Vegan diet, mood disorders, and the methodological issues

Researcher Network member, Manuela Giugliano, explores the issue of inaccurate and unreliable research in the field of nutrition studies.

Mood disorders, including major depressive disorder (MDD), are highly prevalent mental illnesses affecting mood, energy levels, sleep, and cognitive function (Friedrich, 2017; Grande et al., 2016), thus resulting in significant individual and socioeconomic burdens (Wittchen, 2012).

Genetic and environmental factors contribute to the primary causes of mood disorders, with diet being a possible contributor (Sullivan, 2012; Barros et al., 2017). At the International Society for Nutritional Psychiatry Research (ISNPR), experts stated that “diet and nutrition are central determinants of mental health” and that “nutrition is a crucial factor in the high incidence and prevalence of mental disorders”.

However, although our knowledge about the association between vegetarian diets and physical health relies on numerous studies and more reliable sources (Barnard et al., 2006; Le & Sabate’, 2014; Appleby & Key, 2016; Papier et al., 2019; Chiu et al., 2020), data available on the associations between vegetarian/vegan diets and mental health are still inconclusive and sometimes contradictory.

Nutritional science is considered a recent field of research, but nutrition studies tend to attract public and media attention more than any other kind of study. They often claim that a particular diet or food will be able to prevent or cause a certain disease. However, the conclusions and results and their catchy headlines, are not always so reliable and must not be taken for granted.

A recent article published by the Journal of Affective Disorders found that individuals who followed a vegetarian diet appear to have a higher prevalence of depressive episodes than meat-eaters (Kohl et a., 2022). But let’s glance at the article:

The ELSA-Brazil cohort was composed of 14,216 participants, but only 82 did not/rarely ever consume meat, which represents around 0.5% of the overall sample analysed.

In this study, participants were asked to complete a food questionnaire over the preceding 12 months. But, if you were asked how many fruits and vegetables you ate each day one year ago as well as the portion size, your recall and self-reporting would be most likely inaccurate. A study examining food intake measurements found that very few subjects were able to accurately remember the types and amounts of food they had consumed in the previous 24 hours (Todd, 1983). Thus, this methodology likely leads to inaccurate results and the authors’ conclusion may therefore be invalid.

Participants in this study were asked about their intake of food listed in a questionnaire, the Food Frequency questionnaire (FFQ), which even if it is a popular tool in nutritional studies is not designed for vegan diets. For example, in the dairy section, there is only one option, soy milk, but no other vegan dairy options such as alternative milk (almond, oat, coconut), vegan cheese or vegan yogurt, excluding many potential sources of food for vegans.

In addition, in this study, the duration of the specific food habits and the onset of depressive symptoms were not investigated, which makes it very difficult to establish inferences regarding the associations between a vegetarian diet and depressive episodes.

In 2021 an article titled “Meat and mental health: a systematic review of meat abstention and depression, anxiety, and related phenomena”, concluded that the study does not support meat avoidance as a strategy to benefit psychological health.

The review examined 18 research studies: 11 concluded that meat abstention was associated with poorer psychological health, 4 studies were equivocal, and 3 studies demonstrated that meat abstainers had better outcomes.

However, among the selected studies biases were very common and even the authors concluded that “there were numerous issues that reduced the confidence in the published results”. A large number of studies were excluded from the analysis, and, also the selected studies were not fully representative. For example, in one of the studies concluding that the prevalence or risk of depression and/or anxiety (or related symptoms) was significantly greater in participants who avoided meat consumption, of a total of 4,181 study subjects, only 54 were “complete vegetarians” or 1.3% of the sample analysed.

In addition, scrolling to the funding section of the paper it is clearly stated that “This study was funded in part via an unrestricted research grant from the Beef Checkoff, through the National Cattlemen’s Beef Association”, which could represent a conflict of interest. 

A systematic review published in 2022 identified 25 studies that investigated the association between vegetarian or vegan diets and depression also revealing conflicting evidence (Jain et al., 2022)

11 studies concluded that vegetarian or vegan diets are associated with higher rates of depression, 7 suggested that these dietary patterns are associated with lower risk and symptoms of depression and 7 found no association between vegetarian diets and depression.

However, among the 25 studies, only 4 were rated as good in terms of quality of research: one study associated vegetarian/vegan diets with either no risk or a higher risk of depression (depending on the population), and three studies associated these diets with a lower risk of depression.

Nutrition studies have the potential to add critical perspectives to public health, however, they often use inaccurate methodologies and suffer from self-recall bias. They tend to be sensationalised in the media, and it is, therefore, important to scrutinise a study’s methods and ethics and determine if similar studies have been done to ensure their validity and accuracy. 

Considering that this is a relatively new field of research, and given the heterogeneity of the studies, further research is necessary to determine whether a causal relationship exists between vegan diets and mood disorders. Higher-quality studies using consistent tools to measure diet and depression will ensure that the findings are comparable and delineate any relationship if it exists. In addition, longitudinal studies are necessary to investigate the long-term effects of vegetarianism/veganism on mental health, potentially resulting in dietary guidelines on preventive and therapeutic strategies for individuals prone to or suffering from mood disorders.

The views expressed by our Research News contributors are not necessarily the views of The Vegan Society.

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