The role of olive oil and its constituents in mental health: a scoping review

Abstract

Mounting evidence suggests that the Mediterranean diet has a beneficial effect on mental health. It has been hypothesised that this effect is mediated by a variety of foods, nutrients and constituents; however, there is a need for research elucidating which of these components contribute to the therapeutic effect. This scoping review sought to systematically search for and synthesise the research on olive oil and its constituents and their impact on mental health, including the presence or absence of a mental illness or the severity or progression of symptoms. PubMed and OVID MEDLINE databases were searched. The following article types were eligible for inclusion: human experimental and observational studies, animal and preclinical studies. Abstracts were screened in duplicate, and data were extracted using a piloted template. Data were analysed qualitatively to assess trends and gaps for further study. The PubMed and OVID MEDLINE search yielded 544 and 152 results, respectively. After full-text screening, forty-nine studies were eligible for inclusion, including seventeen human experimental, eighteen observational and fourteen animal studies. Of these, thirteen human and four animal studies used olive oil as a comparator. Observational studies reported inconsistent results, specifically five reporting higher rates of mental illness, eight reporting lower and five reporting no association with higher olive oil intake. All human experimental studies and nine of ten animal studies that assess olive oil as an intervention reported an improvement of anxiety or depression symptoms. Olive oil may benefit mental health outcomes. However, more experimental research is needed.

Mental illnesses, including depression, anxiety, schizophrenia and bipolar disorder, are widely prevalent across the globe. In Canada, one in five individuals are affected by mental illness every year, and it remains one of the leading causes of disability in Canada^(1,2) . Mood and anxiety disorders in particular have continued to drastically rise in prevalence in Canada within the past decade⁽³⁾. Mental illnesses have detrimental impacts on quality of life, commonly equivalent or greater than that of other illnesses⁽⁴⁾, through an impact on social support, marital status, occupation and disability. With this increase in mental illness and its substantial impact, there is a need for more research that targets not only treatment options for mental illness but also preventative measures that reduce the global burden of disease.

There is currently abundant research demonstrating the positive effect of the Mediterranean diet on physical health. The Mediterranean diet has been known to have substantial benefits on the prevention of cancer, CVD and metabolic disease⁽⁵⁾. Other studies found that the Mediterranean diet reduces the risk of Parkinson’s disease, Alzheimer’s disease and overall mortality⁽⁶⁾. There is also emerging evidence that it may protect against the development of mental health disorders, as demonstrated by multiple prospective cohort studies⁽⁷⁾. The Mediterranean diet is rich in fruits, vegetables, nuts, fish, white meat and unsaturated fats, namely olive oil. It has also been highly regarded as a palatable and sustainable diet⁽⁸⁾. Clinical trials using the Mediterranean diet have demonstrated the ability to significantly reduce depressive symptoms⁽⁹⁾. One systematic review of observational studies found that higher adherence to the Mediterranean diet was associated with lower rates of Axis I disorders (depression, anxiety, bipolar disorders, schizophrenia, psychosis, eating disorders, obsessive–compulsive disorders, trauma and stress-related disorders, substance abuse and attention-deficit/hyperactivity disorder), with the lowest rates seen in depression and anxiety⁽¹⁰⁾. Although the evidence is mounting for a protective and therapeutic effect of the Mediterranean diet on mental health, it is not fully understood how each of the individual components of this diet contributes to this benefit.

Olive oil, a staple in the Mediterranean diet, is mainly composed of oleic acid (an omega 9 [n-9] fatty acid), and oleuropein, a phenolic compound that gives olives their bitter taste⁽¹¹⁾. It makes up 20–25 % of the total energy intake in the traditional Mediterranean diet⁽¹²⁾. Olive oil is known to have a beneficial association with cardiovascular disease, type 2 diabetes and all-cause mortality⁽¹²⁾. Specific compounds in olive oil, such as oleocanthal, oleacein and oleuropein, have been found to be directly associated with reduced risk of cancer due to their anti-inflammatory and antioxidant properties^(13,14) . Another study observed significant reductions in oxidative stress biomarkers associated with the high phenolic content of olive oil, providing further support for its antioxidant effects⁽¹⁵⁾. One study found significant reductions in symptoms associated with immune-mediated inflammatory diseases, specifically due to the anti-inflammatory and immunomodulatory effects of polyphenols in olive oil⁽¹⁶⁾. Inflammation and oxidative stress have both been proposed as mechanisms which mediate the impact of diet on mental health⁽¹⁷⁾; these mechanisms might explain how olive oil could benefit mental health. To date, the effect of olive oil consumption on mental health has not been thoroughly studied. Due to the limited experimental research and no prior synthesis on the effects of olive oil on mental health, a scoping review is warranted. The objective of this scoping review is to systematically search for and synthesise the research on olive oil and mental health.

Methods

Eligibility criteria

The following eligibility criteria were applied.

Inclusion criteria:

• Human experimental or observational studies, animal studies and meta-analyses.

• Delivery of olive oil or one of its constituents (oleic acid and oleuropein) or assessment of intake of olive oil or measurement of tissue levels of constituents.

• Assessment of any mental health outcome (incidence of mental disorder, severity of mental disorder, treatment or progression of a mental disorder). Eligible mental disorders included: depressive disorders, anxiety disorders, psychotic disorders, bipolar and related disorders, obsessive–compulsive and related disorders, attention-deficit disorders, trauma- and stressor-related disorders, substance use disorders and eating disorders. Any year of publication, language or publication status.

Exclusion criteria:

• Narrative reviews, editorials or opinion articles.

• Delivery or assessment of olive oil in combination with other fatty acids, nutrients or foods

• Assessment of neurodegenerative disorders.

Information sources and search strategy

PubMed and OVID MEDLINE databases were searched on 10 November 2023. The following search strategies were used. No filters or limits were used.

PubMed: (Olive oil OR (“Oleic Acid”[Mesh]) OR “Olive Oil”[Mesh] OR n-9 OR Oleuropein) AND (“Anxiety”[Mesh] OR “Anxiety Disorders”[Mesh] OR depression OR “Depressive Disorder”[Mesh] OR “Depression”[Mesh] OR mental health OR psychiat* OR mental illness)

OVID Medline: (Olive oil or n-9 or oleuropein) and (anxiety or anxiety disorder or depression or depressive disorder or mental health or psychiat* or mental illness)

Record screening and selection

Titles and abstracts were screened independently in duplicate by the two authors using the eligibility criteria. The program Abstrackr⁽¹⁸⁾ was used for organisation. In cases of disagreement, the authors discussed the study until consensus was reached. Studies were then reviewed in full text to confirm eligibility.

Data collection process and data items

Data were extracted using piloted templates. Extraction was completed by one author and checked by the second author. The data extracted included: study design, participant/subject population, sample size, intervention or exposure (including duration), outcome and results.

Data analysis

Data were analysed qualitatively to assess trends and gaps for further study.

Results

The PubMed and OVID MEDLINE search yielded 544 and 152 results, respectively (Fig. 1). After deduplication, 552 results remained. After full-text screening, forty-nine studies were eligible for inclusion, including seventeen human experimental, eighteen observational and fourteen animal studies. Of the studies found, thirteen human intervention studies and four animal studies were designed to assess the impact of a different intervention on mental health outcomes and used olive oil as the comparator. Four human intervention studies that were designed to assess the impact of olive oil on mental health were identified (Table 1)^(19–22). All four were randomised controlled trials. Sample size ranged from seventeen to 129 and duration ranged from three to 12 weeks. The patient populations involved included people with major depressive disorder, fibromyalgia, obesity and healthy subjects. All four studies reported an improvement in mental health outcomes, including symptoms of depression^(19,21) , anxiety⁽²¹⁾, stress⁽²²⁾ and overall mental health status⁽²⁰⁾.

Figure 1. PRISMA flow diagram.

Table 1. Intervention studies designed to assess the impact of olive oil supplementation

EVOO, extra virgin olive oil; MDD, major depression disorder; BDI-II, Beck Depression Inventory II; HAMD-7, Hamilton Depression Rating Scale 7; MCS-12, Mental Component Score 12; HADS, Hospital Anxiety and Depression Scale.

Eighteen observational studies met criteria for inclusion (Table 2)^(23–40). Twelve studies were cross-sectional in design and of these, five assessed olive oil intake using a self-reported diet data collection instrument^(23–27) while seven measured tissue levels of oleic acid^(28–34). The six prospective studies assessed olive oil or oleic acid intake^(35–40). Of the six prospective studies, five reported an association between higher baseline olive oil intake and lower risk of developing a mental health disorder or symptoms at a later time point^{(35,36,38–40)} . The outcomes included diagnosis of anorexia or bulimia nervosa⁽³⁵⁾, symptoms of depression⁽³⁶⁾, incident depression diagnosis⁽³⁸⁾, development of severe depressed mood⁽³⁹⁾ and positive affect⁽⁴⁰⁾. The remaining study found no association between intake and depression symptom severity⁽³⁷⁾. Among the cross-sectional studies, the results were less consistent. Two cross-sectional studies measuring olive oil intake reported significant reductions in symptoms of mental illness (depression and binge eating disorder)^(26,27) with higher intake, while three cross-sectional studies measuring olive oil intake reported a significant increase in symptoms of mental illness (depression and anxiety)^(23–25). Of the seven cross-sectional studies that assessed tissue levels of oleic acid, one reported that higher levels were associated with greater plasticity among children with attention deficit hyperactivity disorder⁽³⁴⁾. Four of the seven cross-sectional studies measuring tissue levels of oleic acid found no assocaition^{(28,29,32,33)} , while two reported an association between higher levels and worse mental health: higher levels were found in criminal offenders compared with control subjects and increased levels were associated with increased diagnosis of depression^(30,31) .

Table 2. Observational studies

TBSP, tablespoon; BDI-II, Beck Depression Inventory II; MDD, major depressive disorder; NS, not significant; PANSS, Positive and Negative Syndrome Scale; MADRS, Montgomery-Åsberg Depression Rating Scale; GAF, Global Assessment of Functioning; NEO-FFI, NEO Five-Factor Inventory; HR, hazard ratio.

Ten animal studies designed to assess the effect of olive oil met criteria for inclusion (Table 3)^(41–50). All of these studies used rodent models. Five studies measured the impact of olive oil supplementation while five assessed the impact of oleuropein supplementation. Nine of ten animal studies reported significant improvements in mental health outcomes with higher olive oil consumption^{(41–47,49,50)} . Three studies reported a decrease in anxiety-like behaviour^(42,44,49) , two studies reported a decrease in depression-like behaviour^(43,50) and three studies reported a decrease in both anxiety and depression-like behaviours^(41,45,46) . One study reported a decrease in cognitive impairments associated with post traumatic stress disorder (PTSD)⁽⁴⁷⁾, while one study reported no improvement in anxiety-like behaviour⁽⁴⁸⁾.

Table 3. Animal studies

LFD, low-fat diet.

Thirteen interventions studies using olive oil as a comparator were eligible for inclusions (Table 4)^(51–63). Of the thirteen studies, twelve compared olive oil supplementation to omega 3 (n-3) fatty acids^{(51–57,59–63)} and one compared olive oil supplementation to flax oil⁽⁵⁸⁾. Sample size ranged from 30 to 132. Olive oil dose ranged from 280 mg to 8 g per day and duration ranged from 35 d to 44 weeks. Populations included individuals diagnosed with major depressive disorder^(51,53,63) , individuals being treated for a current depressive episode⁽⁵⁶⁾, individuals with bipolar disorder^(54,58) , renal transplant recipients⁽⁵²⁾, children with attention deficit/hyperactivity disorder-like symptoms⁽⁵⁵⁾, individuals diagnosed with schizophrenia⁽⁵⁷⁾, adolescents with Tourette’s disorder⁽⁵⁹⁾, community members reporting chronic work stress⁽⁶⁰⁾, pregnant women⁽⁶¹⁾ and healthy subjects⁽⁶²⁾. Primary outcomes include changes in depressive symptoms^{(51,53,56,61,63)} , duration of remission of bipolar disorder⁽⁵⁴⁾, symptoms of mania and depression⁽⁵⁸⁾, quality of life⁽⁵²⁾, attention deficit/hyperactivity disorder symptom severity⁽⁵⁵⁾, severity of both schizophrenic and depressive symptoms⁽⁵⁷⁾, tic severity⁽⁵⁹⁾, changes in stress⁽⁶⁰⁾ and changes in overall mood⁽⁶²⁾. Eight studies reported significant improvement in mental health outcomes in both the olive oil supplementation group and the intervention group^{(53,55,56,58–61,63)} , while four studies reported significant improvements in the intervention group compared with the olive oil supplementation group^{(51,54,57,62)} , and one study reported no significant improvements in either group⁽⁵²⁾. Of the eight studies reporting improvement, four reported significant improvement in severity of depression^{(53,56,61,63)} , one reported improvement in severity of attention deficit/hyperactivity disorder symptoms⁽⁵⁵⁾, one reported significant improvement in severity of mania and depression symptoms associated with bipolar disorder⁽⁵⁸⁾, one reported significant improvement in symptoms of Tourette’s disorder⁽⁵⁹⁾ and one reported significant improvements in stress⁽⁶⁰⁾.

Table 4. Intervention studies using olive oil as a comparator intervention

EVOO, extra virgin olive oil; ml, milliliters; g, grams; MDD, major depression disorder; HAMD, Hamilton Depression Rating Scale; MCS, Mental Component Summary; SF-36, 36-Item Short Form Survey; CES-D-K, Center for Epidemiological Studies Depression Scale Korean version; CGI-S, Clinical Global Impression Scale; CGI-I, Clinical Global Impression Improvements; PANSS, Positive and Negative Syndrome Scale; CDSS, Calgary Depression Scale for Schizophrenia; ADHD, Attention Deficit/Hyperactivity Disorder; PBMC, peripheral blood mononuclear cells; HDRS-SF, Short Form Hamilton Depression Rating Scale; BDI II, Beck Depression Inventory II; ASQ, Abbreviated Symptom Questionnaires; DBD, Disruptive Behaviour Disorders; CES-D, Center for Epidemiological Studies Depression Scale; PSS-10, Perceived Stress Scale; YMRS, Young Mania Rating Scale; CDRS-R, Child Depression Rating Scale – Revised; GAF, Global Assessment of Functioning; YGTSS, Yale Global Tic Severity Scale; EPDS, Edinburgh Postnatal Depression Scale; POMS, Profile of Mood States psychometric scale; HDRS, Hamilton Depression Rating Scale; CY-BOCS, Children’s Yale-Brown Obsessive Compulsive Scale.

Lastly, four animal studies used olive oil as a comparator (Table 5). Three were designed to assess the anti-depressant or anti-anxiety effects of fish oil and reported greater benefit in the groups receiving fish oil^(64–66). The fourth study assessed the effects of a pesticide on animals with an olive oil control; a depressogenic effect of the pesticide was reported⁽⁶⁷⁾. None of the studies reported benefit on anxiety or depressive-like behaviour in the olive oil group.

Table 5. Animal studies using olive oil as a comparator intervention

Discussion

This scoping review identified human and animal studies aiming to assess the mental health impact of dietary olive oil and its constituents. All four human experimental studies designed to assess the impact of olive oil on mental health symptoms demonstrated significant improvements in mental health with olive oil consumption. Ten of thirteen human studies using olive oil as a placebo reported some degree of improvement among participants receiving the olive oil control. Nine of 10 animal studies designed to assess the impact of olive oil on mental health symptoms showed significant improvements in anxiety and/or depression-like behaviour. The observational studies yielded somewhat inconsistent associations between mental health and increased consumption of olive oil or higher oleic acid levels.

Among the observational studies, the findings were more consistent among the prospective cohort studies compared to the cross-sectional studies. Among the cohort studies, five of six studies reported a protective effect of higher intake of olive oil. These prospective studies also had larger sample sizes (range: 610–12 059, mean: 6606) compared with the cross-sectional studies (range: 20–7635, mean: 1066). The heterogenous findings in the cross-sectional studies may be due to smaller sample size, uncontrolled confounding variables, reverse causation or other sources of bias that are more common in this study design. Additionally, many of these studies assessed tissue levels of oleic acid rather than olive oil intake. Given that other foods contain oleic acid, the tissue levels might not have provided an accurate indication of olive oil intake. It is noted that four of the five observational studies that reported an association between higher intake of olive oil constituents and increased psychopathology assessed oleic acid tissue levels or intake and not olive oil. Among the human and animal studies, there was a relatively high degree of consistency in the findings, despite significant heterogeneity in the populations, and outcomes assessed. The most commonly reported benefit was a reduction in depression symptoms; however, studies also reported a reduction in symptoms of anxiety, psychosis, bipolar disorder, Tourette‘s, attention deficit-hyperactivity disorder and stress. The doses used in the studies designed to test the effects of olive oil ranged from 15 to 47 grams per day while the doses used as comparators ranged from 280 mg to 8 grams per day; these lower doses may have contributed to the smaller benefits reported. In one study of the Mediterranean diet and cardiovascular disease, the participants in the highest tertial of olive oil intake were consuming 34·6 ± 27·4 grams per day, suggesting that doses used in the olive oil studies were more aligned with the dose obtained with the Mediterranean diet⁽⁶⁸⁾.

Although the mechanism of olive oil’s therapeutic effects is not fully understood, the benefits are suggested to be due to its anti-inflammatory and antioxidant properties. Hydrophilic polyphenols in olive oil, namely oleuropein, demethyloleuropein and ligstroside, have been found to be protective against oxidative damage caused by redox dyshomeostasis^(69,70) . Other compounds in olive oil, specifically 3,4-dihydroxyphenylglycol and hydroxytyrosol, have been found to act as inhibitors on inflammatory pathways⁽⁷¹⁾. Given the known role of inflammation and oxidative stress in mental illnesses pathogenesis, these mechanisms may be responsible for a beneficial effect. Furthermore, some studies have assessed the impact of olive oil on neurotransmitter metabolism. Neurochemical studies show continual olive oil consumption decreases levels of 5-hydroxytryptamine, 5-hydroxyindoleacetic acid and dopamine in the brain and increases levels of homovalinic acid, a metabolite of dopamine⁽⁴⁶⁾. Based on these findings, it is hypothesised that the anxiolytic effects are due to the reduction of 5-hydroxytryptamine synthesis and metabolism in the brain. Conversely, the anti-depressive effects of olive oil are hypothesised to be due to the increase of dopamine release and turnover⁽⁴⁶⁾.

In this review, our search identified many studies that used olive oil as a placebo to examine the effects of other interventions, such as n-3 supplements, on mental health. Ten of 13 human studies reported statistically significant favourable mental health outcomes in both the intervention group and the placebo group. None of the four animal intervention studies that used olive oil as a placebo reported benefit in the mental health outcomes in the olive oil group. As high-level evidence has suggested an anti-depressant effect of fish oil supplements⁽⁷²⁾, the human studies that reported non-inferiority may further add to the evidence of olive oil’s therapeutic potential; however, given that other studies have reported conflicting or uncertain results⁽⁷³⁾, it is very challenging to draw clear conclusions from these studies. Based on these results, olive oil should be considered a poor choice of placebo in mental health research on fatty acids due to its potential therapeutic benefits; this call has been made by other researchers as well⁽⁷⁴⁾.

This scoping review has many strengths related to its thorough and explicit methodology. The credibility of this review is enhanced by the use of a comprehensive literature search, using multiple databases and search terms. Explicit eligibility criteria and duplicate screening minimised bias in the selection of studies for inclusion. Furthermore, this scoping review addresses a gap in the existing literature on the impact of olive oil consumption on mental health. The findings of this review may be used in the design of dietary intervention studies aimed at improving mental health outcomes, which may lead to public health recommendations regarding mental health support.

This scoping review has a few limitations. First, very few human experimental trials designed to assess the impact of olive oil compared to an inert comparator were eligible for inclusion in the analysis. When considering all of the human experimental and observational studies, significant methodological heterogeneity existed. The included studies had a wide array of study designs and methodologies, including different mental health outcomes and methods of assessing olive oil exposure. Populations also varied greatly, specifically in terms of participant characteristics, sample size and country of origin. Each of these studies used varying doses of olive oil for varying durations. Limited detail was provided about the quality, source, purity or composition of the olive oil used. Studies also varied in whether extra virgin olive oil or olive oil was used as an intervention. There is a need for more randomised controlled trials that assess the impact of olive oil intake compared to a control intervention that is either inert or well understood in its impact on mental health, in order to fully understand the potential of this food in a mental health promoting diet.

Conclusions

Olive oil is considered an important component of the Mediterranean Diet, a diet which has been well researched for its role in promoting mental well-being. The findings of this scoping review provide early support for a role of olive oil in improving mental health; however, more research, particularly in the form of high-quality clinical trials is needed.