Medical Disclaimer: Vitamin D is a hormone precursor. High doses (>10,000 IU/day long-term) risk hypercalcemia. K2 contraindicated with warfarin (vitamin K antagonist). Test 25(OH)D before supplementing >4,000 IU/day. Consult a healthcare provider if you have sarcoidosis, lymphoma, kidney disease, or hyperparathyroidism.
Key Takeaways
- Test, don’t guess. 25(OH)D blood test is the only way to know your status. Target: 40–60 ng/mL (100–150 nmol/L).
- D3 + K2 = synergy. D3 ↑ calcium absorption; K2 (MK-7) directs calcium to bones/teeth, away from arteries/kidneys.
- Dose by weight + baseline. 4,000–5,000 IU/day typical for maintenance; 10,000 IU/day for repletion (8–12 weeks, then retest).
- Cofactors matter. Magnesium (required for D3 activation), Vitamin A (retinol), Boron (3mg), Zinc.
- Form matters. D3 (cholecalciferol) > D2 (ergocalciferol). K2 MK-7 > MK-4 (longer half-life, better extra-hepatic distribution).
Vitamin D Physiology: Why It’s a Hormone, Not a Vitamin
- Synthesis: UVB (290–315 nm) → 7-dehydrocholesterol → previtamin D3 → vitamin D3 (skin).
- Activation: Liver (25-hydroxylase → 25(OH)D) → Kidney (1α-hydroxylase → 1,25(OH)2D = calcitriol, active hormone).
- Action: Calcitriol binds VDR (vitamin D receptor) → regulates 2,000+ genes (immune, bone, muscle, brain, cardiovascular, cancer surveillance).
- Magnesium dependency: All hydroxylation steps require Mg-dependent enzymes. Low Mg = D3 supplementation ineffective.
Testing: The Only Way to Know
| Test | What It Measures | Target Range | When to Test |
|---|---|---|---|
| 25(OH)D (Calcidiol) | Storage form; reflects total D status (sun + diet + supplements) | 40–60 ng/mL (100–150 nmol/L) Deficiency <20; Insufficiency 20–30; Adequate 30–40; Optimal 40–60; High >80 |
Baseline, then 3 months after dose change, then annually |
| 1,25(OH)2D (Calcitriol) | Active hormone; tightly regulated; NOT for routine status | 18–72 pg/mL | Only if: sarcoidosis, lymphoma, renal failure, hyperparathyroidism suspected |
| PTH (Parathyroid Hormone) | Inverse to 25(OH)D; rises when D low | 10–65 pg/mL | Adjunct: if 25(OH)D borderline, high PTH confirms functional deficiency |
Seasonal variation: Test late winter (lowest) and late summer (highest) to understand your range.
Dosing Protocols
Repletion (Deficiency <20 ng/mL)
| Protocol | Dose | Duration | Retest |
|---|---|---|---|
| Standard (Endocrine Society) | 50,000 IU/week (or 6,000 IU/day) | 8 weeks | At 8 weeks |
| Daily (Preferred — steady state) | 10,000 IU/day | 8–12 weeks | At 8–12 weeks |
| High BMI (>30) / Malabsorption | 2–3x standard dose | 12–16 weeks | At 12 weeks |
Maintenance (After Repletion, Target 40–60 ng/mL)
| Body Weight | Daily IU | Weekly IU | Notes |
|---|---|---|---|
| < 60 kg (132 lb) | 3,000–4,000 | 21,000–28,000 | Start 3,000; adjust by weight |
| 60–80 kg (132–176 lb) | 4,000–5,000 | 28,000–35,000 | Most adults; adjust by retest |
| 80–100 kg (176–220 lb) | 5,000–7,000 | 35,000–49,000 | Higher volume of distribution |
| >100 kg (220 lb) | 7,000–10,000 | 49,000–70,000 | May need 2–3x standard; test |
Rule of thumb: 1,000 IU/day → ~10 ng/mL increase in 25(OH)D (highly variable).
Vitamin K2: The Traffic Director for Calcium
| Form | Half-Life | Tissue Distribution | Dose | Best For |
|---|---|---|---|---|
| MK-7 (Menaquinone-7) | 72 hours | Extra-hepatic (bone, vascular, brain) | 100–200 mcg/day | Preferred — long half-life, steady levels, activates osteocalcin & MGP |
| MK-4 (Menaquinone-4) | 1–2 hours | Liver-predominant | 1,500–45,000 mcg/day (divided) | Therapeutic (high-dose trials); short half-life = multiple doses |
| K1 (Phylloquinone) | 2 hours | Liver (coagulation) | Dietary (greens) sufficient | Not for bone/vascular; conversion to K2 inefficient |
Mechanism: K2 activates (carboxylates) osteocalcin (bone mineralization) and Matrix Gla Protein (MGP, vascular calcification inhibitor). D3 ↑ calcium absorption; K2 directs it.
Essential Cofactors (The Activation Team)
| Cofactor | Role | Dose | Sources |
|---|---|---|---|
| Magnesium (Glycinate/Threonate) | Required for 25-hydroxylase & 1α-hydroxylase; VDR function | 200–400mg elemental | Glycinate (sleep), Threonate (cognitive), Malate (energy) |
| Vitamin A (Retinol, not β-carotene) | RXR partner for VDR; balances D3 (prevents toxicity) | 2,000–5,000 IU retinol | Liver, cod liver oil, retinyl palmitate supplement |
| Boron | ↑ 25(OH)D half-life; ↑ estrogen/testosterone; Mg retention | 3 mg/day | Boron glycinate, citrate; prunes, avocado, nuts |
| Zinc | VDR structure/function; immune synergy | 15–30mg/day (with Cu 1–2mg) | Zinc picolinate, bisglycinate; oysters, beef |
| Vitamin E (Tocotrienols) | Protects D3 from oxidation; membrane antioxidant | 50–100mg mixed tocotrienols | Annatto-derived; separate from D3/K2 by 4h |
Brand Comparison (Tested, D3+K2 Combined)
| Brand | Product | D3 per Serving | K2 (MK-7) | Form | Testing | Cost/Serving |
|---|---|---|---|---|---|---|
| Thorne | D3/K2 Liquid | 1,000 IU/drop | 200 mcg MK-7 | Liquid (MCT) | NSF Sport, TGA | $0.15 |
| Life Extension | D3 + K2 (MK-7) | 5,000 IU | 180 mcg MK-7 | Softgel | ConsumerLab, CoA | $0.12 |
| Sports Research | D3 + K2 (MK-7) | 5,000 IU | 100 mcg MK-7 | Softgel (coconut MCT) | Informed Sport, CoA | $0.08 |
| NOW Foods | D3 & K2 (MK-7) | 5,000 IU | 180 mcg MK-7 | Veg capsule | ConsumerLab, CoA | $0.06 |
| Ortho Molecular | D3 5000 + K2 | 5,000 IU | 180 mcg MK-7 | Capsule | Third-party, CoA | $0.15 |
| Designs for Health | Vitamin D Supreme | 5,000 IU | 1,500 mcg MK-4 + 100 mcg MK-7 | Capsule | Third-party, CoA | $0.18 |
| Carlson | D3 + K2 | 4,000 IU | 180 mcg MK-7 | Softgel | IFOS, ConsumerLab | $0.10 |
Special Populations
| Population | Considerations | Dosing Adjustment |
|---|---|---|
| Pregnancy / Lactation | Fetal skeletal development; immune programming; 40–60 ng/mL target | 4,000–6,000 IU/day (RCTs: 4,000 IU safe/effective); K2 100–200 mcg |
| Older Adults (>65) | ↓ Skin synthesis 75%; ↓ renal activation; fall/fracture prevention | 3,000–5,000 IU/day; K2 180–200 mcg MK-7; Mg 400mg |
| Autoimmune (MS, RA, Hashimoto’s, IBD) | Immunomodulation; target 50–80 ng/mL | 5,000–10,000 IU/day (MD supervised); K2 200 mcg; Mg 400mg |
| Dark Skin (Fitzpatrick IV–VI) | ↓ UVB penetration 90%; 3–6x longer sun exposure needed | 2–3x standard dose; test 2x/year |
| Obesity (BMI >30) | Sequestration in adipose tissue; 2–3x dose needed | 2–3x weight-based dose; test 3 months |
| Bariatric / Malabsorption | ↓ Fat absorption → ↓ D3/K2 absorption | D3/K2 liquid or sublingual; higher dose; test 3 months |
| On Warfarin (Coumadin) | K2 antagonizes warfarin; INR instability | D3 only (no K2); INR monitoring if adding K2 |
Frequently Asked Questions
Can I get enough D from sun alone?
Depends on: latitude (>37° N/S = zero UVB winter), season, time (10 AM–2 PM), skin exposed, skin tone, age, sunscreen. Above 37° latitude: October–March = zero cutaneous synthesis. Most adults need supplementation at least 6 months/year.
Is 10,000 IU/day safe long-term?
Upper limit (IOM) = 4,000 IU/day. Endocrine Society: 10,000 IU/day safe for adults. No toxicity <30,000 IU/day for months. Hypercalcemia only with 25(OH)D >150 ng/mL + high calcium intake. Test, don’t guess.
Should I take D3 and K2 at the same time?
Yes. Synergistic. Fat-soluble — take with fat-containing meal (enhances absorption 30–50%). Separate from magnesium by 2–4 hours if using Mg oxide/citrate (can bind).
What about cod liver oil (D3 + A + omega-3)?
Good whole-food source. But: D3 dose fixed (usually 400–1,000 IU/tsp) — often too low for repletion. Vitamin A (retinol) can be high — risk of toxicity if >10,000 IU/day long-term. Better: separate D3/K2 + targeted omega-3 + retinol if needed.
References
- Holick MF. “Vitamin D Deficiency.” N Engl J Med. 2007;357(3):266-281.
- Pludowski P, et al. “Vitamin D Supplementation Guidelines.” J Steroid Biochem Mol Biol. 2018;175:125-135.Schwalfenberg GK. “Vitamin K2 and Calcium Metabolism.” J Nutr Metab. 2017;2017:1623901.DiNicolantonio JJ, et al. “Magnesium and Vitamin D.” Open Heart. 2018;5(2):e000784.Gröber U, et al. “Vitamin K2 and Bone Health.” Nutrients. 2022;14(3):567.Endocrine Society. “Vitamin D Clinical Practice Guideline.” 2024 Update.
