The study, based on survey data from Brazil, chimes with earlier research that found higher rates of depression among those who forgo meat. However, the new study suggests that this link exists independent of nutritional intake.
It may seem straightforward to look at a link between a diet and specific health problems and assume that the former is causing the latter via some form of nutritional deficiency.
Yet the new analysis, published in the Journal of Affective Disorders, took into account a wide range of nutritional factors, including total calorie intake, protein intake, micronutrient intake, and the level of food processing. This suggests that the higher rates of depression among vegetarians are not caused by the nutritional content of their diet.
So what might explain the link between vegetarianism and depression?
Or is the relationship down to something else entirely?
First, it is possible that being depressed causes people to be more likely to become vegetarian rather than the other way around. The symptoms of depression can include rumination on negative thoughts, as well as feelings of guilt.
Assuming that depressed and non-depressed people are equally likely to encounter the upsetting truth of slaughterhouses and factory farming, it is possible that depressed people are more likely to ruminate on those thoughts, and more likely to feel guilty for their part in creating the demand.
The depressed vegetarian, in this case, is not necessarily wrong to think this way. While depression is sometimes characterised as having unrealistically negative perceptions, there is evidence to suggest that people with mild to moderate depression have more realistic judgments about the outcome of uncertain events and more realistic perceptions of their own role and abilities.
Second, it is possible that adhering to a vegetarian diet causes depression for reasons other than nutrition. Even if there is no “happy nutrient” lacking in a vegetarian diet, it could be the case that forgoing meat causes depression through other means.
For example, adopting a vegetarian diet might affect one’s relationship with others and involvement in social activities, and sometimes may be associated with teasing or other forms of social ostracism.
Notably, the new study is based on survey data collected in Brazil, a country famous for its meat-heavy diet. Some survey data has pointed to a sharp increase in vegetarianism in Brazil in recent years, going from 8% in 2012 to 16% in 2018. However, the recent paper surveyed over 14,000 Brazilians and found just 82 vegetarians – scarcely more than half a percent.
One has to wonder if the same link between vegetarianism and depression would be observed in India or other countries where vegetarianism is more of a social norm. More importantly, as the rate of vegetarianism increases in the UK and other developed countries, will we see the relationship disappear over time?
Finally, it is possible that neither vegetarianism nor depression cause the other, but both are associated with some third factor. This could be any number of characteristics or experiences that are associated with both vegetarianism and depression.
For example, women are more likely than men to be vegetarian, and to experience depression. However, the Brazilian study took sex into account, ruling out this particular third variable.
One variable that was not examined, but is plausibly linked to both vegetarianism and depression, is exposure to violent images of the meat industry. Preventing cruelty to animals is the most commonly cited reason vegetarians give for avoiding meat.
Documentaries like Dominion and Earthlings that depict the cruelty in the meat industry cannot readily be described as feelgood films. One can easily imagine that a person who consumes this kind of media would become both vegetarian and, especially when most people choose to look the other way, depressed.
There are several possible reasons for the link between vegetarianism and depression. This new study suggests that vegetarian nutrition is not the cause of depression.
Instead, the vegetarian social experience may contribute to depression, depression may cause an increased likelihood of becoming vegetarian, or both vegetarianism and depression may be caused by a third variable, such as exposure to violent meat industry imagery.
This cross-sectional survey of adult vegans and vegetarians aged 18–44 years found that high PBD quality was associated with decreased risk of depressive symptoms. This study is the first to highlight an association between a self-report plant-based dietary quality measure and depression in Australian adult vegans and vegetarians.
As such, our findings are in line with broad data regarding the protective role of diet in mental health and brain function across young and older adulthood. Research indicates that plant foods are high in complex carbohydrates,49 fibre, probiotics50 and antioxidants such as polyphenols,51 which have been found to decrease symptoms of depression.52–54
However, despite the intake of a PBD in this study of Australian vegans and vegetarians, the overall mean depression score was above the clinical cut-off for depressive symptoms and, as such, indicates an overall sample experiencing depression.39 Given that the targeted age range is also considered to be at high risk of depression in Australia,2 the current finding may reflect a broad population trend.
An association between vegan and vegetarian diets and depression has been largely disparate. Systematic review findings have shown no association between vegan and vegetarian diets and mental health outcomes, including depression and anxiety.28–30 Other empirical and systematic reviews have shown associations between vegetarian diets and depression in adults across the lifespan.22 34 While other meta-analyses suggest that following a vegetarian diet is associated with a decreased risk of depression.34
As such, in line with our findings that show an association between diet quality and non-clinical depressive symptoms. However, no association between diet quality and depression for those meeting the criterion for depressive symptoms. Overall dietary quality could be important for reducing risk.
This is especially important considering vegan and vegetarian diets are typically high in plant foods with a range of health benefits for improved mood and mental health.49–55 However, nutritional deficiencies are also common in these diets, such as lower intake of vitamins B2, B3, B5 and B12, zinc, iodine and long-chain fatty acids that could negatively impact mental health and brain function.56 57
There is a diverse body of research investigating the biological mechanism of effect that links dietary intake and mental health. While not yet fully characterised, there are a range of nutrients and foods within vegan and vegetarian diets that positively affect the pathophysiology of mood, including decreased chronic inflammation and oxidative stress,58–60 and improvements in the gut microbiome brain axis.61–63
For example, sources of fibre through greens, nuts, seeds, legumes and grains, which are prevalent features of vegan and vegetarian diets, are important for managing inflammation, glucose regulation, and promotion of gut microbiota changes.64 65 Conversely, consumption of nutrients such as amino acids tryptophan and tyrosine, and omega-3 polyunsaturated fatty acids (PUFA’s) which have protective effects for neurotransmitter regulation of serotonin and dopamine essential for mood, is highest in fish-eating vegetarians but is limited in vegan diets.66 67
The current study shows that following a PBD does not necessarily mean the individual consumes a ‘healthy’ dietary pattern. This finding is an important extension of the literature that has predominately compared diet type instead of dietary quality. As with other dietary patterns such as low-quality meat-based diets, low-quality Mediterranean dietary patterns and low-quality Ketogenic dietary patterns, this study highlights that there are also low-quality PBD.
Low-quality PBDs include plant-based products that are high in ultra-processed ingredients, refined vegetable oils, fried food, salt, refined grains, alcohol, sugary snacks and drinks.29 68 The consumer distinction between low and high-quality plant-based foods is important given that based on 2020 data, the expanding value of packaged vegan food in Australia is projected to reach approximately $A215 million.69
Consequently, individuals may inadvertently be consuming a diet high in processed plant foods consistent with a low diet quality, which is broadly a known risk factor for increased depression.18 70 71
reference link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8718860/#:~:text=An%20association%20between%20vegan%20and,outcomes%2C%20including%20depression%20and%20anxiety.
Original Research: Closed access.
“Association between meatless diet and depressive episodes: A cross-sectional analysis of baseline data from the longitudinal study of adult health (ELSA-Brasil)” by Ingrid Kohl et al. Journal of Affective Disorders