A review, published in the July edition of the Journal of Epidemiology and Community Health, examined 11 previous studies involving 15,315 participants and concluded that although the research does not prove that low folate causes depression, it does show the two are linked.

Recent research from the same team, led by Dr. Simon Gilbody, published in the American Journal of Epidemiology, showed that people with depression commonly have a gene that processes folate less efficiently.

Folate is linked to the production of some of the "feel good" chemicals in the brain, such as serotonin. The identification of the gene provides a plausible explanation as to why folic acid supplements may help people with depression, according to Gilbody.