Vegetarians are twice as likely to experience depression as those eating a regular balanced diet, a new study finds.
The longer people followed a vegetarian diet, the higher their depression scores, the researchers found.
It is possible that the link is down to poor nutrition.
Vegetarians typically have low levels of vitamin B12 in their diet.
Indeed, around 50% of vegans have a vitamin B12 deficiency, while 7% of vegetarians have the deficiency.
Vitamin B12 is found in red meat and has been linked to mood problems.
Vegetarians also typically eat more nuts, which contain omega-6 fatty acids: these have been linked to mental health problems.
The study's authors write: "Other potential factors include high blood levels of phytoestrogens - consequent mainly on diets rich in vegetables and soya.
Another potential contributing factor is that lower intakes of seafood are thought to be associated with greater risk of depressive symptoms." The results come from a study that examined 350 committed vegetarians among almost 10,000 men.
The study was not able to rule out the possibility that people who are depressed are more likely to become vegetarian.