Methylcobalamin (B12) vs Vitamin D3

Mechanism

Methylcobalamin serves as a cofactor for methionine synthase, transferring a methyl group from 5-MTHF to homocysteine to regenerate methionine and subsequently SAMe. Adenosylcobalamin (the other active B12 form) is a cofactor for methylmalonyl-CoA mutase in mitochondrial energy production and odd-chain fatty acid metabolism. B12 is essential for myelin synthesis, DNA synthesis (thymidylate synthase pathway), red blood cell maturation, and neurological function. Deficiency causes megaloblastic anemia and irreversible subacute combined degeneration of the spinal cord.

Standard Dosing

1000-5000 mcg methylcobalamin daily (sublingual preferred)

Timing

Morning, sublingual for best absorption (bypasses intrinsic factor requirement). Can combine with methylfolate.

Cycle Duration

ongoing

Side Effects

• Acne/skin breakouts (common)
• Anxiety (overmethylation in susceptible individuals)
• Diarrhea
• Headache
• Hypokalemia (during rapid repletion of severe deficiency)

Contraindications

• Leber's hereditary optic neuropathy (cyanocobalamin specifically; methylcobalamin is generally safer)
• Polycythemia vera

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Mechanism

Vitamin D3 (cholecalciferol) is hydroxylated in the liver to 25(OH)D (calcidiol), then in the kidneys to 1,25(OH)2D (calcitriol), the active hormone. Calcitriol binds the nuclear vitamin D receptor (VDR), forming a heterodimer with RXR that regulates >1000 genes. Key actions: upregulation of intestinal calcium/phosphorus absorption (TRPV6, calbindin), modulation of innate immunity (cathelicidin LL-37 antimicrobial peptide production), suppression of adaptive immune overactivation (Th1/Th17 to Treg shift), regulation of PTH and osteocalcin for bone mineralization, and modulation of insulin secretion from beta cells.

Standard Dosing

5000 IU daily (125 mcg)

Timing

With largest fat-containing meal of the day (fat-soluble). Morning preferred.

Cycle Duration

ongoing (lifelong for most people in northern latitudes)

Side Effects

• Hypercalcemia at excessive doses (>10,000 IU/day long-term without monitoring)
• Nausea/vomiting (toxicity)
• Kidney stones (with excessive calcium)
• Metallic taste (toxicity sign)

Contraindications

• Hypercalcemia
• Granulomatous diseases (sarcoidosis, some lymphomas — unregulated 1-alpha hydroxylase conversion)
• Primary hyperparathyroidism (without monitoring)
• Williams syndrome

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