People who have banished the remains of dead animals from their diet are more likely to suffer from depression than omnivores. The American nutritionist Joseph Hibbeln found this out when he analysed data on almost ten thousand English men. The suspected culprit? A vitamin B12 deficiency.

Study

The men also filled in a questionnaire – the Edinburgh Postnatal Depression Scale to be precise – which detects depression. If you score 12 or more points on the questionnaire there’s a good chance that you are suffering from depression.The researchers used the data on 9668 English men that had been gathered in the Avon Longitudinal Study of Parents and Children. The researchers knew which men ate meat, and which ones were vegan or vegetarian.

Results
Vegetarians and vegans were 67 percent more likely to have an EPDS score of 10 or higher.

Vitamin B12 shortage may make vegetarians depressed

Explanation
One possible explanation for the relationship is that vegetarians and vegans do not get enough omega-3-fatty acids because they have cut fish out of their diet, or that the ratio between omega-6-fatty acids and omega-3-fatty acids leans too far over to the omega-6-fatty acids. That’s one of Hibbeln’s hobby horses. [theguardian.com 17 October 2006]

Reading between the lines the researchers actually are most in favour of the theory that meat avoiders – despite all the information that’s around – still suffer from a vitamin B12 shortage.
Australian researchers stumbled across the relationship between vegetarianism, depression and vitamin B12 five years ago [Psychother Psychosom. 2012;81(3):196-8.], and according to a British study from 2010, 52 percent of vegans and 7 percent of vegetarians have a vitamin B12 deficiency. [Eur J Clin Nutr. 2010 Sep;64(9):933-9.]Vitamin B12 shortage may make vegetarians depressed

Conclusion
“The results presented here do indicate that male British vegetarians did have a greater risk for depressive symptoms after adjusting for multiple confounding variables,” the researchers summarise.

“We also cannot rule out the possibility of reverse causation, that is that having depressive symptoms might change dietary habits and increase the likelihood of being a vegetarian.”

“This study does not resolve the question of whether adoption of a vegetarian diet will increase, or decrease the risk of depressive symptoms or affect mental well-being or what specific nutrients, if any, may influence those risks, but does suggest that a randomized controlled trial of selected nutrients or foods may be warranted.”

Source:
J Affect Disord. 2017 Jul 28;225:13-7.