Surprisingly enough, this partially exposes the link between depression and some of the autoimmunal diseases. One example is how patients with psoriasis have significantly elevated levels of proteins from the IL-17 family (namely, IL-17A, IL-17C, and IL-17F) - up to 4 to 8 times above nominal values.

At the same time, bimekizumab, one of the bleeding-edge psoriasis and psoriatic arthritis treatments, suppresses production of IL-17A and IL-17F (methotrexate does that, too, albeit to a much smaller degree). As a result, people receiving IL-17 suppressors become happier over the course of years, and not only due to months-long remission - I had a chance to see this in one of the experimental treatment programs.

Related paper on receptors: https://www.cell.com/cell/abstract/S0092-8674(25)00279-X

Paper suggesting mood alteration: https://www.cell.com/cell/abstract/S0092-8674(25)00278-8

I read a study a while ago, the study found common vitamin deficiency in depressed humans.

I researched more about it and looks like some of the deficiency in the vitamins in the study are correlated with higher IL-17 levels.

Carnitine: Seems to have some effect in decreasing IL-17?

Vitamin D: Highly impacts IL-17 (https://pmc.ncbi.nlm.nih.gov/articles/PMC4609465/)

COQ10: Decreased plasma levels of IL-17 (https://www.clinicalnutritionespen.com/article/S2405-4577(22...)

Folic Acid: Reduces IL-17 (https://pmc.ncbi.nlm.nih.gov/articles/PMC8839991/)

Lutein: I didn't find any study that seems to study the effect on IL-17

Citrulline: No relevant studies

This is the study I'm referring to: https://www.nature.com/articles/s41398-023-02696-9

I think we're getting closer to find some blood level correlations, so we can fix them!

I can say for a fact when I take CoQ10, I feel my energy levels going up so it really has some effect.

Related paper on the immune system triggering anxiety: "Stress-Induced Metabolic Disorder in Peripheral CD4+ T Cells Leads to Anxiety-like Behavior" https://doi.org/10.1016/j.cell.2019.10.001

Related: "N-acetylcysteine as a new prominent approach for treating psychiatric disorders" https://doi.org/10.1111/bph.15456 section "2.3 Regulation of inflammatory mediators".

N-acetylcysteine (NAC) is considered safe and is sold as a diet supplement in USA and many other countries. If you suffer from anxious feelings, 1200mg/day NAC may reduce them a lot.

There are multiple theories that focus on different ethiologies of depression. Besides brain networks, early life adversities, sleep disturbances and the now obsolete monoamine hypothesis I read about the inflamation hypothesis. On a higher level that ties into all the other theories quite well and I don't oppose it. But there has been research into TNF-a, IL-6, IL-4, COX-2, and so on. We know there is inflammation, but we don't know where it starts and why. SSRIs are antiinflammatory in general, but why? Is this a downstream effect? Where is the original target we want to hit?

All those possible targets need to be verified by actual treatments. Treating the increased diabetes type 2 risk in depressed people is still advantagous but won't help the depression much. It's like changing tires on a broken car. Pushing is easier, but won't run by itself.