A small but increasing number of people in Western countries are choosing to restrict meat for various reasons. While in countries such as India a high proportion (35%) of the population follows a vegetarian diet due to cultural and religious traditions [1], rates in Western countries are much lower. However, a considerable minority of populations in Western countries do not consume meat. In a methodologically sound study [2] 1.6% of respondents in a representative sample of 20.000 Germans reported being vegetarians [2]. Estimates for US and UK samples are slightly higher (3%) [3, 4].

During the past decades, increasing knowledge has emerged about the effects of vegetarian diet on nutritional status and physical health. Taken as a whole, studies have shown that vegetarians are in good physical health compared with national averages and as healthy as non-vegetarians with a comparable background and lifestyle [5, 6]. This outcome can be explained by the more health-conscious behaviors of vegetarians and by the fact that vegetarian diets are often healthy with the respect to such factors as fat composition [7] and fiber [8].

Although our knowledge about the association between vegetarian diet and physical health is based on numerous studies, relatively little data is available on the associations between vegetarian diet and mental health. Diverse processes could in principle produce differences between vegetarians and non-vegetarians in rates of mental disorders. On a biological level, nutrition status resulting from vegetarian diet may affect neuronal function and synaptic plasticity, which in turn influences brain processes relevant for onset and maintenance of mental disorders [9, 10]. For example, there is strong evidence that long-chain n-3 fatty acids causally affect risk for major depressive disorders [11, 12]. Moreover, although evidence is less unequivocal, vitamin B 12 levels appear to be causally linked to major depressive disorders. Studies have reported that vegetarians show lower tissue concentrations of long-chain n-3 fatty acids [13, 14] and vitamin B 12 [15, 16] which may elevate risk for major depressive disorder.

Besides differences in nutrition status, vegetarians and non-vegetarians differ in a number of psychological and socio-demographic characteristics that may influence their risk for mental disorders. Vegetarians are predominantly female [17], are more likely to live in urban areas and to be single [18]. All these socio-demographic factors are correlates of the presence of mental disorders [19]. Moreover, vegetarians tend to be more aware of the factors influencing their dietary intake and of the importance of a healthy lifestyle in general. A number of studies have shown that vegetarians tend to be slimmer, drink less alcohol, and exercise more than non-vegetarians [20–23]. In addition, vegetarians tend to define themselves negatively by emphasizing what they do not do; they tend to stress their dissimilarity from others and thereby accentuate their differences from the general society [24]. Moreover, some vegetarians base the choice of their diet more on an ethical motivation [25]. Thus, some psychological and socio-demographic characteristics of vegetarians, such as negative self-definition and dysfunctional eating attitudes, could have detrimental effects on mental health; other characteristics could have beneficial effects, such as a healthy lifestyle and ethical motivation.

In summery, any associations that may be found between vegetarian diet and mental disorders could be attributable to several possible causal mechanisms: (a) the biological effects of diet have an influence on brain processes that increases the chance for the onset of mental disorders, in which case it could be expected that adopting a vegetarian diet would precede the onset of mental disorders; (b) relatively stable psychological characteristics independently influence the probability of choosing a vegetarian diet pattern, and developing a mental disorder, in which case the adoption of the diet and the onset of a mental disorder would be unrelated; or (c) developing a mental disorder increases the likelihood of choosing a vegetarian diet, in which case the onset of the mental disorder would precede the vegetarian diet. Although published findings on that type of relationship are missing, it is conceivable that individuals with mental disorders are more aware of suffering of animals or may show more health-oriented behaviors (e.g. adopting a vegetarian diet) in order to positively influence the course of their mental disorder.

Few empirical studies have directly tested associations between vegetarian diet patterns and mental health. We could locate only seven studies of mental health in vegetarians which all were limited in their measures of mental health, as they were based only on self-report, and with no formal diagnosis. Moreover, participants were not broadly representativeness of the community population, because they were either adolescents, young adults, or drawn from a special population. Five of seven studies analyzed characteristics of vegetarian adolescents compared to non-vegetarian adolescents: (a) Using a single item measure of depression, Larsson et al. [17] found that vegetarian adolescents were more often depressed during the previous week; (b) in the study of Perry, McGuire, Neumark-Sztainer, and Story [26] vegetarian adolescents were more likely to have contemplated and attempted suicide (assessed with a single item) than non-vegetarians. However, no significant differences were found in current depression symptoms (assessed with a 7-item scale) between the groups. Furthermore, vegetarians more frequently reported having been told by a physician that they had an eating disorder; (c) in another study by Neumark-Sztainer, Story, Resnick, and Blum, adolescent vegetarians reported more deviant eating behaviors, including higher rates of dieting, intentional vomiting and laxative use (all single item measures) [27]; (d) in addition, Bas, Karabudak and Kiziltan [28] found abnormal eating attitudes, low self-esteem, high social anxiety, and high trait anxiety (all assessed with psychometrically sound self-report measures) in Turkish vegetarian adolescents; (e) in a study with adolescents and young adults (age: 15–23 years) Robinson-O'Brien, Perry, Wall, Story, and Neumark-Sztainer, [29] showed that current vegetarians may be at increased risk for binge eating with loss of control (assessed with two questions), while former vegetarians may be at increased risk for extreme unhealthful weight-control behaviors (assessed by asking participants whether they showed one of the following behaviors during the past year: “took diet pills,” “made myself vomit,” “used laxatives,” and “used diuretics”).

Baines et al. [18] investigated mental health in a representative sample of young (22–27 years old) Australian women. Vegetarians reported higher levels of depression during the previous 12 months (single item measure) and were more often diagnosed with depression by a doctor than were their non-vegetarian counterparts. It should be noted that vegetarian and non-vegetarian women also differed in socio-demographic characteristics (i.e., vegetarians were more likely to live in urban areas and to be single).

The only study we found to report better mental health in vegetarians was conducted by Beezhold, Johnston, and Daigle in a sample of Seventh Day Adventist (i.e., members of a Christian denomination) adults [30]. In this study, vegetarian Adventists reported significantly less negative emotions than did non-vegetarian Adventists, as assessed by the Depression Anxiety Stress Scales and the Profile of Mood States [31]. The discrepancy between this study and the previously cited studies may be due to the special population in the Beezhold et al. study of member of a small religious community, unlike participants in the other studies who were drawn from more general populations. Although not all Seventh Day Adventists are vegetarians, vegetarianism is highly valued and commonly practiced among Seventh Day Adventists, giving followers of this diet strong coherence and high status in their peer group. In contrast, most vegetarians in Western societies consciously have to reject the opinions of the majority (i.e., eating meat), an act possibly more isolating from than joining with others [24].

Previous studies on mental health of vegetarians have been restricted to specific groups, young age ranges, and mostly in people unrepresentative of the community. This poses a problem with regard to the average adult population, for whom dietary issues or vegetarian orientation may have a different significance compared to individuals of younger age. No study to date has investigated mental health in a representative adult sample encompassing a broad range of age groups. Furthermore, the studies so far are limited in the assessment of mental health exclusively by self-report questionnaires. No study to date has used clinical diagnoses of mental disorders based on standardized diagnostic interviews. In addition, most studies are limited to a set of specific behaviors (e.g., depressive symptoms, deviant eating behavior) and thus fail to provide a broader perspective on the mental health status of vegetarians in Western cultures.

The present study aimed to investigate associations between vegetarian diet and mental disorders in a representative sample of adults between 18 and 65 years of age. We used data from the German National Health Interview and Examination Survey and its Mental Health Supplement (GHS-MHS), a nationwide epidemiological study of both somatic and mental health in Germany in a representative community sample [32]. Extending previous research, we were interested in a broad spectrum of mental disorders (unipolar depressive disorders, anxiety disorders, somatoform disorders and syndromes, and eating disorders) which were assessed by a standardized individual face-to-face diagnostic interview for mental disorders by clinically trained interviewers according to the criteria in the Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV) [33]. Although the GHS-MHS was cross-sectional, participants’ retrospective age of onset of their mental disorders and of their vegetarian diet enabled us to investigate whether diet change precedes or follows the onset of mental disorders. In addition, we analyzed the consumption of various food products (meats, vegetables and fruits, fast food, fish) in individuals with different mental disorders. We expected that, consistent with previous studies on vegetarian diet and mental health, vegetarians would exhibit more mental disorders. We further predicted that people with mental disorders would have lower frequencies of meat intake.