More research needs to be carried out to determine the role diet may play in the development of depression, scientists have said.

They believe that this research should be similar to that carried out into diet and cardiovascular disease, as 'depression is similar in many aspects to heart disease'.

"Both are associated with low-grade inflammation, endothelial dysfunction and worse lipid (fat) profiles. This tends to suggest that the underlying causes, such as a diet high in trans fats, are also the same," the Spanish scientists said.

They noted that research already indicates that fast food may increase the risk of depression, while healthier diets, such as the Mediterranean diet, which includes a high intake of fruit, vegetables and olive oil, reduce the risk. However, most of these studies do not show reasons for this.

"It is difficult to be sure that the diet is responsible for depression. It could be that depressed people make bad food choices," the scientists suggested.

Other factors which may influence dietary habits include marital status, lifestyle, medical conditions and genetics.

"To address these issues we need long-term, randomised clinical studies similar to ones successfully conducted for diet and cardiovascular disease risk. Only then will we really understand the impact of diet on depression," the team from the Universities of Las Palmas and Navarra concluded.

They made their comments in the journal, BMC Medicine.

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