Vitamin D: The Complete Supplementation Guide
Latitude, skin tone, lifestyle, season, your D needs change. Here's how to actually get it right.
Vitamin D is the most-deficient nutrient in the modern world and one of the most clinically impactful to correct. But the right dose varies wildly by individual, and over-supplementing without K2 can backfire. Here's the complete guide.
Get a blood test first
Order a serum 25(OH)D test (not 1,25-OH-D, that's a different marker). Available at any commercial lab, often $40-80 out of pocket if your doctor won't order it.
Targets
| Level (ng/mL) | Status |
|---|---|
| < 20 | Deficient, treat aggressively |
| 20-30 | Insufficient, most adults |
| 30-50 | Sufficient, basic adequacy |
| 50-80 | Optimal for most health outcomes |
| > 100 | Approaching the toxicity range |
Most longevity-oriented clinicians aim for 50-60 ng/mL serum 25(OH)D.
Dose by lifestyle and latitude
Tropical / outdoor lifestyle, lighter skin
1,000-2,000 IU daily, mostly seasonal.
Temperate latitude (30°-50°), mixed indoor/outdoor
2,000-4,000 IU daily, year-round.
High latitude or mostly indoor lifestyle
4,000-5,000 IU daily, year-round.
Darker skin
Add 1,000-2,000 IU above baseline (melanin reduces UVB-driven synthesis).
Obesity (BMI > 30)
Double the baseline, D3 is sequestered in adipose tissue.
Older adults (65+)
Add 1,000 IU above baseline, skin synthesis efficiency drops with age.
D3 vs D2
D3 (cholecalciferol) is the form your skin produces from sunlight and the form your body uses. D2 (ergocalciferol) is plant-derived and less bioavailable. Always pick D3 unless you require a strict vegan source (lichen-derived D3 is available).
Why K2 matters with D3
D3 increases calcium absorption. Without enough K2, that calcium can deposit in arteries instead of bones. K2 directs calcium correctly.
Pair every 1,000 IU of D3 with at least 45-90 mcg of K2 MK-7. Most quality D3 products include K2 in the same softgel.
Take with fat
D3 is fat-soluble. Absorption is 30-50% better when taken with a meal containing fat (eggs, avocado, olive oil) vs on empty stomach.
Loading vs daily
For severe deficiency (< 20 ng/mL), some protocols use loading doses (50,000 IU weekly for 8 weeks). Daily smaller doses (5,000 IU) reach the same end state in 8-12 weeks and are gentler. Avoid mega-loading without medical supervision.
Retest at 3 months
Recheck serum 25(OH)D after 3 months of consistent supplementation. Adjust dose accordingly.
Toxicity
Vitamin D toxicity is rare but possible above serum levels of 100 ng/mL, sustained. Symptoms include hypercalcemia, kidney stones, and fatigue. Doses up to 10,000 IU/day are considered safe per the Endocrine Society, but routine doses above 5,000 IU without testing aren't necessary for most people.
Special populations
- Pregnancy, 4,000 IU/day appears safe and beneficial
- Sarcoidosis or other granulomatous diseases, D3 supplementation can trigger hypercalcemia; avoid without medical supervision
- Primary hyperparathyroidism, avoid without endocrine guidance
Sunshine, briefly
10-30 minutes of midday sun on bare skin (face + arms) several times a week produces ~5,000-10,000 IU of vitamin D for lighter skin tones. Darker skin needs longer exposure. Sunscreen blocks UVB synthesis. Sun is the best source, supplementation is the backup.
Bottom line
Test once. Take D3 with K2 at the dose that gets you to 40-60 ng/mL serum. Retest in 3 months. Most adults need 2,000-5,000 IU year-round. It's the single highest-EV supplement most people don't take consistently.
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