Research on the association between diet and depression has focused primarily on isolated nutrients. Recent years have seen a move away from analyzing associations between isolated nutrients and health to consideration of the effects of dietary pattern¹. To the best of our knowledge, our study² was the first to investigate prospectively the association between dietary patterns and the occurrence of depressive symptoms five years later, assessed by the well validated CES-D scale³ (Center for Epidemiologic Studies Depression), in a large British middle aged population, by taking into account a large range of potential cofounders.

Main findings. Analyses were carried on the 3,486 participants (26.2% women, mean age 55.6 years) from the prospective Whitehall II study (WIIS), which originally included London based office workers⁴. Two dietary patterns, derived from using principal component analysis, were identified: the “whole food” heavily loaded by high intake of vegetables, fruits and fish; and the “processed food” which was heavily loaded by high consumption of sweetened desserts, chocolates, fried food, processed meat, pies, refined grains, high fat dairy products and condiments.

After adjusting for a large range of sociodemographic and socio-economic factors, health behaviors and health status factors, participants in the highest tertile of the “whole food” pattern had lower odds of CES-D depression (Odds Ratio=0.74, 95% CI:0.56-0.99) than those in the lowest tertile. By contrast, participants with a high intake of “processed food” had higher odds of CES-D depression compared to those with the lowest intake (Odds ratio = 1.58, 95% CI: 1.11-2.23). Furthermore, no association was observed between previous reports of depression (using depression subscale of General Health Questionnaire and use of antidepressive drugs) and dietary patterns assessed 6 years later (p=0.24 for the “whole food” pattern and p=0.92 for the “processed food” pattern) suggesting that the dietary pattern – CES-D depression associations described in this study were due to an effect of diet on depression and not the reverse.

Plausible mechanisms

The association between the whole food pattern and self-reported depression can be explained by the protective effect of:

1. high level of antioxidants found in fruits and vegetables, as previous studies have shown higher antioxidant levels to be associated with lower risk of depression⁵.
   
2. folate found in large amounts in green vegetables and dried legumes⁶. It has been suggested that low levels of folate may reduce the availability of Sadenosylmethionine, which can result in impaired formation of myelin, neurotransmitters and membrane phospholipids, and might increase the risk of depression⁷.
   
3. fish on low incidence of depression⁸, which is traditionally attributed to its high long chain omega 3 poly-unsaturated fatty acids content⁹.

Finally, it is also possible that the protective effect of diet on depression comes from the cumulative and synergic effect of nutrients from different sources of foods rather than from the effect of one isolated nutrient.

The deleterious effect of “processed food” on self reported depression is a novel finding. Its high content of sugar can be one underlying pathway as a positive correlation between sugar consumption and the annual rate of depression¹⁰. Furthermore the “processed food” diet is very close to the “Western” pattern identified in American population1 which has been shown to be associated with higher risk of CHD1 and inflammation¹¹ possibly involved in pathogenesis of depression. Further studies are needed to better understand the association between “processed food” intake, the inflammation process and depression.

Conclusion

The results suggest that fruits, vegetables and fish consumption afford protection against the onset of depressive symptoms five years later, while a diet rich in processed meat, chocolates, sweet desserts, fried food, refined cereals and high fat dairy products increases vulnerability. These findings suggest that existing healthy eating policies will generate additional benefits to health and well-being, and that diet should be considered as a potential target for the prevention of depressive disorders.