Is There a Relationship between Vegetarianism and Seasonal Affective Disorder? A Pilot Study

Seasonal affective disorder (SAD), winter type, is an almost yearly recurring depression, which occurs in autumn or winter with recovery in spring or summer [1]. In the first study describing this syndrome, Rosenthal et al. [2] mentioned decreased physical activity along with affective symptoms. Other common symptoms are hypersomnia, increased appetite, carbohydrate craving, and increased weight. The treatment of first choice for SAD, winter type, is light therapy (LT). The effectiveness of LT is well-established with high response rates and minor adverse effects [3,4].

Generally, in winter, people with SAD complain of fatigue and a lack of energy. Fatigue and tiredness are common symptoms for a variety of other diseases that vary in degrees from mild to severe, and to disabling complaints like chronic fatigue syndrome [5], but they can also be caused by physiological deficits. A vitamin B₁₂ deficiency is a well-known cause of a lack of energy [6], and people who do not eat animal food are at risk for vitamin B₁₂ deficiency [7,8].

During their depression, patients with SAD selectively eat more carbohydrates, particularly sweets, but also starchy and fibre-rich foods [9,10]. Clarys et al. [11] showed that vegetarians eat less protein and fat and more carbohydrates and fibre than non-vegetarians. Thus, patients with SAD and vegetarians would seem to eat more carbohydrates on average than the normal population. SAD patients often have appetite and weight issues, but also dysfunctional eating attitudes. A significant correlation was found between seasonal body weight changes and body dissatisfaction [12].

Baines et al. [13] reported that, even though vegetarians are physically healthier than non-vegetarians, they seem to have poorer mental health. Vegetarians show a higher prevalence of depression, anxiety disorders, and severe tiredness. In Western cultures, it appears that a vegetarian diet is associated with an elevated risk for mental disorders, cancer, or allergies [14]. However, there is no evidence for a causal role of a vegetarian diet in the aetiology of mental disorders [15]. Some researchers have found that adolescent vegetarians are more likely to attempt self-harm than non-vegetarians [16]. Since vegetarians are more prone to develop mental health issues and SAD patients are more likely to have dysfunctional eating attitudes, it would be interesting to investigate whether a relationship between SAD and vegetarianism exists. In the Finnish general population, 1.4% is vegetarian [17] and 3.6% suffer from SAD [18], and in the Dutch general population, 4.5% are either vegetarians or vegans [19] and 3% suffer from SAD [20]. So far, no research has been done on the relationship between SAD and vegetarianism.

We hypothesize that there are relatively more vegetarians in the SAD population than in the general population. We also hypothesize that the seasonal variations in mood, sleep, social activities, weight, appetite, and energy levels in vegetarian SAD patients are greater than in non-vegetarian SAD patients. Finally, we hypothesize that among the people who follow a vegetarian diet, there are more SAD sufferers than among those who follow a non-vegetarian diet.

Data available from a Dutch SAD outpatient clinic and the Finnish national FINRISK 2012 study were used. In both groups, the seasonal variations in mood and behaviour were assessed with the Seasonal Pattern Assessment Questionnaire (SPAQ) [21].

In Finland, the FINRISK 2012 study included 9,905 randomly drawn individuals aged 25-74 years from the population register of 5 large regions [22]. Of these, 5,827 (59%) invited agreed to participate in a health examination survey. They were given a self-administered questionnaire that included the SPAQ and a Food Frequency Questionnaire (FFQ) containing a question on adherence to a vegetarian diet. The FFQ was based on self-assessment and was to be filled out at home. In total, 4,905 (49.5%) returned the SPAQ and 4,860 (49.1%) returned the FFQ. The dataset was collected between January and April 2012. The total number of participants who filled out the SPAQ and the FFQ was 4,578 (2,062 males aged 53.3 ± 13.7 years; 2,516 females aged 51.2 ± 13.9 years). FINRISK 2012 was conducted according to the guidelines of the Declaration of Helsinki and the Ethics Committee of the Hospital District of Helsinki and Uusimaa approved the research protocol. Written informed consent was obtained from all participants.

The outpatient clinic for SAD patients of the University Center for Psychiatry (UCP) of the University Medical Center Groningen (UMCG), The Netherlands, has existed for nearly 3 decades. Many patients who follow the programme of the outpatient clinic have done so for a number of consecutive years. Patients fill out the SPAQ every year at the start of the season. In this paper, we only use the information of the first season that the patient filled out the SPAQ.

From September 2013 to April 2016, 257 patients (71 males aged 39.7 ± 15.5 years; 186 females aged 36.7 ± 12.8 years) received the diagnosis of major depressive disorder with seasonal pattern according to the criteria of the DSM-IV [1], and filled out the SPAQ [21]. These patients were not on antidepressant medication. In this population, another question was added, asking patients whether they were vegetarian or not.

The SPAQ is a retrospective instrument, which aims to measure seasonal mood changes [21]. This self-rating instrument consists of 7 sections on seasonal changes of some key variables. Part of the SPAQ is the Global Seasonality Score (GSS), consisting of 6 questions and providing a composite measure for change across the seasons of mood, social activities, appetite, sleep, weight, and energy. Item scales range from (0) “no change” to (4) “extremely marked change”. The total scale ranges from 0-24. The existence and severity of seasonal complaints and a certain GSS score as measured with the SPAQ are used in epidemiological studies to assess the prevalence of SAD [20,23].

The validated FFQ in the FINRISK 2012 study measured the participants' habitual diet over the previous 12 months [24]. The FFQ also contained a question about self-assessed diet adherence, including vegetarianism. However, although individuals may report themselves as being vegetarian, they may still consume meat [17]. We therefore checked the meat consumption of the participants who reported being vegetarian on the FFQ forms. Those who consumed meat (beef, game, lamb, meat products, offal, pork, or poultry) were removed from the dataset. In order to measure the influence of this correction on the results, the results for vegetarians were reported with 2 different variables: the self-reported proportion of vegetarians (based solely on a participant's own assessment) and the corrected proportion of vegetarians. The term “no special diet” refers to omnivores, i.e., they do not follow any special diet but eat all kinds of food.

The Finnish data was presented for the total sample and separated according to sex. Participants were divided into vegetarians and non-vegetarians among SAD sufferers and non-SAD sufferers, to examine whether there were more vegetarians among SAD sufferers than among non-SAD sufferers based on their scores on the SPAQ. The data was then divided into SAD sufferers and non-SAD sufferers among vegetarians and non-vegetarians, to examine whether there were more SAD sufferers among the vegetarians.

The data on the Dutch group was divided into vegetarians and non-vegetarians. Comparisons were made using the total scores and the item scores of the SPAQ. Descriptive statistics and logistic regression analysis were carried out with SPSS v21. Logistic regression analyses were adjusted for gender in the Finnish data and for age and gender in the Dutch data.

In the Finnish population, the proportion of vegetarians among SAD sufferers was 4.9% (men 2.8%; women 5.6%) and their corrected proportion was 3.8% (men 0%; women 4.8%), while among those not suffering from SAD, the proportion of self-reported vegetarians was 1.2% (men 0.6%; women 1.7%) and their corrected proportion was 0.8% (men 0.3%; women 1.7%) (Table 1). In a logistic regression analysis, SAD was also a significant predictor of vegetarianism in the total sample (p = 0.000, OR 3.9, 95% CI 1.84-8.45) as well as in women (p = 0.006, OR 3.2, 95% CI 1.39-7.23) but not in men (p = 0.16).

In the Finnish general population, the proportion of self-reported vegetarians was 1.4% (corrected 1.9%) and 12.9% (men 7.1%; women 14.6%) of the self-reported vegetarians suffered from SAD (Table 2). In a logistic regression analysis, vegetarianism was a significant predictor of SAD in the total sample (p = 0.000, OR 3.9, 95 CI 1.81-8.36) and in women (p = 0.007, OR 3.1, 95 CI 1.37-7.16) but not in men (p = 0.16).

The GSS on average ± standard deviation (SD) was 5.7 ± 3.7 (men 4.0 ± 3.4; women 6.0 ± 3.6) for the self-reported vegetarians; after correction, it was 6.2 ± 3.6 (men 4.6 ± 2.8; women 6.5 ± 3.7); and for the non-vegetarians, it was 5.2 ± 3.2 (men 4.5 ± 3.1; women 5.8 ± 3.2). The FFQ-corrected numbers for the vegetarians emphasize the differences on an item level. In a logistic regression analysis predicting vegetarianism with the separate GSS item scores and total GSS score, none of the GSS item scores or the total GSS showed a significant relationship with vegetarianism (p > 0.05).

The number of vegetarians in the SAD population of the Dutch SAD outpatient clinic was 12.5%, higher than in the general population (4.5%) [19]. The GSS on average ± SD was 12.7 ± 4.4 for the vegetarians and 13.5 ± 3.5 for the non-vegetarians. A logistic regression analysis predicting vegetarianism in the SAD population with the separate GSS item scores and total GSS score, with age and gender as a co-variate, showed a significant relationship between seasonal loss of energy and vegetarianism (p < 0.05). No significant contributions of age (p = 0.23) and gender (p = 0.09) were found. Based on the explained slope values, an increase of 1 point on seasonal loss of energy relates to an OR of 1.5, showing that, when SAD patients have 1 point more energy loss they are 1.5 times more likely to belong to the group of vegetarians. None of the other GSS item scores or the total GSS showed a significant relationship with vegetarianism.

The percentage of people who are vegetarian and suffer from SAD in Finland was 4 times higher than the percentage of people suffering from SAD in the normal population, and also higher than in people suffering from SAD with other diets. The percentage of vegetarian SAD patients in an outpatient clinic in the Netherlands was 3 times higher than the percentage of vegetarians in the normal population [19]. These findings suggest that there is a link between vegetarianism and SAD. In the Dutch population, a significant effect was found for the value of the seasonal loss of energy on the chance of belonging to the group of vegetarians. Seasonal loss of energy is the key symptom in SAD, and this finding therefore supports the idea that the preference for this diet is, in fact, related to SAD.

It has been hypothesized that the neurotransmitter serotonin plays a role in SAD and a seasonal variation in serotonergic functions has been described [25,26]. Animal studies show that protein in meals can influence the tryptophan concentrations and serotonin synthesis in the brain [27], and that this serotonin synthesis may have some influence on serotonin-linked behaviour [28]. In humans, positive effects of the augmentation of a tryptophan-rich hydrolyzed protein in the diet on mood under physiological stress have been described [29]. However, the role of tryptophan in the serotonergic system of SAD patients is not clear [30], as is the case with the role of serotonergic systems in the pathogenesis of SAD [31]. The amount of tryptophan in a group of vegetarians was higher than in a group of carnivores, but the relationship between food intake and plasma concentration was questioned [32].

Vegetarians are more preoccupied with health [33] and often have particular views on killing animals [34]. Nowadays, health concerns would seem to also play a role in the decision to become a vegetarian [35]. It may be that, in line with these views and concerns, vegetarians object to antidepressants and therefore look for non-pharmacological treatments. This may be a reason why we found there to be a high percentage of vegetarians in the SAD outpatient-clinic.

The goal of this study is heuristic and its design has its limitations. The Dutch SAD outpatient clinic population is not representative of all SAD sufferers in the Netherlands. In both the Netherlands and Finland, the question about special diet adherence (is a subject vegetarian or not?) is based on self-ratings and can be subject to misinterpretation (e.g., [17,35]). The participants in the Dutch group were diagnosed SAD sufferers according to the DSM criteria; the Finnish SAD subjects were defined according to their SPAQ scores. It is well-known that the use of self-rating instruments overestimates the number of SAD sufferers when compared to the use of a clinical interview [36]. The figures presented in this paper are far from conclusive, but provide similar data on populations from 2 different countries, indicating that food preferences may influence mood among subjects suffering from SAD. Therefore, the data should at least be considered as a starting point for further investigation, as vegetarians may be at a greater risk of developing SAD. One way of determining this may be a prospective longitudinal study using a mood and eating-habits diary.

The authors are grateful to Josie Borger for the improvement of the English.