Inflammation is the body’s response to illness or injury; the swelling around a cut or a bruise is part of this process. The inflammatory model of depression suggests that, for some people, depression may be caused or exacerbated by inflammation.
Supporting this theory is the finding that people with depression tend to have higher levels of markers of inflammation in their blood. In addition, inducing an inflammatory immune response (by injecting participants with particles of bacteria) promotes the development of depressive symptoms in placebo-controlled trials.
Though inflammation is most obviously the response to a pathogen, it also occurs in response to other non-injury factors such as prolonged stress, an imbalance of bacteria in the gut, or poor diet.
The Dietary Inflammatory Index® (DII®) is a scale that measures the inflammatory potential of dietary patterns. On this scale diets high in saturated fatty acids, trans fats and cholesterol are ranked as pro-inflammatory. In contrast, diets higher in vegetables, fruits, fibre/wholegrains, herbs and spices such as ginger, garlic and turmeric, green and black tea, and dietary patterns such as the Mediterranean diet are associated with lower biomarkers of inflammation. It should be noted that the DII® scores overall dietary intake rather than highlighting specific nutrients.
Landmark studies such as the ‘SMILES Trial’ and ‘HELFIMED’ have demonstrated that dietary modification may be a useful depression intervention for many patients. A new study, using the DII®, seems to add to these results, indicating a role for nutrition in depressive illness. This study followed people for eight years, tracking their diet and their risk of developing depressive symptoms.
3,648 participants (1,577 males, 2,071 females; mean age: 60.6 years) with/at risk of knee osteoarthritis took part. Participants filled out a food frequency questionnaire, indicating how often they ate certain foods from ‘never’ to ‘every day’. This data was used to give each participant a ranking on the DII®. They were also assessed for depression.
At the start of the study (baseline), those with the most pro-inflammatory (higher scores on the DII®) diets also had higher depression scores. Over the course of the eight-year follow-up period 837 people developed depressive symptoms, and the incidence of depression was significantly higher in people with the most pro-inflammatory diets at baseline, potentially indication a cumulative effect of inflammation on depression. In further analysis, being female and having a higher BMI were also associated with increased incidence of depressive symptoms during follow-up. The authors report that participants with the highest DII score had a 24% increased risk of developing depressive symptoms over the course of the study.
More research is required to clarify and confirm these results and, since depression is one of the leading global causes of disability, I look forward to more research in this important area of mental health.
Shivappa, N., Hébert, J. R., Veronese, N., Caruso, M. G., Notarnicola, M., Maggi, S., ... Solmi, M. (2018). The Relationship Between the Dietary Inflammatory Index (DII®) and Incident Depressive Symptoms: A Longitudinal Cohort Study. Journal of Affective Disorders. DOI: 10.1016/j.jad.2018.04.014
Shivappa, N., Steck, S. E., Hurley, T. G., Hussey, J. R., & Hébert, J. R. (2014). Designing and developing a literature-derived, population-based dietary inflammatory index. Public Health Nutrition, 17(8), 1689–1696. http://doi.org/10.1017/S1368980013002115