Hypothesis

Chronic obesity creates a hepatic choline sink that diverts circulating choline toward phosphatidylcholine synthesis for VLDL export, thereby lowering plasma choline availability for brain Kennedy pathway activity. This reduction impairs neuronal phosphatidylcholine production, elevates diacylglycerol‑derived lipid droplets, and promotes microglial activation, ultimately driving synaptic loss and cortical atrophy independent of genetic Kennedy pathway defects.

Mechanistic Rationale

• Hepatic choline demand rises in obesity – Expanded adipose tissue secretes leptin and inflammatory cytokines (TNF‑α, IL‑6) that upregulate hepatic CTP:phosphocholine cytidylyltransferase (CCTα) and PEMT activity to meet increased VLDL phosphatidylcholine needs for triglyceride export [1].
• Plasma choline falls – As hepatic choline is shunted into VLDL, less choline remains available for transport across the blood‑brain barrier via choline transporters (CHT1). Low plasma choline has already been linked to faster brain aging and elevated NfL [1].
• Brain Kennedy pathway slows – Reduced substrate limits CCTα‑catalyzed conversion of phosphocholine to CDP‑choline, decreasing phosphatidylcholine synthesis. The compensatory PEMT pathway cannot fully offset this deficit under choline‑limiting conditions, especially in aging brains [3].
• Lipid droplet accumulation and microglial activation – Impaired Kennedy flux raises diacylglycerol, promoting lipid droplet formation in neurons and astrocytes. These droplets act as danger‑associated molecular patterns that trigger microglial NF‑κB signaling, amplifying neuroinflammation and tau pathology [4].
• Synaptic retraction and atrophy – Loss of phosphatidylcholine compromises membrane curvature and vesicle fusion, weakening synaptic integrity. Chronic microglial activation then drives synaptic pruning and neuronal loss, measurable as cortical thinning on MRI.

Testable Predictions

1. Cross‑sectional association – In a cohort of mid‑life adults (40‑60 yr), plasma choline concentration will negatively correlate with hepatic phosphatidylcholine secretion rate (measured by VLDL‑TG turnover using stable‑isotope tracers) and positively correlate with brain Kennedy pathway activity (assessed via PET with a CCTα‑specific tracer).
2. Longitudinal trajectory – Individuals with high hepatic choline diversion (high VLDL‑TG secretion / low plasma choline) will show accelerated increase in cortical thinning and NfL over 3‑year follow‑up, independent of baseline APOE‑ε4 status.
3. Intervention effect – Supplementation with CDP‑choline (500 mg BID) for 12 weeks will raise plasma choline, reduce hepatic VLDL‑TG secretion fraction, and normalize brain Kennedy pathway PET signal. Cognitive composite scores will improve relative to placebo, with mediation analysis indicating that changes in hepatic choline flux account for ≥40 % of the cognitive benefit.
4. Mechanistic biomarker – Elevations in neuronal lipid droplet markers (e.g., perilipin‑2‑positive puncta on PET‑compatible lipid droplet tracer) will increase in parallel with declining Kennedy pathway flux and predict subsequent microglial activation (TSPO‑PET signal).

Falsifiability

If longitudinal data show no relationship between hepatic choline diversion metrics and brain Kennedy pathway activity, or if CDP‑choline supplementation fails to modify hepatic VLDL secretion or brain phosphatidylcholine synthesis despite raising plasma choline, the hypothesis would be refuted. Likewise, a lack of mediation by hepatic flux in the cognitive improvement pathway would challenge the proposed mechanistic chain.

References

[1] https://news.asu.edu/20251125-health-and-medicine-surprising-connection-between-obesity-choline-and-brain-inflammation [2] https://www.mcgill.ca/oss/article/medical-student-contributors/ups-and-downs-choline-supplements [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC7762932/ [4] https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2023.1148166/full [5] https://www.menshealth.com/health/a69645530/choline-egg-yolk-brain-aging-study/