Sickness behaviour
When you are ill, you withdraw, lose appetite, lose interest, and want to sleep. This is not incidental — it is an organised, evolutionarily conserved response driven by inflammatory cytokines acting on the brain. It conserves energy for the immune response.
The overlap with depressive symptoms is close enough that researchers have asked the obvious question: what if some depression is sickness behaviour without an infection?
The supporting data is reasonable. Elevated IL-6, TNF-alpha and CRP appear in roughly a third of people with depression. People treated with interferon for hepatitis develop depression at high rates. And elevated baseline inflammatory markers predict poorer response to conventional antidepressants.
Where the inflammation comes from
Chronic psychological stress raises inflammatory markers directly and measurably. This is one of the clearest mind-to-body pathways in the literature.
Sleep loss does it fast — a few nights of restriction is enough to raise IL-6 in healthy people.
Intestinal permeability allows bacterial components into circulation, triggering an immune response. This is the mechanistic core of the gut-brain conversation, and it is why gut-targeted compounds appear in mood discussions at all.
Visceral adiposity is itself an inflammatory organ, secreting cytokines continuously.
The compounds on this pathway
Thymosin alpha-1 has by far the best evidence, with regulatory approval in over thirty countries and randomised trials in sepsis. It modulates rather than suppresses. No mood endpoint has ever been tested.
KPV inhibits NF-κB and reduces gut inflammation in animal models, with no human trials. Selank affects IL-6 alongside its anxiolytic effects. BPC-157 has anti-inflammatory gut effects in the rodent literature.
None of these has been trialled for depression. The pathway is legitimate; the compounds on it are, for this purpose, untested.