Vitamin D

Your intake

What each level of vitamin d does

Approximate dose-response bands. Individual response varies — these are starting points, not prescriptions.

1. Severely lowYOU ARE HERE

   0 mcg – 6.6 mcg

   Bone-mineralization and immune-modulation functions impaired. Common in low-sun climates and people who cover most of their skin.

2. Insufficient

   6.6 mcg – 20 mcg

   Sub-optimal serum 25(OH)D likely. Most adults need 1000–2000 IU daily to lift levels into the adequate band.

3. Adequate

   20 mcg – 30 mcg

   FDA Daily Value (20 mcg / 800 IU) met. Supports calcium absorption and baseline immune function.

4. Therapeutic

   30 mcg – 40 mcg

   Common range for raising blood 25(OH)D when measured-low. Test after 8–12 weeks before staying here long-term.

5. High

   40 mcg – 100 mcg

   Approaching the 100 mcg / 4000 IU UL. Pair with magnesium + K2 and consider a blood panel.

6. Over upper limit

   100 mcg – +

   Hypercalcemia risk — kidney stones, calcification. Stop supplementing and re-test.

Overview

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.

Functions

• ●Drives intestinal calcium and phosphate absorption
• ●Regulates osteoblast/osteoclast bone remodelling
• ●Modulates innate and adaptive immune signalling
• ●Influences gene expression across ~3% of the human genome

Mechanism

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.

Benefits

• ●Lowers rickets/osteomalacia risk; reduces fracture risk when combined with calcium in older adults
• ●Reduces acute respiratory infection incidence in deficient individuals (meta-analyses)
• ●Possibly modulates autoimmune disease risk (MS, type 1 diabetes); evidence still maturing
• ●VITAL trial: 2,000 IU/day did not lower CV events or cancer incidence but reduced cancer mortality and autoimmune disease incidence

Deficiency

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).

Excess

Toxicity is rare and almost always from supplements >10,000 IU/day for months. Sun exposure cannot cause toxicity (photodegradation is self-limiting).

Forms

• Cholecalciferol (D3)
  From animals/UVB; raises serum 25(OH)D ~2× more efficiently than D2
• Ergocalciferol (D2)
  From fungi; prescription 50,000 IU caps; shorter half-life
• Calcifediol (25-OH-D3)
  Rx; bypasses liver hydroxylation, fastest serum rise
• Calcitriol (1,25-(OH)2-D3)
  Rx active hormone for renal failure; bypasses both hydroxylations

Food sources

• Wild salmon (cooked) · 3 oz15 mcg (600 IU)
• Cod liver oil · 1 tsp11 mcg (450 IU)
• Fortified milk · 1 cup3 mcg (120 IU)
• Canned sardines (with bones) · 3 oz4 mcg (165 IU)
• Egg yolk · 1 large1 mcg (45 IU)
• UV-exposed mushrooms · 1 cup10 mcg (400 IU)

Supplement forms

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.

Bioavailability

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.

Longevity relevance

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.

Relationships

References

• NIH ODS — Vitamin Dguideline
• Linus Pauling Institute — Vitamin Dreview
• VITAL trial (NEJM 2019)study
• Martineau — D supplementation and respiratory infection (BMJ 2017)review

About Vitamin D

Calcium absorption, bone mineralization, immune modulation.

Role

Bone · immune

Daily target

20 mcg (DV)

Upper limit

100 mcg

Also called

vitamin d, vitamin d3, vitamin d2, cholecalciferol, ergocalciferol

Forms with lower absorption: d2, ergocalciferol. Prefer better-absorbed forms when supplementing.