Depression is a major cause of disability worldwide and the impact of diet on mental health is raising increasing interest. Two studies presented in this issue have examined the relationship between dietary patterns and the presence of depressive symptoms in large samples of adults.

Both studies evidenced that dietary patterns rich in processed food, sugar and refined grains were associated with higher levels of depressive symptoms. By contrast, the British study showed that a healthy diet characterized by high intake of vegetables, fruits, and fish was associated with lower depressive symptoms, whereas no protective dietary pattern could be evidenced in the Australian study. A common limitation of these studies is their cross-sectional design which cannot rule out reverse causality.

Nevertheless, as explained in the accompanying review, these findings have biological plausibility. Depression is more prevalent among people with high plasma homocysteine. Folate and other B vitamins found in green leafy vegetables and whole grain can lower plasma homocysteine. Long-chain omega 3 polyunsaturated fatty acids found in fish play a major role in brain function and structure. Conversely, a high glycemic load diet is associated with worsening of mood.

The clear suggestion is that dietary intake of vegetables, fruits and fish, provide a set of nutrients that could contribute to improve well-being. As often noted in nutrition, the sum of the parts is greater than the whole.

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