People who eat a lot of fish are less likely to be depressed, new research shows.

Men who had a diet rich in fish saw their risk of depression reduce by a fifth (20 per cent) while women saw their risk decrease by a sixth (16 per cent).

Researchers suggested the omega-3 fatty acids found in fish may alter the structure of brain membranes. They may also modify the activity of the neurotransmitters (chemical messengers), dopamine and serotonin, both of which are thought to be involved in depression. The high quality protein, vitamins, and minerals found in fish may also help stave off the disease, while eating a lot of fish may be an indicator of a healthy and more nutritious diet. However, the researchers cautioned that further studies are needed. Depression affects an estimated 350 million people worldwide, and is projected to become the second leading cause of ill health by 2020, according to the World Health Organisation.

Diet has been recognised as playing an important role in the risk of getting depression, with regular consumption of fruits, vegetables, fish and whole grains thought to have an impact. But no studies have so far looked at the individual components, and the link between fish and depression risk has remained controversial. 

Professor Dongfeng Zhang at the Medical College of Qingdao University, Shandong, China, said: 'The association between fish consumption and risk of depression is controversial.

'Many studies have investigated the associations between food consumption and depression risk. 'Furthermore, a meta-analysis published recently indicated that a healthy dietary pattern, characterised by a high intake of fruits, vegetables, fish and whole grains, was significantly associated with a reduced risk of depression. 

'However, it is not yet clear which component of the dietary pattern would be responsible for the protective effect. He added that fish, as an important source of  n-3 polyunsaturated fatty acids (n-3 PUFAs), which may play important roles in the structure and function of brain and nerve cells, have been reported to be associated with depression in several studies.

However, other studies did not find an association between fish consumption and the risk of mental illness. The new study looked at all relevant worldwide research examining the association between fish consumption and the risk of depression published between 2001 and 2014. Only the European studies backed up the link between high fish consumption and lower depression risk. Professor Zhang said eating fish may be beneficial in in preventing depression, but more studies are needed to investigate whether this association varies according to the type of fish. The exact biological mechanisms by which a high intake of fish reduces the risk of depression is not fully understood, he continued.

He said n-3 PUFAs may chance the brain's structure and alter the activity of serotonin and dopamine. 'In addition, high-quality protein, vitamins and minerals may have a protective effect on depression,' he said. 'Finally, high-fish consumption may also be related to a healthier diet and better nutritional status, which could contribute to the lower risk of depression.'  The specific mechanisms will require large studies to confirm, he concluded.

The findings were based on 16 articles that included 26 studies and involved a total of 150,278 participants. The analysis did not define how much fish should be eaten weekly or how it was prepared. The study was published in the Journal of Epidemiology & Community Health.