP-1 and IGFBP-2 by 19.5% and 12.5% with no influence on IGFBP-3 nor either IGF-1 or IGF-2 hormone levels.^[9]

Although there is no influence on IGF hormone levels, there may be reduced overall IGF-like effects in the body due to an increase in their binding proteins; more research needs to be conducted here.

10.2. Testosterone

Oleuropein at 0.1% of the diet for 28 days in male rats was able to increase testicular concentrations of testosterone in a linear relation with overall dietary protein intake (with rats consuming 40% dietary protein experiencing a larger increase than 25% or 10%, the latter experiencing no increase) and the highest protein group experienced a decrease in urinary nitrogen excretion by 19.7% with oleuropein.^[70] These changes were associated with a dose-dependent increase in luteinizing hormone (LH) seen with oleuropein.^[70]

Possible enhancement of testosterone synthesis, requires some human evidence

11.1. Skin

Olive products (leaves and oils) appear to have historical usage as emollient for rhematoid arthritis (pain and inflammation symptoms) in Portugal^[89] while in the USA and in both Bulgaria and Italy topical olive application is recommended for burn healing.^[90]

Application of olive leaf water extracts (1% via ointment) to wounds on rats appears to accelerate wound healing rates and improve tensile strength of the skin over 21 days, although to a lesser degree than the reference drug (0.1% centella asiatica). ^[91]

Topical application of oleuropein is able to reduce the damage done in skin cells from UV(B) damage^[92] which appears to extend to oral ingestion of 25-85 mg/kg twice daily in mice (or 1000 mg/kg olive leaf extract twice daily).^[93] 1000 mg/kg olive leaf or 85 mg/kg oleuropein twice daily in this study abolished the changes in skin thickness seen with UV(B) radiation and greatly attenuated changes in elasticity and appears to be due to anti-inflammatory mechanisms^[93] and has been replicated elsewhere following oral ingestion.^[94]

There appears to be protective effects on the skin following oral consumption of high dose olive leaf (human equivalent dosage of 80 mg/kg; 30 mg/kg ineffective) or topical application of oleuropein.

12.1. Urinary tract

60 mg/kg olive leaf extracts to rats appears to have weak diuretic effects, underperforming to the reference drug (Hydrochlorothiazide at 25 mg/kg). The mechanism is thought to be via inhibition of Na⁺ and K⁺ reabsorption in the early portion of the distal tubule.^[3]

12.2. Liver

In rats fed a high carbohydrate and fat diet to induce obesity and comorbidities, the increase seen in serum liver enzymes (ALT and AST) was attenuated with ingestion of 20 mg/kg oleuropein (3% olive leaf in the feed) and the fibrosis scores of the liver were similarly reduced significantly.^[14] This has been noted in toxin-induced diabetic rats with 500 mg/kg olive leaf, with no significant influence on the liver enzymes of otherwise healthy rats.^[88]

Olive leaf extracts may be hepatoprotective in instances of liver damage or metabolic syndrome without significantly affecting serum liver enzymes at other times.

13.1. Mechanisms

The antioxidative effects of olive leaf are able to reduce the genotoxic effects of some cancer initiating agents that work via oxidative damage.^[95] This is thought to be practiaclly relevant at low doses; one human intervention using 3-12 mg of phenolics (40% oleuropein and 6.5% hydroxytyrosol) noted that the levels of the oxidative biomarker 8-oxo-dG decreased 49.2% (mitochondrial measurement) and 51.67% (urine) after 4 days at rest, and reduced the postprandial increase in mitochondrial 8-oxo-dG.^[60]

Appears to reduce genomic damage secondary to antioxidative effects, and is thought to be relevant following oral ingestion of very low doses of olive phenolics. This may have a role in cancer prevention (rather than treatment)

Antiproliferative effects have been noted in glioblastoma cells with olive leaf extract.^[96]

Antiproliferative effects have been noted in promyelocytic leukemic cells (HL-60; associated with the apigenin glucoside content^[97]) and Jurkat leukemic cells^[98] with both antiproliferative and pro-apoptotic effects have been noted with colon cancer cells (HT29 and Caco-2) attributed to a triterpenoid.^[99] IC₅₀ values of inhibition with the olive leaf extract tend to be high (ineffective), in the 1mg/mL range^[96] or the millimolar range.^[99]

Olive leaf extract has been noted to enhance the cytotoxicity of temozolomide in T98G glioblastoma cells^[96] although another study has noted antagonism with temozolomide yet synergism with cisplatin and paclitaxel.^[100] 

May have anticancer properties in cancer cells, but this does not appear to be overly significant and no in vivo evidence currently exists

14.1. General

Studies on olive leaf extract have failed to note any significant side-effects with doses up to 1000 mg daily for 8 weeks^[2] or lower doses.^[9]

In general, supplemental dosages of olive leaf extracts are not connected to significant side effects.

14.2. Allergies

Olea europaea (olive leaf) appears to be associated with polinosis (pollen-based allergies), and eight allergens have been detected from olive pollen (named Ole e 1 through 8).^[101][102] Ole e 7 appears homologous to lipid transfer protein (seen with some apple allergens^[103]) which may explain how those sensitive to olive pollen and Ole e 7 have a high frequency of cross-reactivity to the rosaceae family (containing apples).^[102] There also appears to be some allergens in olea europaea that are similar to Birch allergens^[104] (which itself is highly associated with apples^{[105][106][107]}).

It is possible to be allergic to the pollen of olive plants, but this has not been highly linked to consuming olive leaf based supplements or phenolic containing olive oils