Jakobsson 2026 — Microbial bile acids correlate with anti-SGC IgG and disease severity in FM

One-paragraph summary

The most mechanistically consequential paper in the autoimmune anchor set. Building on the same group's prior anti-SGC IgG work (Krock, Kosek, Svensson — Karolinska), Jakobsson et al. quantified 24 bile acids and 11 short-chain fatty acids in serum from 35 FM patients and 32 matched healthy controls using LC-MS, and correlated these against anti-SGC IgG titers, FM symptom severity, and mental-wellbeing scores. FM patients had significantly elevated levels of non-conjugated, microbially produced (secondary) bile acids vs. controls. Total BA was higher in the high-anti-SGC-IgG subset of FM patients vs. the low-anti-SGC-IgG subset. Specific BAs correlated with disease severity and with poorer mental wellbeing. This provides the missing molecular intermediate in a previously bridging-tier hypothesis: gut microbiome → secondary bile acids → IgG autoantibody production → SGC activation → FM symptoms. It is the first paper in the project's evidence base to instantiate the full B2 (microbiome → glia) chain with a specific molecular mechanism.

Claims as triples

• bile_acid — present_in → fm_autoimmune [evidence: secondary BAs elevated in FM vs HC; confidence: emerging]
• bile_acid — correlates_with → IgG_fm [evidence: total BA higher in high-anti-SGC-IgG subset; confidence: emerging]
• bile_acid — correlates_with → widespread_pain [evidence: specific BAs ↔ disease severity; confidence: emerging]
• bile_acid — correlates_with → depression [evidence: specific BAs ↔ mental wellbeing scores; confidence: emerging]
• gut_brain_axis_disruption — causes → bile_acid (alteration) [evidence: secondary BAs are by definition microbial products; elevation indicates dysbiosis; confidence: emerging]

Methods note

Cross-sectional case-control design. n = 35 FM, 32 matched HC. Serum quantification via LC-MS/HRMS for 24 BAs and 11 SCFAs. Anti-SGC IgG quantified by immunocytochemistry. Correlation analyses against clinical scales. Karolinska group with established expertise in the autoimmune-FM hypothesis.

Limitations

• Cross-sectional — cannot establish temporal direction. Microbiome dysbiosis may cause IgG autoantibody production, IgG might cause microbiome shifts via gut-immune interaction, or both may share an upstream cause.
• Small n (35 FM); replication needed for the high-IgG subset stratification (sample size of high-IgG group not stated in available abstract).
• The correlation between BAs and anti-SGC IgG is suggestive but doesn't establish causality. The mechanistic chain (BA → immune dysregulation → autoantibody production) is implied by the literature but not directly demonstrated in this study.
• 11 SCFAs were also quantified but the abstract focuses on BAs — SCFA findings need to be checked in full text.

Open questions raised

• What is the temporal sequence? Longitudinal microbiome + IgG measurements would discriminate microbiome → IgG vs. IgG → microbiome.
• Does microbiome modulation (FMT, prebiotic, antibiotic) reduce anti-SGC IgG titers? (Closing experiment for B2.)
• Which specific BAs correlate most strongly with disease severity? (Druggability question — bile acid receptors TGR5 and FXR have specific agonists/antagonists already in clinical use for other conditions.)
• Do the same BA elevations occur in long COVID patients with anti-SGC IgG positivity?

Triangulation notes

• This paper closes part of bridge B2 (microbiome → glia). The chain is now: gut_brain_axis_disruption → bile_acid → IgG_fm → satellite_glia → autoantibody_mediated_pain. Each link has at least one direct evidence paper. The bridge is no longer purely speculative — it has a candidate molecular intermediate.
• Combined with Hanani 2026 (multi-ganglia activation) the autoimmune subtype now has both an upstream mechanism (microbiome → BA → IgG) and a downstream anatomical map (DRG, TG, NG, SCG → multi-system symptoms).
• Key triangulation: this is the most plausible candidate for a causal cure path in the project so far. If microbiome modulation reduces anti-SGC IgG titers, and anti-SGC IgG is the proximal cause of multi-system FM symptoms (per Hanani 2026), then microbiome-targeted therapy is a causal, not symptomatic, intervention.

Bridges

• B2 (Microbiome ↔ FM via SCFA → vagal → glia) — substantially closed. The bile-acid intermediate is more specific than SCFA and provides direct evidence in FM patients. Recommend updating the bridge entry to "closing" status.
• B4 (ME/CFS ↔ FM via post-viral neuroimmune) — if BA elevations are also present in ME/CFS, this strengthens the shared-mechanism hypothesis.