Approximate dose-response bands. Individual response varies — these are starting points, not prescriptions.
Bone-mineralization and immune-modulation functions impaired. Common in low-sun climates and people who cover most of their skin.
Sub-optimal serum 25(OH)D likely. Most adults need 1000–2000 IU daily to lift levels into the adequate band.
FDA Daily Value (20 mcg / 800 IU) met. Supports calcium absorption and baseline immune function.
Common range for raising blood 25(OH)D when measured-low. Test after 8–12 weeks before staying here long-term.
Approaching the 100 mcg / 4000 IU UL. Pair with magnesium + K2 and consider a blood panel.
Hypercalcemia risk — kidney stones, calcification. Stop supplementing and re-test.
You're at 0% of your vitamin d target. The biggest single-serving sources to top it off:
A pro-hormone, not a classical vitamin. Skin synthesises it from UVB on 7-dehydrocholesterol; the liver hydroxylates it to 25(OH)D (the storage form measured on labs); the kidney converts to 1,25(OH)2D (calcitriol, the active hormone) that controls calcium, immune, and bone biology. Modern indoor lifestyles make insufficiency one of the most common micronutrient gaps in temperate latitudes.
Calcitriol binds the vitamin D receptor (VDR), a nuclear transcription factor expressed in nearly every tissue. VDR-RXR heterodimers bind vitamin D response elements (VDREs) to regulate calbindin (calcium transport), cathelicidin (antimicrobial peptide), CYP24A1 (auto-degradation feedback), and many others. Serum 25(OH)D is the clinical proxy for stores because 1,25(OH)2D is tightly homeostatically controlled.
Defined clinically by serum 25(OH)D: <20 ng/mL (50 nmol/L) = deficient; 20–30 = insufficient. Affects 30–40% of US adults; higher at high latitudes, with dark skin, in elderly, and in obesity (sequestered in adipose tissue).
Toxicity is rare and almost always from supplements >10,000 IU/day for months. Sun exposure cannot cause toxicity (photodegradation is self-limiting).
D3 (cholecalciferol) raises serum 25(OH)D more efficiently than D2 (ergocalciferol). Take with a fat-containing meal for absorption. K2 (MK-7) pairing is reasonable but optional. Typical maintenance for adults with low baseline: 1,000–2,000 IU/day; correct deficiency with 4,000–5,000 IU/day under monitoring.
Requires bile and dietary fat for micellar absorption. Obesity reduces bioavailability — same oral dose produces lower serum rise. UVB synthesis is highly variable: 10–30 min of midday summer sun on arms/legs can produce 1,000–10,000 IU in fair skin, near-zero through window glass.
U-shaped mortality curve: both deficiency (<20 ng/mL) and excess (>60 ng/mL) associate with higher all-cause mortality in observational data. Adequacy (30–50 ng/mL) reduces fall, fracture, and infection burden in older adults — high-leverage in geriatric healthspan. Megadosing does not extend the benefit.
Calcium absorption, bone mineralization, immune modulation.
Forms with lower absorption: d2, ergocalciferol. Prefer better-absorbed forms when supplementing.
The mechanisms and systems this nutrient feeds. Click any to drill into what runs on it.
★ = load-bearing / primary cofactor. Track these in My Journey.
Whole foods that contribute meaningfully (≥10% DV per 100 g serving). Click any food to see its full nutrient profile and what else it brings to the table.
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