Inflammation is the body’s response to illness or injury; the swelling around a cut or a bruise is part of this process. But inflammation also happens within and around cells in response to other non-injury factors such as poor diet, prolonged stress or an imbalance of bacteria in the gut.
Inflammatory diseases such as rheumatoid arthritis and inflammatory bowel disease are commonly associated with psychological and behavioural changes such as lack of energy or interest, loss of appetite, and depression.
And this consistent relationship between inflammation and depression has led to the inflammation theory of depression - that for some sufferers inflammation may be contributing to the severity of their depressive symptoms and, in the West, diet quality may be a significant factor in depression risk.
There is a robust association between diet and risk of depression. The Dietary Inflammatory Index (DII) is a scale of how likely certain foods are to promote inflammation in the body.
Broadly, processed foods increase inflammation while whole foods, fruits, vegetables, nuts and oily fish reduce it. A higher score on the DII is linked to higher markers of inflammation in the body.
In one study of over 4000 people researchers found that women with high DII scores were much more likely to develop recurrent depression then those with lower DII scores, and this effect was independent of other health factors.
The SUN Project, a large Spanish cohort study of the relationship between diet quality and mental illness found that participants in the highest quartile for free sugar consumption had the greatest depression risk. This study also found that diets high in nutrients were linked to a reduced risk of depression.
These observational studies highlight the correlations between diet and depression, but questions around the direction of causality remain a limitation of observational analyses. Only randomised controlled trials (RCTs) can show causality.
In December 2017 a paper was published in the journal Nutritional Neuroscience. This 3-month RCT compared dietary modification (FOOD) with social support (SOCIAL) in people with a diagnosis of depression.
Participants in the FOOD group were given cookery lessons, fish oil supplements, and were provided with a Mediterrenean-style diet food hamper every fortnight for the duration of the study. The SOCIAL group also attended fortnightly social meet ups (social support is known to reduce depression severity).
At the end of three months both groups saw improvements in mental health, however the FOOD group had significantly greater improvements in depression and overall mental health-related quality of life compared to the SOCIAL group. These improvements were significantly correlated with improvements in diet quality.
Not only did the FOOD group improve more than the SOCIAL group, but, of the people in the FOOD group, those who adhered more closely to the diet saw greater improvements.
This study was a replication of the earlier SMILES Trial, a landmark study demonstrating that improving diet alongside standard treatment improved depression recovery.
Depression is one of the leading causes of global disease burden and researchers and clinicians alike have struggled to understand why rates of treatment-resistant depression have continued to rise.
But a growing body of research seems to show that diet quality and nutritional status are important factors in mental health. Is it time that psychology made more space for the body in the consulting room?
Akbaraly, T. N., Kerlau, C., Wyart, M., Chevallier, N., Ndiaye, L., Schivappa, N., Hébert, J. R., Kivimäki, M. (2016). Dietary Inflammatory Index and Recurrence of Depressive Symptoms: Results From the Whitehall II Study. Clinical Psychological Science, first published on August 8, 2016 doi:10.1177/2167702616645777
Sánchez-Villegas, A., Henríquez-Sánchez, P., Ruiz-Canela, M., Lahortiga, F., Molero, P., Toledo, E., & Martínez-González, M. (2015). A longitudinal analysis of diet quality scores and the risk of incident depression in the SUN Project. BMC Medicine, 13, 197-197.
Sanchez-Villegas, A., Zazpe, I., Santiago, S., Perez-Cornago, A., Martinez-Gonzalez, M., & Lahortiga-Ramos, F. (2018). Added sugars and sugar-sweetened beverage consumption, dietary carbohydrate index and depression risk in the Seguimiento Universidad de Navarra (SUN) Project. British Journal of Nutrition, 119(2), 211-221. doi:10.1017/S0007114517003361
Parletta, N., Zarnowiecki, D., Cho, J., Wilson, A., Bogomolova, S., Villani, A., Itsiopoulos, C., Niyonsenga, T., Blunden, S., Meyer, B., Segal, L., Baune, B. T. & O’Dea, K. (2017). A Mediterranean-style dietary intervention supplemented with fish oil improves diet quality and mental health in people with depression: A randomized controlled trial (HELFIMED). Nutritional Neuroscience.
Jacka, F. N., O’Neil, A., Opie, R., Itsiopoulos, C., Cotton, S., Mohebbi, M., Castle, D., Dash, S., Mihalopoulos, C., Chatterton, M. L., Brazionis, M., Dean, O.M. Hodge, A. M. & Berk, M. (2017). A randomised controlled trial of dietary improvement for adults with major depression (the “SMILES” trial). BMC Medicine, 15, 23. http://doi.org/10.1186/s12916-017-0791-y