The occurrence of depression has increased drastically in the past few decades.
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Suicide, the most serious outcome of depression, is one of the leading causes of death in young men. Several links have been drawn between a shift to poorer diets and an increased risk of depression.
The relationship between food and mental health
Diets have changed drastically in the past decades, creating a global movement away from complex carbohydrates and fiber towards processed foods, saturated fats, and refined sugar. The correlation between diet and depression appears unrelated to the type of cuisine commonly eaten and dietary restrictions, such as vegetarianism.
The pattern also persists for children and teenagers, where poor diets at young ages are more likely to lead to poorer mental health outcomes. There is some debate on whether significant gender differences in the effect of diet on depression exist, with different studies finding support both for and against.
Several studies that have looked at changes in diets to reduce depression have also involved other lifestyle changes, such as exercise and sleep changes. However, these studies did also find the pattern that lifestyle changes do reduce depressive symptoms, meaning improved diet in combination with improved lifestyle can potentially be an effective intervention strategy.
Why diet can be an important factor
Depression has increased by 30% in American teenagers over the past 10 years. Teenagers and children are particularly vulnerable to developing habits that can lead to worse mental health outcomes, and intervention in lifestyle at an early age can help reduce the chance of depression.
Diet intervention presents a viable way to influence a seemingly contributing factor to depression. Certain food components in particular, such as sodium, are generally considered biomarkers of an unhealthy diet. Potassium, on the other hand, can be an indicator of a healthy diet high in vegetables and whole grains. This knowledge can be used to intervene with those groups who appear most vulnerable to depression and poor diet.
Uncertainties and limitations
While several studies and meta-analyses have found correlations and linear relationships between poor diet and depression, the evidence for this relationship being causal is still lacking. So, while poor diet may have some form of direct or indirect relationship to the risk of depression, there is still not much evidence that a poor diet in itself can cause depression.
One of the main drawbacks of studies linking diet to depression is how depression is measured. Several studies use self-reported measures of depressive symptoms or similar, and those studies that do use a clinical depression diagnosis as an outcome measure do not find an association between poor diet and incidence of depression. Only one study so far has found, using a randomized controlled trial, that diet alterations can reduce clinical depression levels.
Furthermore, when a psychiatrist diagnosed depression was used, there was no observed association with diet quality. This means the evidence for any causal relationship is further reduced, and it implies that the outcomes of studies that have used other methods to classify depression have merely shown reductions in symptoms of depression, rather than the mental illness itself.
A third major limitation of many studies that have found a relationship between diet and depression is the fact that baseline depression severity was not always controlled for. When baseline severity was controlled for, no association between diet and depression was found.
Some studies have looked at diet as an intervention strategy, wherein study subjects self-assess their dietary habits. These types of questionnaires are biased, which can help explain the mixed results from longitudinal studies of diet and depressive symptoms.
Another limitation of longitudinal studies of diet and depression is that there can be a shortcoming of appropriate active control groups to compare with. For example, it would generally be unethical to request subjects to eat more unhealthy food to compare. Regardless, the lack of an active control group means that such studies do not have an opposite to compare to, and can only compare to those who continued their typical diet as normal.
Francis, H.M. et al. (2019). A brief diet intervention can reduce symptoms of depression in young adults – A randomized controlled trial. PLoS ONE. https://doi.org/10.1371/journal.pone.0222768
Mrug, S. et al. (2019). Sodium and potassium excretion predicts increased depression in urban adolescents. Physiological Reports. https://doi.org/10.14814/phy2.14213