A meta-analysis of 21 prospective cohort studies found that adherence to a healthy dietary pattern characterised by whole grains, fruits, vegetables, legumes, fish, and limited ultra-processed food was associated with a 35 percent lower risk of depression. The dietary pattern most associated with depression risk was the Western pattern characterised by ultra-processed foods, refined grains, sugar, and red meat. The associations held after controlling for reverse causation and socioeconomic factors.
The Biological Mechanisms
- Gut-brain axis: 90 percent of serotonin is produced in the gut; plant fibre supports the gut microbiome composition that supports optimal serotonin production; disrupted gut microbiome from ultra-processed food diets reduces serotonin production capacity
- Neuroinflammation: depression is increasingly characterised as an inflammatory condition; elevated CRP, IL-6, and TNF-alpha are found in depressed patients; plant-based diets that reduce systemic inflammation reduce neuroinflammation
- Folate and B vitamins: folate from dark leafy greens and legumes is required for serotonin and dopamine synthesis; folate deficiency is significantly more common in depressed populations
- Magnesium: deficiency produces glutamate receptor overactivation that contributes to anxiety and depression; adequate intake from plant foods normalises NMDA receptor function
The SMILES Trial
The SMILES RCT (Supporting the Modification of lifestyle In Lowered Emotional States) randomised 67 adults with major depression to either dietary counselling toward a Mediterranean whole food diet or social support. After 12 weeks, 32 percent of the dietary intervention group were in remission from depression compared to 8 percent in the social support group. This is one of the first RCTs to demonstrate that dietary change can produce remission from major depression.