Vitamin D Supplement Guide: D3 vs D2, Dosage, and K2 Pairing
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D3 vs D2: Why It Matters
Vitamin D3 (cholecalciferol) raises 25-OH-D levels approximately twice as effectively as D2 (ergocalciferol) and sustains levels for longer. D3 is the form made by human skin and found in animal foods. D2 is found in plants and used in some prescription supplements. Always choose D3 for supplementation.
Finding Your Optimal Dose
1000–2000 IU/day: prevents deficiency in most adults with some sun exposure. 2000–4000 IU/day: appropriate for those with limited sun exposure (indoor workers, northern latitudes, darker skin). 5000–10,000 IU/day: short-term repletion of deficiency (confirmed by blood test). Testing: get 25-OH-D before supplementing. Optimal target: 40–60 ng/mL. After 3 months, retest to confirm.
Why Pair D3 with K2 MK-7
Vitamin D3 increases calcium absorption significantly. Without K2, absorbed calcium may not be directed to bones and could deposit in arteries (arterial calcification). Vitamin K2 (especially MK-7) activates osteocalcin (bone matrix protein) and Matrix Gla Protein (arterial protection). Recommended pairing: D3 (2000–4000 IU) + K2 MK-7 (100–200 mcg). Take both with fat (fat-soluble vitamins).
Magnesium: The Third Partner
Magnesium activates vitamin D (converts it from inactive 25-OH-D to active 1,25-OH-D). Low magnesium impairs vitamin D conversion — you can supplement D3 and still see minimal benefit if magnesium deficient. Take magnesium glycinate (200–400 mg) as part of the D3/K2/Mg stack.
Who Needs to Supplement
People with limited sun exposure (office workers, northern residents October–March). Darker skin tones (require 3–6× more sun exposure for equivalent synthesis). People over 60 (skin synthesis declines with age). Exclusively breastfed infants (breast milk is low in D). Anyone with 25-OH-D below 30 ng/mL on a blood test.
Safety and Upper Limits
Toxicity from vitamin D is possible but requires sustained very high doses (> 10,000 IU/day for months). The UL for healthy adults is 4000 IU/day by the Institute of Medicine (though many experts argue this is conservative). At standard doses of 1000–4000 IU/day, toxicity is not a real concern. Hypercalcemia (too much calcium from excess D) is the actual mechanism of toxicity.
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