Vitamin D3.
The hormone your skin makes — that indoor life blocks, that almost no food carries?
Vitamin D is not really a vitamin — it is a prohormone: a substance your skin manufactures from cholesterol and sunlight, which the liver and kidneys then convert into a hormone that regulates calcium, bone remodeling, immune function, and hundreds of genes. The modern problem is structural: indoor life, high latitudes, sunscreen, and aging skin have made the sun-driven route unreliable for much of humanity, and food carries the vitamin in only trace amounts — fatty fish, fortified milk, and egg yolks cover a fraction of the need. Roughly 1 billion people worldwide run low. The honest calibration: D3 supplementation at maintenance doses safely supports bone, muscle, and immune function — the benefits concentrate in the deficient, and the dosing has a real ceiling with real overdose consequences.
Cholecalciferol (D3)
A fat-soluble secosteroid your skin synthesizes from 7-dehydrocholesterol under UVB light — then the liver adds one oxygen (25-OH) and the kidney adds another (1,25-OH) to create calcitriol, the active hormone. The 'vitamin' label is a historical accident: D3 is a hormone precursor your body can manufacture, given the sun it mostly no longer gets.
The mechanism that made the vitamin famous: calcitriol upregulates intestinal calcium transport — without adequate D, dietary calcium absorbs at a fraction of its potential. Our osteoporosis page's partnership story.
Regulates the osteoblast-osteoclast balance — the renovation crew of bone. The combined calcium+D trials cut fractures in institutionalized elderly populations.
Vitamin D receptors sit on immune cells; the hormone supports antimicrobial peptides (cathelicidins) and balanced T-cell function. The deficient-subgroup infection data builds on this.
Muscle carries vitamin D receptors; deficiency weakens and destabilizes — the falls connection that made the megadose trials' harm signal so meaningful.
Low D tracks with worse asthma outcomes and depression risk — the associations are strong, the correction evidence modest and concentrated in deficiency. Honest grading throughout this library.
Vitamin D response elements sit across the genome — the hormone touches hundreds of genes, which is why deficiency has diffuse effects and correction has diffuse benefits.
The two-step activation is the key to understanding everything about D3: the skin/liver/kidney pathway is a hormone-synthesis cascade, and the 25-OH intermediate — the form blood tests measure — is a STORAGE form that the body holds for weeks. This storage is why daily dosing works (steady 25-OH levels) and why megadose boluses fail (a single spike disrupts the regulation the system was built around — the falls-and-fractures lesson). The body controls the final kidney step tightly, which is why moderate D3 intake is safe and the 25-OH level is the honest gauge of status.
Capsules
Standard — with fat
Softgels
Oil-based — ideal for fat-soluble D
Liquid Drops
Dose-precise — titration and children
Gummies
Dose-capped — read the label carefully
D3 · Cholecalciferol ✅
The form human skin synthesizes. Raises and sustains blood 25-OH levels roughly twice as effectively as D2 per unit dose. The form used in the pivotal trials, the form this library recommends, the form in every product on this page. Sourced from lanolin (sheep wool grease) or lichen (the vegan source).
D2 · Ergocalciferol
The fungal/plant form (UV-irradiated yeast; what UV-exposed mushrooms carry). Still in some prescriptions and vegetarian products — it works, but dose-for-dose it sustains blood levels less well. The label should say which form you're buying: D3 is the default choice.
The Sunshine Paradox — Why a Sun-Made Vitamin Runs Short
The paradox at the center of vitamin D: your skin can make everything you need — in minutes of midday sun — and modern life blocks almost all of it. UVB photons don't pass through glass (the office window delivers light, not vitamin D), sunscreen (SPF 30 blocks ~95% of synthesis), clouds, winter-at-latitude atmospherics (above ~37°, winter sun is too weak for months), and melanin-rich skin (a built-in SPF that slows synthesis several-fold). Add aging — older skin synthesizes ~75% less D from the same exposure — and the result: the natural route that worked for sun-exposed ancestors fails the indoor, clothed, latitudinal, long-lived modern human.
The population result: an estimated 1 billion people worldwide have inadequate levels (<20 ng/mL), with the highest rates in older adults, people with darker skin at high latitudes, the institutionalized, and — a point of honest tension — populations advised to avoid sun for skin-cancer prevention. The dermatology-versus-endocrinology debate is real: both fields are right about their own risk. The practical resolution most bodies converge on: brief sensible sun when available, supplementation as the reliable baseline — the supplement exists precisely because the natural route became unreliable.
The dosing discipline that follows from the hormone biology: daily maintenance (1,000–2,000 IU for most adults), test-guided correction above that, a hard ceiling (4,000 IU unsupervised), and never megadose timing. The 25-OH blood test — the storage form the body maintains — converts the whole question into numbers: deficient (<20), insufficient (20–29), sufficient (30–50+ ng/mL in most reference ranges), and the zone where more stops helping.
- RDA: 600–800 IU daily (adults, per IOM) — a floor for bone health, not an optimum; many clinicians target 1,000–2,000 IU.
- Unsupervised ceiling: 4,000 IU/day — above it, physician and blood test required.
- The test: 25-OH vitamin D — the storage form; the number that guides every dosing decision this page teaches.
- The form: D3 (cholecalciferol), not D2 — better at raising and sustaining levels, the trial material.
- The partners: calcium (the bone mechanism), magnesium (the activation enzymes) — D3 never works alone.
What It Does NOT Do
The honest negative space — the aisle's claims versus the trial record:
- It does not prevent fractures alone. Vitamin D without calcium did not reduce fractures in pooled analyses — the partnership is the intervention. The combined trials worked; the solo ones didn't.
- It does not prevent cancer or heart disease in the general population. VITAL — 2,000 IU daily in 25,871 generally healthy adults — found no significant reductions. The null that drew the boundary.
- It does not cure or treat depression. The deficiency-mood association is real; correction trials in deficient patients show modest benefit; it is not an antidepressant. Our depression page grades it Tier 3.
- It does not treat asthma. The airway associations are strong; deficient-patient exacerbation data is encouraging; inhalers remain the treatment. Our asthma page grades it Tier 1 only for deficient patients' exacerbation reduction.
- It does not have a 'more is better' curve. The megadose trials proved the opposite: 500,000 IU annually INCREASED falls and fractures ~15%. This is the one supplement where the overdose data is as instructive as the deficiency data.