Fruit and Vegetable Intake and Mental Health in Adults: A Systematic Review

1Mental disorders (referred by the International Statistical Classification of Diseases and Related Health Problems (ICD-10) as F00–F99) are indicated as a global problem by the World Health Organization (WHO). One in four individuals suffers from a mental disorder during some period of life, and hence, the WHO has recognized mental disorders as a health burden that should be no longer neglected. Within the Comprehensive Mental Health Action Plan 2013–2020 implemented by the WHO, it was indicated that there is a need for, inter alia, evidence-based practice for a life course and a multidisciplinary approach to solve this problem. Taking into account the recommendation of multisectoral actions that should be taken by various specialists from the sectors of health and social services, the potential influence of nutrition should be included.

2Some published studies have concluded that following a properly balanced diet allows maintaining a better well-being and lowers the risk of mental disorders in adolescents and students. This finding was also confirmed by O’Neil et al. in their systematic review. At the same time, a therapeutic role was indicated for the Mediterranean diet in 2 randomized controlled trials—HELFIMED and SMILES—for the adults suffering from depression.

3However, it is not easy to assess single elements of a whole dietary pattern and recognize their effect, as each individual consumes a wide range of various products at the same time. Nonetheless, some attempts have been made to identify the elements of a properly balanced diet, which are also typical for the Mediterranean diet, that may be defined as food products with beneficial effects on mental health and aiding in recovery from mental illness; these include fish and seafood, legumes, leafy greens and other vegetables, olive oil, dairy beverages, and nuts. Similarly, the highest Antidepressant Food Score (AFS), calculated based on the content of 12 nutrients that are related to the prevention and treatment of depressive disorders, was given for vegetables, followed by organ meats and fruits.

4So far, some reviews have assessed the influence of the intake of fruit and vegetable, including a meta-analysis by Liu et al., which focused exclusively on the risk of depression and aroused a great deal of controversy, and a recent meta-analysis by Saghafian et al., which also focused exclusively on the risk of depression. In addition, a systematic review of prospective research, analyzing the effect on psychological health, was performed by Tuck et al., but it focused exclusively on vegetables. A review by Rooney et al. analyzed the role of fruits and vegetables in the broad aspects of psychological well-being, but it was not a systematic review and the authors emphasized the need for a more exhaustive synthesis of studies in the form of a systematic review.

5Fruit and vegetables are beneficial for general health and the recent studies indicate that they may be even more important than it was previously supposed and that to obtain prevention of cardiovascular disease, cancer, and premature mortality even the higher intake than the generally-recommended 400 g is needed. However, the mechanism of their influence on mental health is still unknown, while a number of possible factors that may contribute to the positive impact are indicated. Among them, there are specific nutrients, which are known as such that may be related to mental health and for which fruit and vegetables are indicated as a valuable sources in diet, such as complex carbohydrates and fiber, being associated with glycemic index, C vitamin, B vitamins, carotenoids, potassium, and polyphenols. The other explanations are associated with either possibility of reverse mechanism (higher level of mental health may promote better diet, including higher fruit and vegetables intake), or psychological explanation (following better diet, including higher fruit and vegetables intake may promote more positive emotions and better mental health).

6Taking this into account, the present study aimed to conduct a systematic review of the observational studies analyzing the association between the intake of fruit and vegetables and the broad aspects of mental health in adults.

7The search used Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and the review was registered in International Prospective Register of Systematic Reviews (PROSPERO) database (CRD42019138148). A systematic review of peer-reviewed observational studies published until June 2019 was performed in PubMed and Web of Science databases with the additional manual search conducted via references of found studies.

8The present systematic review analyzed the association between the intake of fruit and/or vegetable and mental health in adults. The studies included were those that assessed the intake of fruit or vegetables (conducted using various methods) to specify the amount consumed (supposed reason), together with an assessment of mental health (various factors, both self-assessed and diagnosed) (supposed consequence) and an analysis of the association between the indicated factors. The studies which analyzed the reverse association (assessment of the intake of fruit or vegetables as influenced by mental health) were not included, but those that did not define the supposed reason and consequence and considered only the intake of fruit or vegetables and mental health as coexisting factors were included.

9With respect to the intake of fruit and/or vegetables, studies that assessed the intake of fruits and/or vegetables, or their processed products (e.g., juices), either as a measure expressed in grams or as the number of portions, were included. Studies involving the analysis of highly processed products (e.g., jam, beverages other than juices) were excluded. Only those studies presenting the observations of habitual intake, with no short-term interventions, were included. The studies that did not assess single products (intake of a specific fruit or vegetable) or groups (total intake of fruits and/or vegetables) but assessed the general dietary patterns were analyzed; however, they were not included in the present analysis if there was no separate assessment of the intake of fruit or vegetables.

10With respect to mental health, studies that assessed all the aspects of mental health in both healthy participants and those with physical health problems were included. However, studies conducted in groups of patients with (1) intellectual disabilities, (2) dementia, and (3) eating disorders were excluded. Only studies conducted in humans and especially adults were included.

11The included studies presented the research conducted in all the countries, independent of the location and income, but only those published in English, in peer-reviewed journals were taken into account.

12The search for studies was performed in PubMed and Web of Science databases; the strategy applied for electronic search in both databases is presented in Supplementary Table S1. In addition, a manual search for publications was conducted via analyzing the references of the found studies. The search focused on gathering observational studies published until June 2019.

13Studies were independently extracted by 2 researchers and screened based on the inclusion and exclusion criteria using a 2-stage procedure: (1) Studies were verified based on the title; and (2) studies included based on the title were verified based on the abstract. Any disagreement between the two researchers over including a particular study was resolved through discussion with a third researcher. The full texts of these potentially eligible studies were retrieved (by asking the corresponding author for the full text, if needed) and independently assessed for eligibility by the two researchers. Again, any disagreement between them over the eligibility of a particular study was resolved through discussion with a third researcher.

14The procedure of identification, screening, assessment of eligibility, and inclusion is presented in Figure 1.

15Data extraction was carried out independently by two researchers. Any disagreement between them over specific data was resolved through discussion with a third researcher. Missing data were requested from the authors of the studies, if possible, while the total number of 46 individual e-mails have been sent to corresponding authors of the included studies, as well as to first authors (if the first author was not corresponding one, but his e-mail address was provided in the full text of article). For 6 studies, authors not only answered, they also provided the requested data (in tables those data are referred as provided by authors on request).

16Data extracted from the studies included the following: General details of the study (author, study design), observation (country/location, study group, time), participants (number of participants, gender proportions, age, inclusion criteria, exclusion criteria), exposure (method of assessment, measure of fruit and vegetable, other fruit and vegetable products included), outcomes (method of assessment, psychological measure), and findings (observations, conclusions).

17In order to assess bias and the general quality of the studies, based on the Cochrane recommendations for the tools for assessing the methodological quality or the risk of bias in non-randomized studies, the Newcastle–Ottawa Scale (NOS) was applied, which is used commonly. Each included study was assessed for the following criteria: For case-control studies—selection (scale from 0 to 4), comparability (scale from 0 to 2), and exposure (scale from 0 to 3); and for cohort studies—selection (scale from 0 to 4), comparability (scale from 0 to 2), and outcome (scale from 0 to 3). The results were interpreted based on the commonly assumed criteria and attributed to the following categories: very high risk of bias (0–3 NOS points), high risk of bias (4–6 NOS points), and low risk of bias (7–9 NOS points).

18The following types of outcomes were included to the presented systematic review: General and mental well-being, quality of life, sleep quality, life satisfaction, and mood (general outcomes); flourishing, self-efficacy, curiosity, creativity, optimism, self-esteem, and happiness (positive outcomes); stress, nervousness, anxiety, minor psychiatric disorders, distress, depressive symptoms, depression, and attempted suicide (negative outcomes). Due to the fact that numerous various outcomes were included, while the number of studies for each outcome differed from only one study, to a lot of studies (as for depression or depressive symptoms), it was not possible to summarize the results in the form of meta-analysis, as it requires including comparable studies only (for one type of outcome). As not only outcomes, but also way to express risk factor (namely fruit and/or vegetable intake), studied populations, and settings were not comparable, the studies may not be treated as sufficiently similar to be able to reanalyze the data in the form of meta-analysis, but in the future, while the number of such studies will be higher, for each outcome, an adequate meta-analysis will be valuable to conclude. Taking it into account, based on the data extracted and the assessment of the general quality of the study, a narrative synthesis of the findings from the included studies was performed, which was structured around the type of outcome.

19The list of studies included to the systematic review is presented in Supplementary Table S2. The basic study details and design of observation for the studies included to the systematic review is presented in Table 1. The studies presented in all the tables are listed accordingly based on the year of publication. Among 61 included studies, the majority were conducted for European countries (20 studies), Asian countries (14 studies), or United States of America (USA) (11 studies), but also those conducted for African (1 study), or South American country (1 study), Australia or New Zealand (7 studies), or Canada (3 studies) were included and some of them were conducted for mixed countries (4 studies). The studied populations were mainly adults (including young, middle aged, and older ones, or only some of indicated groups) (37 studies), as well as only young ones (12 studies), middle aged ones (3 studies), or old ones (9 studies). It should be indicated that 3 included studies were conducted in a specific populations, while the health-related inclusion criteria for the study were defined as prediabetes and/or prehypertension, being after coronary artery bypass grafting surgery, or having excessive body mass.

20The characteristics of the study participants for the studies included to the systematic review is presented in Supplementary Table S3. The characteristics of the study exposure and outcomes for the studies included to the systematic review is presented in the Table 2. Among 61 included studies, for the majority of them, a food frequency questionnaire, or a rapid screener was applied (32 studies), or a specific questionnaire to assess the diet quality (1 study), or a simple question about fruit and vegetable intake (22 studies), but for 4 studies, the method of dietary recall or record was applied, either alone or combined with previously indicated methods, and for 2 studies the applied method was not specified. To assess the mental health, various aspects of it were analyzed in the studies, while either a single aspect or some aspects combined were assessed, including mainly depressive symptoms (18 studies), depression (15 studies), general and mental well-being (9 studies), stress (8 studies), distress (8 studies), quality of life (7 studies), mood (5 studies), or anxiety (5 studies), but also in some of them happiness (4 studies), life satisfaction (3 studies), or optimism (2 studies) were analyzed, and in single studies only: sleep quality, flourishing, self-efficacy, curiosity, creativity, self-esteem, nervousness, minor psychiatric disorders, or attempted suicide were indicated.

21The characteristics of the study findings for the studies included to the systematic review accompanied by the quality assessment based on the total score for the Newcastle–Ottawa Scale is presented in the Table 3. The detailed results of the quality assessment based on the total score for the Newcastle–Ottawa Scale for categories of selection, comparability and exposure/outcome are presented in Supplementary Table S4. Among 61 included studies, for the majority of them, a statistically significant influence of fruit and/or vegetable consumption on mental health was proven, but there were also some studies, for which it was proven only in case of some factors associated with mental health and not for all assessed ones and only few studies, for which it was not proven that there is any association. While concluding, authors of the included studies emphasized the existing association, but also among most important remarks, they indicated risks resulting from not following a recommendation to consume at least 5 portions of fruit and vegetable each day (2 studies), or they indicated especially beneficial groups of fruit and vegetables (2 studies).

22The systematic review of the observational studies analyzing the association between the intake of fruit and vegetable and mental health in adults revealed the possible beneficial influence of the indicated products. This association was studied for various aspects of mental health, ranging from general and mental well-being, quality of life, sleep quality, life satisfaction, flourishing, mood, self-efficacy, curiosity, creativity, optimism, self-esteem, stress, nervousness, or happiness, to anxiety, minor psychiatric disorders, distress, depressive symptoms, depression, or attempted suicide.

23Moreover, the indicated effect was studied and stated not only for fresh fruits and vegetables but also for fruit and vegetable products, such as juices, dried, and canned fruits and vegetables, salads, soups, or ketchup, and even for potatoes in some studies that included it to the total intake of fruits and vegetables. This corresponds to the general conclusions of some studies that not only the intake of fruit and vegetable should be increased, but also at least 5 portions of fruits or vegetables must be taken daily as recommended to observe a positive influence on the general mental health.

24The reason behind promoting the consumption of 5 portions of fruits or vegetables daily is that the Food and Agriculture Organization of the United Nations (FAO) and WHO have recommended to consume a minimum of 400 g of fruits and vegetables per day, excluding potatoes and other starchy tubers, with an estimated serving size of 80 g. This recommendation is also supported by a number of prominent experts, boards, and associations, such as the National Health Service (NHS) in Great Britain, American Heart Association (AHA), Centers for Disease Control and Prevention (CDC), and Office of Disease Prevention and Health Promotion (ODPHP) in the United States of America. Moreover, not only the role of raw fruits and vegetables but also that of the processed fruits and vegetables, including frozen, canned, or cooked ones, as well as juices, is emphasized to meet the recommended intake.

25Increasing the intake of fruits and vegetables to the recommended level may result in a noticeable and measurable effect, as was stated in some included studies. An increase in the consumption of fruits and vegetables by one portion a day leads to a 0.133-unit improvement in the mental well-being assessed by GHQ-12 scale, while the consumption of 7–8 servings a day leads to meaningful changes in positive affect and consumption of 8 portions a day leads to a 0.24-unit increase in life satisfaction (equivalent to the psychological gain of moving from unemployed status to employed). However in general, any increase in the consumption of fruits and vegetables results in the improvement of well-being, enhances happiness, and decreases depressive symptoms, with the strongest effect observed for 6 servings a day, 7 servings a day, or more than 8 servings a day (combined with breakfast every day and 3 meals in addition to 1–2 snacks per day), depending on the studied group.

26It must be indicated that among the included studies, some highlighted not only the general effect of fruits and vegetables but also the influence of specific types, such as citrus, berries, green leafy vegetables, green salad, and tomatoes. Simultaneously, among the raw fruits and vegetables, the following were indicated as specially related to better mental health: Bananas, apples, citrus, berries, grapefruit, kiwifruit, carrots, lettuce, cucumber, and green leafy vegetables, particularly spinach. Authors of the indicated studies have no definite explanation why those fruits or vegetables may be especially valuable, similarly as the general mechanism of the influence on mental health is still unknown. However, it may be supposed that the positive influence should be attributed to a specific nutritional value, as a combination of high content of compounds positive for mental health and, at the same time, low content of those negative for mental health, as indicated by LaChance and Ramsey. However, such assumption does not take into account the potential interactions between nutrients in food product and between food products in the diet, so it must be also emphasized that in fact estimating the independent effect of only one type of food products (fruits and vegetables), or nutrients may be hard to conduct, due to other food products, or nutrients, that are interfering.

27While presenting the results of the included studies analyzing the association between the intake of fruit and vegetable and mental health in adults, it must be emphasized that the risk of bias varied from very high to low (defined based on the commonly assumed criteria) as shown by the total NOS score. Taking this into account, the highest attention must be paid to those studies interpreted as having a low risk of bias and the highest quality. Within such studies, it was observed that high total intake of fruits and vegetables and some of their specific subgroups, including berries, might be associated with a high level of optimism, while this association is independent of interfering factors. High intake of fruits and vegetables was also associated with a higher level of self-efficacy, as well as a low level of psychological distress, ambiguity, and cancer fatalism. In addition, it was observed that high total intake of fruits and vegetables and some of their specific subgroups, including citrus and green leafy vegetables, might be associated with a lower risk of depression, but other health-related factors may also play a role in this association. Furthermore, it was highlighted that increasing physical activity may be necessary to benefit from the positive effect of the consumption of fruit and vegetable to protect against depressive symptoms.

28In spite of the fact that the vast majority of the included studies presented compatible conclusions that mental health benefits can be attained with increased consumption of fruits and vegetables, some limitations of this systematic review must be indicated. The main issue results from the fact, that association between fruit or vegetables intake and mental health does not allow to state unambiguously that the intake influences mental health, as the reverse causation is also possible, so only based on experimental study it may be concluded with no doubts, that such influence exists. Moreover, it must be emphasized that no uniform definition of fruit and vegetables was established in analyzed studies as various fruit and vegetable products were either included or excluded from this group, that may also have influenced the observed association. Last but not least, the included studies were conducted using various methodologies, in different populations, and involved diverse measures of intake of fruits and vegetables. Therefore, further studies should be conducted covering all the aspects of mental health in various populations and using the similar methodology, to analyze the association between the consumption of fruits and vegetables and mental health in detail.

29The vast majority of the included studies indicated that the intake of fruits and/or vegetables and their specific subgroups, as well as processed fruits and vegetables, seems to have a positive influence on mental health. Therefore, the general recommendation to consume at least 5 portions of fruit and vegetable a day may be beneficial also for mental health.