Studies of diet and depression have focused primarily on individual nutrients.
To examine the association between dietary patterns and depression using an overall diet approach.
Analyses were carried on data from 3486 participants (26.2% women, mean age 55.6 years) from the Whitehall II prospective cohort, in which two dietary patterns were identified: ‘whole food’ (heavily loaded by vegetables, fruits and fish) and ‘processed food’ (heavily loaded by sweetened desserts, fried food, processed meat, refined grains and high-fat dairy products). Self-reported depression was assessed 5 years later using the Center for Epidemiologic Studies – Depression (CES-D) scale.
After adjusting for potential confounders, participants in the highest tertile of the whole food pattern had lower odds of CES-D depression (OR = 0.74, 95% CI 0.56-0.99) than those in the lowest tertile. In contrast, high consumption of processed food was associated with an increased odds of CES-D depression (OR = 1.58, 95% CI 1.11-2.23).
In middle-aged participants, a processed food dietary pattern is a risk factor for CES-D depression 5 years later, whereas a whole food pattern is protective.
I wonder how much of this is confounded by IQ and conscientiousness, i.e. a healthy user effect. Those more prone to depression may be less intelligent, less conscientious, and less apt to care about whether their food is healthy or not. Nevertheless, a healthy diet is likely to supply the brain with proper nutrition, so a link is plausible. From the full paper:
There are several plausible mechanisms underlying the association we observed between the whole food pattern and self-reported depression. The high content of antioxidants in fruits and vegetables could be protective,20 as some studies have shown higher antioxidant levels to be associated with lower depression risk.21 The potential protective effect of the whole food diet could also come from folate found in large amounts in some cruciferous vegetables (broccoli, cabbage, Brussels sprouts), leafy vegetables (spinach), other green vegetables (asparagus, avocado) and dried legumes (lentil, chickpea).22 It has been suggested that low levels of folate might increase the risk of depression and result in reduced availability of S-adenosylmethionine, a universal methyl donor, which can result in impaired formation of myelin, neurotransmitters and membrane phospholipids.23 In line with this, a large study of Finnish middle-aged men found an increased risk of depression in participants with lower dietary intake of folate.7 However, some studies have found no association between folate levels and depression in elderly populations.6 A further plausible mechanism involves fish consumption. The whole food dietary pattern includes a high intake of fish and there is evidence suggesting an association between high levels of fish consumption and low incidence of depression.24 This protective effect of fish consumption has been traditionally attributed to its high content of long-chain ω-3 polyunsaturated fatty acids.25 These are a major component of neuron membranes and have vascular and anti-inflammatory properties. Evidence of this association has come from observational studies that have shown an inverse association between ω-3 fatty acid levels measured in blood or estimated from intake and depression.3,4 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. A previous cross-sectional study has shown a correlation between sugar consumption and the annual rate of depression in six countries.26 Furthermore, the processed food diet is very close to the ‘ Western’ pattern defined in the American population,8 which has been shown to be associated with higher risk of coronary heart disease and inflammation.8,27 Several lines of investigation have suggested that coronary heart disease and inflammation are involved in the pathogenesis of depression.28,29 However, further studies are needed to improve our understanding of the association between processed food intake, the inflammation process and depression.