Updated: Apr 28, 2022
Key reference: Lee MF, Eather R, Best T. Plant-based dietary quality and depressive symptoms in Australian vegans and vegetarians: a cross-sectional study. BMJ Nutrition, Prevention & Health. 2021;e000332. Doi: 10.1136/bmjnph-2021-000332
Depression and other mood disorders are significant predictors of suicide and are responsible for over 800,000 global suicide-related deaths per year (WHO, 2017). Pharmacology and psychotherapy represent the primary treatments for depression, however, approximately one-third of patients do not respond to these interventions (Al-Harbi, 2012). Lifestyle factors such as a healthy diet and exercise are increasingly recognised for their favourable impact on symptoms of depression and may represent a cost-effective supplement to existing treatment options.
A large number of studies have explored the relationships between different dietary patterns and mental health disorders such as depression. However, few studies have yet considered how diet quality within these specific dietary patterns influences their relationship with mental health disorders. A recent cross-sectional study published in BMJ Nutrition, Prevention and Health explored this question in the context of plant-based diets (PBDs).
What is diet quality?
The term ‘diet quality can be challenging to define, and there is disagreement on exactly how to measure it. Broadly, it refers to the nutritional adequacy of an individual’s dietary pattern. Dietary patterns scoring high on diet quality are generally considered to involve a healthy intake of fresh fruit, vegetables, nuts, seeds, legumes, whole grains, water, lean meats, and dairy, along with a low intake of red and processed meats and foods high in lower-quality ingredients such as refined vegetable oils, salt, refined grains, alcohol, sugary snacks, and drinks.
PBDs represent one category of dietary patterns and can be divided into two primary categories: vegetarian and vegan diets. Australia has seen a surge in the number of people adopting plant-based lifestyles, with over 2 and a half million Australians choosing to go meat-free. This equates to 12% of the total population, with the largest proportion of plant-based dieters being young adults between 18 and 45.
Adopting a vegetarian or vegan diet offers environmental benefits, including reduced greenhouse gas emissions and land and water use (Scarborough et al. 2014). There are also clear animal welfare benefits, and many reported health benefits. However, whilst there is growing evidence on the association between PBDs and lowered disease risk, these diets are not innately synonymous with a “healthy” diet.
A healthy PBD is characterised by high consumption of fresh fruit, vegetables, nuts, seeds, legumes, whole grains and water, and the omission of meat and/or dairy. In parallel, unhealthy PBDs are characterised by the omission of meat and/or dairy, but also by consumption of foods high in lower quality ingredients such as refined sugars and fats.
Diet and depression
There is strong evidence from dietary intervention studies that healthy diets rich in fresh fruits, vegetables, nuts, seeds, whole grains, and legumes are associated with reduced symptoms of depression (Firth et al. 2019). However, evidence regarding the association specifically between PBDs and depression is inconsistent and conflicting. According to a number of literature reviews, some studies observe that vegans and vegetarians are at increased risk of depression whilst others indicated those who adhered to PBDs had decreased depressive symptoms (Iguacel et al. 2021, Jain et al. 2020). The reasons for these conflicting results remain unclear.
From a mechanistic standpoint, adoption of a PBD may improve mood and reduce risk of depression due to the rich abundance of health-promoting nutrients in plant foods, such as complex carbohydrates, fibre, polyphenols, and antioxidants. Plant foods containing these nutrients have been found to decrease chronic inflammation and oxidative stress, and improve the gut microbiome-brain axis. In addition, PBDs may increase risk of deficiency of certain micronutrients, including B vitamins, zinc, and iodine, as well as certain amino acids and polyunsaturated fatty acids. These deficiencies could negatively impact mental health and brain function, for example by disrupting regulation of serotonin and dopamine.
The studies responsible for these conflicting results give little attention to diet quality and its influence on depression, and instead group different people’s vegetarian or vegan diets together. However, as discussed, the potential benefits and risks of following a PBD may be dependent on the foods and nutrients eaten within the confines of a PBD, rather than the PBD itself. This need for greater consideration into diet quality rather than whole dietary patterns underlies the purpose of this study by Lee et al. (2021).
Study design and results
The study explores the association between diet quality of vegans and vegetarians and depressive symptoms in 219 Australian adults aged 18-44. It used a cross-sectional design and took the form of an online, anonymous, quantitative survey. The Centre for Epidemiological Studies Depression (CESD) survey was used to measure symptoms associated with depression, whilst dietary quality was measured using an adapted Dietary Screening Tool. Results from this dietary survey of vegans and vegetarians revealed a great deal of variation in diet quality between participants. This supports the idea that following a PBD does not necessarily mean that person is following a ‘healthy’ diet.
Results also showed a significant relationship between diet quality, BMI, and depressive symptoms when analysed as categorical valuables. Comparing BMI, CESD score and diet quality scores as continuous variables revealed that variation in diet quality was significantly associated with the variation in CESD score in individuals below the clinical cut-off for depression. However, for individuals with scores over the clinical cut-off for depression, diet quality score did not significantly influence CESD score, although BMI did. This suggests that plant-based followers above the clinical cut-off score for depressive symptoms tended to have higher BMI’s and this covariant could be associated with diet quality and/or prevalence of depressive symptoms, with the exact dynamics of this relationship warranting further investigation.
"These findings suggest that a high-quality plant-based diet may be protective against depressive symptoms in vegans and vegetarians."
Thus the findings show an association between diet quality and non-clinical depressive symptoms. Concerningly, the expanding value of packaged vegan food in Australia is projected to reach approximately $A215 million (Hinton T, 2021) and consumption of vegan food high in lower-quality ingredients such as refined vegetable oils, fried food, salt, refined grains, alcohol, sugary snacks, and drinks is increasing. Individuals may therefore inadvertently be consuming a diet high in plant-based foods consistent with lower diet quality, which is broadly a known risk factor for increased depression. Consumer understanding of and access to high-quality plant-based foods may therefore be important if dietary interventions are to support traditional therapies for depression and other mental health disorders.
It is important to consider the limitations of this study. The cross sectional-nature of the design means causality cannot be inferred. Specifically, little attention has been given to the motivations for following a PBD. Individuals may decide to adopt a PBD after the onset of a mental disorder symptoms, and thus results may be a product of reverse causality. Similarly, vegans and vegetarians are likely to have concerns regarding animal welfare and environmental sustainability, and may therefore carry a burden of awareness that could contribute to depressive symptoms.
Furthermore, as data were self-reported, recall bias may exist that relates to the adherence to the diet. Finally, with over 90% of participants identifying as female, and a large majority being partnered, having a university degree, and feeling they were healthier than their peers, there was a lack of heterogeneity in the sample, which limits the generalisability of the study’s interpretation.
Whilst depression and other mood disorders remain to be significant health challenges, emerging research is revealing how diet and other healthy lifestyle factors may positively influence such conditions. Much of the research investigating the relationship between PBDs and mental health disorders has considered whole dietary patterns, and little attention has been paid to factors such as diet quality. Using tools to measure depression and diet quality, this study observed a relationship between high-quality vegetarian and vegan diets and decreased depressive symptoms. This study is the first to highlight such an association in this population of Australian adult vegans and vegetarians. As such, the findings are in line with broad data regarding the protective role of diet in mental health and brain function.
Following a PBD does not necessarily mean following a “healthy” diet.
Future studies looking at the relationship between dietary patterns and mental health disorders should consider diet quality.
In line with existing research on diet quality and mental health disorders, there appears to be a relationship between high-quality PBDs and reduced depressive symptoms.
References and further reading
Al-Harbi KS. Treatment-resistant depression: therapeutic trends, challenges, and future directions. Patient Prefer Adherence. 2012;6:369–88. Doi: 10.2147/PPA.S29716
Firth J, Marx W, Dash S, Carney R, Teasdale SB, Solmi M, Stubbs B, Schuch FB, Carvalho AF, Jacka F, Sarris J. The Effects of Dietary Improvement on Symptoms of Depression and Anxiety: A Meta-Analysis of Randomized Controlled Trials. Psychosom Med. 2019;81(3):265-280. Doi: 10.1097/PSY.0000000000000673.
Hinton T. Value of packaged vegan food Australia 2016-2020, 2021.
Iguacel I, Huybrechts I, Moreno LA, Michels N. Vegetarianism and veganism compared with mental health and cognitive outcomes: a systematic review and meta-analysis. Nutr Rev. 2021;79:361–81. Doi:10.1093/nutrit/nuaa030
Jain R, Degremont A, Philippou E, Latunde-Dada GO. Association between vegetarian and vegan diets and depression: a systematic review. Proc Nutr Soc. 2020;79. Doi:10.1017/S0029665119001496
World Health Organization. Depression and other common mental disorders: global health estimates, 2017. Available from: https://apps.who.int/iris/bitstream/handle/10665/254610/W?sequence=1