⌑ I · The MechanismHow it actually works.
Vitamin D is not, strictly speaking, a vitamin — it is a prohormone. Synthesized in the skin upon exposure to UVB radiation from sunlight (specifically wavelengths between 290-315 nm), cholecalciferol is hydroxylated in the liver to 25-hydroxyvitamin D [25(OH)D] — the form measured in blood tests — and then further hydroxylated in the kidneys to the active form, 1,25-dihydroxyvitamin D [calcitriol].[1]
Calcitriol binds the vitamin D receptor (VDR), which has been identified in nearly every tissue of the human body, including bone, intestine, kidney, parathyroid, muscle, immune cells, brain, and reproductive organs. The VDR regulates over 200 genes involved in calcium homeostasis, immune modulation, cell proliferation, and inflammatory response.[1][2]
The function most people associate with vitamin D — calcium absorption and bone mineralization — is real and important, but represents a small fraction of what calcitriol actually does. Receptor distribution is the reason vitamin D status correlates with outcomes ranging from immune function to muscle strength to cardiovascular markers.
The body produces approximately 10,000-20,000 IU of vitamin D within 15-30 minutes of full-body summer sun exposure at midday — orders of magnitude more than typical dietary intake. Modern lifestyles (indoor work, sunscreen, latitude, melanin) have effectively decoupled humans from the synthesis pathway evolution shaped us around.
⌑ II · The EvidenceWhat the research actually shows.
Vitamin D deficiency is one of the most prevalent nutrient deficiencies in modern populations. NHANES data analyzed by Forrest and Stuhldreher (2011) found that 41.6% of U.S. adults had serum 25(OH)D below 20 ng/mL — the clinical threshold for deficiency. Prevalence was significantly higher in individuals with darker skin (82.1% in non-Hispanic Black adults) and in northern latitudes.[3]
Key consolidated findings:
- Bone health. Vitamin D combined with calcium reduces fracture risk in adults over 50, particularly those who are deficient at baseline. Effect is smaller in already-replete individuals.[1][4]
- Acute respiratory infection. A 2017 individual participant data meta-analysis by Martineau et al. (n = 10,933 across 25 RCTs) found that vitamin D supplementation reduced the risk of at least one acute respiratory tract infection (adjusted odds ratio 0.88). The protective effect was strongest in those with severe baseline deficiency (< 25 nmol/L).[5]
- Muscle function. Deficiency is associated with proximal muscle weakness, particularly in older adults. Repletion improves strength and reduces fall risk in this population.[6]
- All-cause mortality. A 2019 meta-analysis by Zhang et al. (50 RCTs, n = 74,655) found vitamin D3 supplementation reduced all-cause mortality (relative risk 0.95) and cancer mortality (relative risk 0.84) — particularly in adults over 50.[7]
- Testosterone (in deficient men). Pilz et al. (2011) reported that 3,332 IU/day of vitamin D3 over one year significantly increased total testosterone in middle-aged men with baseline deficiency. Effect is repletion-dependent — supraphysiological dosing in replete men does not increase testosterone further.[8]
⌑ III · The ProtocolHow to actually use it.
What dose actually gets you to target?
Enter your current serum 25(OH)D level and target. The chart shows your projected weeks to target at the recommended dose. Adjust to see different timelines.
Step one — test
Order a serum 25-hydroxyvitamin D test. This is the only meaningful measure of vitamin D status. Available through any standard lab; widely covered by insurance. Direct-order options (Quest, LabCorp, on-demand services) are typically $40-80 USD.
Step two — interpret
Endocrine Society reference ranges:
- Deficient: < 20 ng/mL (50 nmol/L)
- Insufficient: 21-29 ng/mL (52-72 nmol/L)
- Sufficient: 30-100 ng/mL (75-250 nmol/L)
- Toxicity: > 150 ng/mL (375 nmol/L) — extremely rare from supplementation
Most research on functional outcomes uses a target of 30-50 ng/mL. Optimal ranges remain debated; the Vitamin D Council suggests 40-80 ng/mL, the Endocrine Society 30-60 ng/mL. Within that band, more is not clearly better.[1]
Step three — dose
Starting dose depends on baseline:
- Deficient (< 20 ng/mL): 5,000 IU/day for 8-12 weeks, then retest
- Insufficient (20-30 ng/mL): 2,000-4,000 IU/day, retest at 3 months
- Sufficient (30+ ng/mL): 1,000-2,000 IU/day for maintenance
The Institute of Medicine RDA of 600-800 IU is calibrated for bone health endpoints only and is widely regarded as too low for broader optimization.[1]
With what
Vitamin D is fat-soluble. Absorption requires dietary fat. Take with your largest meal of the day for maximum bioavailability — Mulligan and Licata (2010) demonstrated a 32% increase in serum 25(OH)D when D3 was taken with the largest meal vs. fasting or with a small meal.[9]
Co-factors
Vitamin D activates calcium absorption. Without adequate vitamin K2 (which directs calcium into bone rather than soft tissue), some practitioners caution about arterial calcification at high D doses. Evidence is suggestive, not conclusive. Reasonable practice: ensure adequate dietary K2 (fermented foods, hard cheeses, egg yolks) or supplement 90-200 mcg MK-7 daily.[10]
⌑ IV · Form and QualityD3, not D2.
Two forms are commercially available: vitamin D2 (ergocalciferol, plant-derived) and vitamin D3 (cholecalciferol, animal-derived or lichen-derived for vegan formulations). Tripkovic et al. (2012) meta-analyzed direct head-to-head trials and found that D3 was significantly more effective at raising serum 25(OH)D levels than D2 at equivalent doses, with the disparity widening at higher doses.[11]
D3 is the form humans synthesize endogenously, and is the form preferentially metabolized. There is no compelling reason to choose D2 unless required by religious or dietary constraints — and even then, lichen-derived D3 is available.
⌑ V · Contraindications & ConsiderationsWhat to watch for.
- Hypercalcemia. Conditions causing baseline hypercalcemia (primary hyperparathyroidism, certain granulomatous diseases like sarcoidosis, some lymphomas) can be exacerbated by vitamin D supplementation. Physician guidance required.[1]
- Toxicity threshold. Vitamin D toxicity is rare and typically requires sustained intake exceeding 50,000 IU/day for months. The Institute of Medicine sets the tolerable upper intake level at 4,000 IU/day; observational and trial data suggest 10,000 IU/day is well-tolerated for most adults with monitoring.[1]
- Drug interactions. Thiazide diuretics, glucocorticoids, anticonvulsants, and some HIV medications affect vitamin D metabolism. Consult prescriber.
- Baseline obesity. Higher BMI sequesters fat-soluble vitamin D in adipose tissue. Obese individuals may require 2-3x higher doses to achieve equivalent serum levels.[1]
Sunlight exposure, dietary intake, and supplementation dose are not reliable predictors of serum 25(OH)D. Two individuals on the same dose can have wildly different levels depending on body composition, gut absorption, season, and genetic variation in VDR and metabolic enzymes. The blood test is the only way to know.
⌑ VI · StackingWhat pairs well.
- Magnesium. Required as a cofactor for vitamin D activation. Magnesium deficiency can blunt the response to vitamin D supplementation. See the magnesium protocol →
- Vitamin K2 (MK-7). Theoretical synergy for directing calcium to bone over soft tissue. Evidence suggestive, not definitive.[10]
- Omega-3 (EPA/DHA). Fat-soluble synergy: taking D3 alongside a fish oil capsule with a meal increases absorption of both. See the omega-3 protocol →