‘Vegetarians have around twice as many depressive episodes as meat-eaters, according to a new study.

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? Is there some non-nutritional mechanism that makes the former cause the latter? 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…

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…

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…’

— via IFLScience