A new meta-analysis on depression in vegetarians and non-vegetarians found that on average, vegetarians had a more depressed mood.
There is a complex relationship between our mental health and what we eat. On the one hand, certain diets appear to increase the risk of developing a mental disorder. On the other hand, suffering from a mental disorder could lead to eating more or eating less, or eating different kinds of foods.
One particularly mysterious association is that between depression and being a vegetarian. While some studies showed that vegetarians are more depressed than meat-eaters, others showed the exact opposite.
To unravel this mystery, a German research team (which I am a part of) conducted a new meta-analysis on vegetarian diet and depression that has now been published in the Journal of Affective Disorders (Ocklenburg & Borawski, 2021). A meta-analysis is a form of statistical analysis that integrates the results of many different scientific studies. It has the advantage of a larger sample size, increasing statistical power, and rendering the analysis less likely to be affected by characteristics of individual studies.
In the meta-analysis, data on individual scores in depression questionnaires from 13 different empirical studies that compared vegetarians and non-vegetarians were included. Overall, data from 49,889 participants (8,057 vegetarians and 41,832 non-vegetarians) were included in the analysis, making this a large and statistically robust analysis.
The meta-analysis revealed a statistically significant difference between vegetarians and non-vegetarians, with vegetarians showing higher depression scores than non-vegetarians. Thus, vegetarians, on average, showed a more depressed mood than meat-eaters. But correlation is not causation—and from the association observed in the meta-analysis, it is not possible to determine whether depressive mood leads to a higher probability of becoming a vegetarian, or whether being vegetarian increases the chances of experiencing depressive episodes.
However, the results of a previous longitudinal study on vegetarian diet and depression, anxiety, and somatoform disorders suggest that depressive mood may come first (Michalak et al., 2012). In this study, it was shown that many people started to follow a vegetarian diet only after they got a diagnosis for a mental disorder, not the other way around. Thus, a vegetarian diet may not lead to depression at all. Instead, it may be more likely that people become vegetarian after developing mental health issues. Michalak and co-authors (2012) suggested three possible reasons for this behavior: