A. Strongly Defensible, Direct CNS Evidence
These thresholds are either explicitly supported in CNS literature or converge tightly with well‑established neurobiological risk data.
Serum B12 ≥ 500–550 pg/mL
Status: Strongly defensible
- WHO lower limit ~480 pg/mL; Japan ~500 pg/mL explicitly targets nervous system health
- Neurologic injury (white matter changes, slowed conduction, cognitive decline) documented in patients with “normal” U.S. levels <500pg/mL
- U.S. cutoffs were historically designed to prevent anemia, not CNS injury
[nyheadache.com], [perniciousanemia.org]
Verdict: Gold‑standard TMP example.
Copper ≥ 100 µg/dL
Status: Very strong
- Copper deficiency causes myelopathy, peripheral neuropathy, cognitive impairment, very often without anemia
- Neurologic injury may be irreversible if prolonged
- CNS demyelination parallels B12 deficiency syndromes
[mayoclinic…edings.org], [cmaj.ca]
Verdict: Strongly CNS‑protective and patient‑safety justified.
Fasting Homocysteine ≤ 10.0 µmol/L
Status: Strong
- Review literature increasingly supports ≥10 µmol/L as a neurotoxic threshold
- Associated with:
- White‑matter loss
- Cognitive decline
- Depression, psychosis, dementia
- Many modern reviews explicitly question older 15 µmol/L “normal” cutoffs
[mdpi.com], [discovery.ucl.ac.uk]
Verdict: Defensible for CNS protection.
Whole Blood Thiamine ≥ 150 nmol/L
Status: Strong
- Thiamine deficiency directly impairs cerebral glucose metabolism
- Marginal deficiency produces neurocognitive symptoms well before Wernicke syndrome
- Whole‑blood testing targets active thiamine diphosphate
[emedicine….dscape.com]
Verdict: Clearly CNS‑protective.
Omega‑3 Index ≥ 6%
Status: Strong
- Psychiatric risk rises sharply below 4–5%
- Depression, psychosis, dementia consistently associated with low index
- 6% represents a safety buffer, not an extreme target
[frontiersin.org]
Verdict: Defensible and conservative.
B. Well‑Supported, Conservative CNS Protection
These values exceed minimal deficiency definitions but are still squarely grounded in neurobiology.
25‑OH Vitamin D ≥ 50 ng/mL
Status: Reasonable
- Vitamin D influences neurotrophins (BDNF), immune modulation, and white‑matter integrity
- This threshold aims to avoid chronic insufficiency, not push into toxicity
[mdpi.com], [academic.oup.com]
Ferritin ≥ 50–75 ng/mL
Status: Defensible
- Brain iron is required for:
- Myelination
- Dopamine synthesis
- Mitochondrial function
- Psychiatric symptoms occur long before anemia
- Lower ferritin correlates with depression, anxiety, fatigue, cognitive inefficiency
[link.springer.com], [jaacap.org]
RBC Folate ≥ 500
Status: Defensible
- RBC folate better reflects CNS tissue availability than serum
- Supports methylation, myelin maintenance, monoamine synthesis
- Defensive buffer against homocysteine neurotoxicity
[bestpractice.bmj.com]
C. Biologically Rational, CNS‑Protective Extrapolations
Ascorbate ≥ 40 µmol/L
- Deficiency impairs:
- Myelin formation
- Neurotransmitter synthesis
- Antioxidant defense
- Cognitive performance correlates with plasma sufficiency above ~30 µmol/L
[frontiersin.org], [mdpi.com]
Comment: biologically coherent but less guideline‑codified.
Zinc ≥ 80 µg/dL
Status: Reasonable but context‑dependent
- Zinc is essential for:
- Synaptic plasticity
- NMDA modulation
- Neurogenesis
- Lower levels associated with depression, cognitive impairment
[journals.plos.org], [ods.od.nih.gov]
Must be balanced carefully with copper.
Prealbumin ≥ 20 mg/dL
Status: Indirectly CNS‑protective
- Proxy for protein availability and systemic anabolic state
- Chronic low protein impairs neurotransmitter synthesis and immune stability
- Not specific, but rational in CNS‑risk populations
