Eating junk food increases the risk of becoming depressed, a study has found, prompting calls for doctors to routinely give dietary advice to patients as part of their treatment for depression.

In contrast, those who follow a traditional Mediterranean diet are much less likely to develop depression because the fish, fruit, nuts and vegetables that diet involves help protect against one of the commonest mental health problems, the research suggests.

Published in the journal Molecular Psychiatry, the findings have come from an analysis by researchers from Britain, Spain and Australia who examined 41 previous studies on the links between diet and depression.

Dr Camille Lassale, the study's lead author, said that bad diet heightens the risk of depression to significant extent.

The analysis found that foods containing a lot of fat or sugar, or was processed, led to inflammation of not just the gut but the whole body, known as "systemic inflammation". In that respect the impact of poor diet could be compared with that of smoking, obesity and lack of exercise.

One in six adults in the UK are thought to experience depression, often alongside anxiety. The Centre for Mental Health has estimated the illness's overall cost to society, including lost productivity as well as treatment, is £105bn a year.

Increasingly, more GPs are recommending that their patients try to make sensible diet and lifestyle changes as part of a holistic approach to the management of chronic diseases because of the positive effects on patient's physical as well as mental health.