This large and well designed clinical study performed in men confirms results previously reported from surveys in women by showing that people who identify themselves as vegetarians or vegans are more likely to be depressed. Previous studies had shown that such an increased risk also applies to anxiety disorders.
Like all published studies, the current report doesn’t address the question of causation: Are individuals with increased depression or anxiety more likely to chose a vegetarian diet, or do the nutritional deficiencies associated with a strict vegetarian diet increase the risk to develop mental disorders. Not knowing the scientific answer to this question, I personally favor the former hypothesis.
There is no question that largely plant based diets have significant and well documented health benefits for a wide range of disorders, including mental disorders (https://www.amazon.com/Mind-Gut-Connection-Conversation-Impacts-Choices/dp/0062376551/). However, such diets as the traditional Asian diets, the Mediterranean diet all have a component of meat consumption, even though it comes predominantly from fish and poultry sources.
Thus, if you chose the vegetarian diet exclusively for health reasons, and possibly environmental reasons, you may want to consider switching to one of the predominantly plant based diet with a small component of fish and possibly poultry. if you made the decision primarily based on ethical consideration, you should pay close attention to supplement your diet with essential nutrients.