Olive leaf extract

Olive leaf extract comes from the leaves of olive plants, and is distinct from Olive Oil; the leaf extract contains phenolics such as oleuropein, and appears to have highly protective effects against LDL oxidation and may also benefit glucose metabolism and skin health.

This page features 108 unique references to scientific papers.

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All Essential Benefits/Effects/Facts & Information

Olive leaf extract is a supplement derived from the leaves of the plant that bears olive (a fruit from which the cooking oil is derived from) and contains the main bioactives of hydroxytyrosol/tyrosol and oleuropein/ligstroside.

Olive phenolics in general, which are present in high levels in olive leaf supplements, appears to potently protect LDL cholesterol from oxidation; this is attributed to the anti-inflammatory effects of hydroxytyrosol, and appears to be active at a low enough dose to apply to consumption of olive product consumption. Olive leaf may also benficially influence the levels of lipoproteins in a beneficial manner (LDL, HDL, total cholesterol) although the actual changes in lipoprotein content is rather small. Overall, olive phenolics are more anti-artherosclerotic than they are cholesterol reducing.

There appears to be beneficial effects on glucose metabolism, which may be related to the pancreas. The mechanisms of olive leaf and glucose metabolism are not fully elucidated at this point in time, but appears to be more of a preventative supplement than a rehabilitative one.

Although olive leaf has been implicated in various fat burning effects (increased thyroid hormones and adrenaline) there does not appear to be any overall fat lost in trials using olive leaf supplements; the reason for this is not known, but may be related to a decrease in levels of the receptor that adrenaline works through which may normalize the effects.

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Things To Know

Also Known As

Olive polyphenols

Do Not Confuse With

Olive Oil (the fatty acids from the fruits), Oleuropein (main bioactive)

Things to Note

  • The oleuropein content correlates well with the pungency of oil and fruit based olive products, although leaf extracts tend to be quantitatively standardized for oleuropein

Caution Notice

It is possible to be allergic to olive products, and in that instance it may be possible to have a reaction to olive leaf supplements

Examine.com Medical Disclaimer

How to Take

Recommended dosage, active amounts, other details

Supplemental olive leaf is taken in the 500-1000mg range daily, although supplements with even as low as 10mg (as seen in olive oil products) may confer good protection against LDL oxidation. At least for LDL oxidation, olive food products may suffice rather than supplementation.

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Human Effect Matrix

The Human Effect Matrix looks at human studies (it excludes animal and in vitro studies) to tell you what effects olive leaf extract has on your body, and how strong these effects are.

Grade Level of Evidence
Robust research conducted with repeated double-blind clinical trials
Multiple studies where at least two are double-blind and placebo controlled
Single double-blind study or multiple cohort studies
Uncontrolled or observational studies only
Level of Evidence
? The amount of high quality evidence. The more evidence, the more we can trust the results.
Outcome Magnitude of effect
? The direction and size of the supplement's impact on each outcome. Some supplements can have an increasing effect, others have a decreasing effect, and others have no effect.
Consistency of research results
? Scientific research does not always agree. HIGH or VERY HIGH means that most of the scientific research agrees.
Blood Pressure Notable High See all 4 studies
One study assessing the potency in hypertensive persons noted it was comparable to Captopril, and olive leaf appears to be potent when it can reduce blood pressure. It does not appear to reduce blood pressure in normotensive persons, however
Oxidation of LDL Notable Very High See all 8 studies
The decrease in LDL oxidation seen with olive oil phenolics appears to be of notable potency due to its reliability (occurring in both healthy persons and diseased persons, chronically and acutely) and a reduction in LDL oxidation rates can exceed 25% in some studies with low intakes of olive phenolics (enough from virgin olive oil consumption)
HDL-C Minor Moderate See all 7 studies
The increase in HDL seen with olive phenolics is somewhat inconsistent and not to a large magnitude when it occurs
LDL-C Minor Moderate See all 5 studies
A decrease in LDL-C seems to be somewhat consistent, but the magnitude of decrease is not overly impressive relative to other agents
Total Cholesterol Minor Moderate See all 4 studies
There appears to be somewhat of a decrease in total cholesterol (mostly due to LDL) with olive leaf consumption; it is not overly reliable
Triglycerides - Very High See all 5 studies
The majority of the evidence suggests that there is no impressive effect on triglycerides with olive phenolic consumption
Cell Adhesion Factors Notable - See study
A decrease was noted in CD40 and MCP1 adhesion factors, and this is notable as this downregulation was dose-dependently related to ingested tyrosol/hydroxytyrosol and may explain the reduced LDL oxidation
DNA Damage Notable - See study
The one study to measure DNA damage (via 8-oxo-dGF as a biomarker) noted up to 50% reductions in mitochondrial and urine measurements; a fairly significant reduction.
Anti-Oxidant Enzyme Profile Minor Very High See 2 studies
Appears to induce the activity of the enzyme known as glutathione peroxidase
Blood Glucose Minor Moderate See 2 studies
There appears to be significant interactions with olive leaf and diabetes, and research is too preliminary to come to conclusions. Olive leaf may reduce blood glucose in diabetics only
General Oxidation Minor - See study
Is able to reduce oxidative parameters in the blood when they are measured.
HbA1c Minor - See study
A minor decrease in HbA1c has been noted with olive leaf consumption
IGF Binding Protein Minor - See study
An increase in IGF binding proteins has been noted in one study (which would sequester and reduce the activity of IGF hormones); potency of this interaction unable to be determined (no reference drug)
Insulin Minor Very High See 2 studies
A decrease of fasted and postprandial insulin with olive leaf consumption has been noted, not to a remarkable degree
Insulin Sensitivity Minor - See study
An increase in insulin sensitivity has been noted in otherwise healthy persons (Matsuda Index and oral glucose tolerance test) and thought secondary to the pancreas; may be relevant to otherwise healthy persons
Lipid Peroxidation Minor Very High See 2 studies
Appears to reduce biomarkers of lipid peroxidation.
IGF-1 - - See study
No significant influence on circulating IGF-1 nor IGF-2 levels
Inflammation - - See study
No significant effects on biomarkers of inflammation when measured
Liver Enzymes - Very High See 2 studies
No significant influences on liver enzymes have been noted in trials that assess them (safety reasons)
Weight - Moderate See 2 studies
Overall, there does not appear to be a significant effect of olive phenolic consumption on weight
Carbohydrate absorption Minor - See study
There appears to be a decrease in either the rate or overall amounts of carbohydrate absorption seen with olive leaf consumption

Studies Excluded from Consideration

NOTE: The rubric contains studies comparing high phenolic Olive Oil against control (low or no phenolic olive oil) due to the practical difference being merely phenolics found in both leaf and oil extracts

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Scientific Research

Table of Contents:

  1. 1 Sources and Composition
    1. 1.1 Sources
    2. 1.2 Composition
  2. 2 Molecular Targets
    1. 2.1 PPARs
  3. 3 Pharmacology
    1. 3.1 Absorption
    2. 3.2 Metabolism
  4. 4 Neurology
    1. 4.1 Mechanisms
    2. 4.2 Neuroprotection
    3. 4.3 Pain
  5. 5 Cardiovascular Health
    1. 5.1 Cardiac Tissue
    2. 5.2 Blood Pressure
    3. 5.3 Cholesterol and Triglycerides
    4. 5.4 Artherosclerosis
  6. 6 Fat Mass and Obesity
    1. 6.1 Mechanisms
    2. 6.2 Interventions
  7. 7 Inflammation and Immunology
    1. 7.1 Cold and Flu Interactions
    2. 7.2 Bacterial Interactions
    3. 7.3 Virological Interactions
  8. 8 Interactions with Oxidation
  9. 9 Interactions with Glucose Metabolism
    1. 9.1 Absorption
    2. 9.2 Mechanisms
    3. 9.3 Interventions
  10. 10 Interactions with Hormones
    1. 10.1 IGF
    2. 10.2 Testosterone
  11. 11 Interactions with Aesthetics
    1. 11.1 Skin
  12. 12 Interactions with Organ Systems
    1. 12.1 Urinary tract
    2. 12.2 Liver
  13. 13 Interactions with Cancer Metabolism
    1. 13.1 Mechanisms
  14. 14 Safety and Toxicology
    1. 14.1 General
    2. 14.2 Allergies

1Sources and Composition

1.1. Sources

Olive (Olea europaea of the family Oleaceae) leaves appear to have medicinal usage for the treatment of diabetes,[1] blood pressure, and artherosclerosis mostly[2] with some less frequent claims of olive leaf being a diuretic, hypotensive, emollient, febrifuge and tonic, for urinary and bladder infections and for headaches.[3] Traditional usage seems to be localized to around the mediterranean area (Spain, Italy, France, Greece, Israel, Morrocco, Tunisia, Turkey[3]) and in Africa by the Sotho, Xhosa, and Zulu tribes where it holds 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 towards being anti-diabetic and cardioprotective. 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 and hydroxytyrosol, elenolic acid, and mostly the 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; oleuropein correlates to the pungency of the olive product

2Molecular Targets

2.1. PPARs

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

While oleuropein is technically an inhibitor of PPARγ, the concentration required for this action seems quite high in adipocytes and may not be relevant to oral ingestion of the compound (as oral doses of oleuropein are relatively low to reach this concentration in peripheral tissue)


3.1. Absorption

Hydroxytyrosol appears to be passively absorbed through the intestinal wall in a concentration and time dependent manner[36] and appear to be 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] but did not differentiate between phenolics 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 in vitro to occur in the small intestine.[42]

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

Tyrosol and hydroxytyrosol are very readily absorbed in the small intestines, and the dialdeyde 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 buld 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 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 with brain neurotrophic factors, but the practical relevance of this information is not currently known

4.2. Neuroprotection

Oleuropein appears to exert antioxidative effects in PC12 cells and reduce the rates of apoptosis induced by 6-hydroxydopamine at 20-25µg/mL (olive leaf at 400-600µg/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) it does not appear to be an overly 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 300-500mg/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 in diabetic rats but was preserved to 84.78% and 85.97% (300mg/kg and 500mg/kg; 100mg/kg ineffective).[49] These changes were associated with reductions in blood glucose in the two higher doses but not 100mg/kg over 4 weeks.[49]

May reduced diabetic neuropathy secondary to aiding glucose levels in diabetic rats

5Cardiovascular Health

5.1. Cardiac Tissue

In rats fed an obesogenic diet with olive leaf (20.7+/-0.3mg/kg oleuropein and 4.3+/-0.1mg/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, potency of these effects unknown (no comparators)

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 26μM (IC50 value),[10] but the degree of absorption of oleacin is not certain.

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

In animal studies, rats who 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] and the leaf extract has shown benefit in L-NAME (Nitric Oxide inhibitor) induced hypertensive rats (100mg/kg olive leaf)[50] and normotensive rats under anaesthesia (180mg/kg olive leaf).[51] Conversely, a high fat and high carbohydrate fed obsesogenic model of high blood pressure failed to find reductions in blood pressyre with 20.7+/-0.3mg/kg oleuropein and 4.3+/-0.1mg/kg hydroxytyrosol equivalents (3% of feed) over 4 weeks.[14] This study that failed to find a reduction in blood pressure did find increase 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 (51.1mg 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 1000mg of olive leaf daily has noted reductions in systolic blood presssure by 8% (500mg daily was ineffective)[11] and 500mg of olive leaf extract twice daily (1000mg 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 50mg if needed).[2] Some studies that compare low phenolic Olive Oil against high phenolic olive oil also note slight blood pressure reductions associated with the consumption of olive phenolics when the study population is hypertensive persons[52] with similarly structured studies in normotensive persons failing 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 reducing protective effects of olive leaf extract.[3]

In overweight men without any significant metabolic abnormality, oral ingestion of olive leaf extract (51.1mg oleuropein; 9.7mg 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) tends to note a dose-dependent reduction in LDL oxidation rates.[57] Trials following this methodology note a 5.2% reduction (3mg) and 28.2% reduction (20mg) while control experienced a 3.2% increase,[52] a 3% (4mg) and 6.5% (9mg) reduction when control elevates 2.6%,[53] an 8.9% increase (6mg) and 15.2% decrease (15mg) when control experienced a 20.9% increase (study assessed oxidation following a meal),[58] a reduction of 8% with 9mg relative to 1mg,[59] a 5% (3mg) and 35% (12mg) reduction when no change occurred in control,[60] and a 12% (2mg) and 34% (4mg) reduction, although this trial noted that control decreased by 18% (performing similarly to 2mg polyphenolics).[54]

When looking at supplements, oral ingestion of olive leaf conferring 51.1mg oleuropein and 9.7mg 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] (which 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 35mL 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

6Fat Mass and Obesity

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 one study giving 100, 250, or 500mcg 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%) and no significant influence on circulating T4.[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 thought to be due to the oleuropein[69] which 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] although this was detected in endothelial cells rather than fat cells.

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

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, routinely fail 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] and another trial assessing the effects of olive leaf on blood pressure (which found benefit) failed to find a weight reducing effect.[2]

Neither choosing extra virgin olive oil over refined oil nor supplemental olive leaf extracts have been demonstrated to have a fat reducing effect at this moment in time

7Inflammation and Immunology

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 125-250µg/L against Staphylococcus aureus, Staphylococcus epidermidis, and Streptococcus pyogenes, with essentially no effect (MICs 500-2000µg/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 4.4mg/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 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 300μg/mL,[78] as well as completely abolish infectivity of haemorrhagic septicaemia rhabdovirus (which infects farmed and sea fish) at 54μg/mL,[79] and can inhibit HIV-1 replication with an EC50 of 0.2μg/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.

8Interactions with Oxidation

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)

9Interactions with Glucose Metabolism

9.1. Absorption

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

In rats, 20mg/kg of olive leaf extract was able to reduce postprandial glucose but to a lesser degree than the active controls of 1mg/kg oleanolic acid and 0.1mg/kg luteolin; in volunteers who consumed 300g 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 (100mg/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 not overly remarkable

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 (100mg/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 (200mg/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 500mg/kg of olive leaf over 14 days (but not 100-250mg/kg) has been found to outperform 600mcg/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 500mg/kg.[88] However, one (human) study has noted a 28% improvement in pancreatic β-cell responsiveness following oral ingestion of an olive leaf extract (51.1mg oleuropein; 9.7mg hydroxytyrosol) for 6 weeks in otherwise healthy persons.[9]

Olive leaf appears to be more potently preventative in animal models where diabetes is induced via a toxin, and may be due to protecting 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 1.42µM (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 study then demonstrated that 100mg/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 (51.1mg oleuropein; 9.7mg 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 500mg 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]

Preliminary evidence in diabetics and healthy persons, but it appears that olive oil appears to exert protective effects on glucose metabolism

10Interactions with Hormones

10.1. IGF

One intervention in overweight men using olive leaf extract (51.1mg oleuropein; 9.7mg 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

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

11Interactions with Aesthetics

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 here and in both Bulgaria and Italy topical olive application appears to be recommended for burn healing.[90]

Application of olive leaf water extracts (1% via ointment) to wounds on rats appears to accelerate wound healing rates over 21 days, although to a slightly lesser degree than the reference drug (0.1% centella asiatica) and improved tensile strength of the skin.[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 25-85mg/kg twice daily in mice (or 1000mg/kg olive leaf extract twice daily);[93] 1000mg/kg olive leaf or 85mg/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 80mg/kg; 30mg/kg ineffective) or topical application of oleuropein

12Interactions with Organ Systems

12.1. Urinary tract

60mg/kg olive leaf extracts to rats appears to have weak diuretic effects, underperforming to the reference drug (Hydrochlorothiazide at 25mg/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 20mg/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 500mg/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

13Interactions with Cancer Metabolism

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] which is thought to be practiaclly relevant at low doses; one human intervention using 3-12mg 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

14Safety and Toxicology

14.1. General

Studies on olive leaf extract have failed to notice any significant side-effects when doses up to 1000mg 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

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