Gaps, cross-references & frontiers
A critical map of what we don't know, the cross-domain connections no one has tested, and the research running right now. The opportunity: depression science is rich in single-exposure associations and poor in causal, interaction-aware evidence.
The biggest gaps & contradictions
- Serotonin was dismantled, not replaced — no falsifiable successor mechanism.
- Neuroinflammation causal direction is contested — IL-6 MR is positive in some studies, null in others; instrument validity is debated.
- BDNF contradicts itself by brain region (down in hippocampus, up in amygdala).
- Antidepressants: same effect size (SMD≈0.3), opposite conclusions; published trials overstate by ~50%.
- Gut microbiome: strong in animals, ambiguous and heterogeneous in humans.
- Exercise & psychedelic trials can't be blinded — expectancy may explain much of the effect.
- Childhood adversity is the strongest risk factor yet the hardest to prove causal (can't be genetically instrumented).
- Vitamin D & omega-3 associations shrink to null in prevention mega-trials.
- Missing heritability: twin ≈37% vs GWAS ≈8–10%; PRS predicts ~2–3%.
- One label, hundreds of disorders: DSM-5 admits 227 symptom combinations; biotypes don't map onto it.
Unexplored cross-references (testable now)
Microplastics → gut → kynurenine
Dietary microplastics may shift the microbiome to divert tryptophan to neurotoxic kynurenine. Test: fecal microplastics + 16S + plasma kynurenine/tryptophan + PHQ-9.
PM2.5 × genetic risk → kynurenine
A confirmed gene×pollution interaction (OR≈3.2) may be mediated by kynurenine. Test: UK Biobank PM2.5 × PRS × metabolomics.
Ultra-processed food as a bundled vector
Emulsifiers + microplastics + low fibre + glycaemic load + aluminium may jointly raise IL-6. Test: composite UPF index + IL-6 + PHQ-9.
Cadmium as the hidden smoking mediator
Much of smoking's effect may run through cadmium, not nicotine. Test: NHANES cadmium + selenium + PHQ-9 mediation.
PFAS × thyroid × circadian
PFAS suppress thyroid hormones that entrain clock genes. Test: NHANES PFAS + free T4 + actigraphy (all variables already exist).
Fatty liver (MASLD) → brain
UPF-driven liver inflammation may cause an anhedonic "metabolic depression." Test: UK Biobank liver-fat MRI + PNPLA3 Mendelian randomization.
What's running now — the pipeline
- Psilocybin (COMP360) met its endpoint in both Phase 3 trials for treatment-resistant depression; a possible first psychedelic approval is on the 2026–27 horizon. It works via neuroplasticity, not "detox."
- Inflammation-stratified treatment (e.g. tocilizumab in elevated-CRP patients) is in trials — pushing toward biomarker-guided care.
- PGC depression GWAS (688,808 cases, 697 loci) now implicates specific neuron types — a GWAS-to-drug-target pipeline.
- Exposome at scale (EU EXPANSE; NIEHS HHEAR) makes ExWAS feasible across millions.
- Metabolic psychiatry — Oxford's ketogenic-diet RCT reported a remission signal in treatment-resistant depression.
- Microplastics & brain — moving from confirmed bioaccumulation toward exposure–outcome studies.