Longitudinal research shows association between progressively higher glycaemic index diet and incidence of depressive symptoms. Experimental exposure to diets with high glycaemic load increases depressive symptoms in healthy volunteers, with moderately large effect.

Mechanism could be repeated and rapid changes in blood glucose, triggering counter regulatory hormones such as cortisol, adrenaline, growth hormone, glucagon.

Appears to be an inflammatory response to high glycaemic index foods too. Adherence to Mediterranean diet reduces markers. Mood disorders have been linked to heightened inflammation, although only in a minority. Observational studies show people with depression score higher for “dietary inflammation” viz trans fats, refined carbohydrates, lower intake of omega 3 fats. Mediated through polyphenols, polyunsaturated fatty acids?

Diet also affects microbiome, which interacts with the brain in bidirectional ways using neural, inflammatory and hormonal signalling pathways. High fibre, polyphenol, unsaturated fats promotes microbial taxa that generate anti-inflammatory metabolites such as short chain fatty acids.

Study of probiotics in healthy volunteers found altered response to a task that requires emotional attention, and may even reduce symptoms of depression.

But no benefit in large trial of Medierranean diet with subclinical depressive symptoms, only small trials of current depression showed benefit. Note context of people’s expectations regarding food/diet, which will likely have a marked effect on wellbeing.

Danger too of stigmatisation if trying to change an individual’s dietary choices.

[Joseph Firth, BMJ 2020;369:m2382]]