Vitamin D — Complete Supplement Guide (D3 vs D2, dose, safety)

SupplementsVitaminsVitamin DFat-soluble

The “sunshine vitamin”. At northern latitudes many people run low, especially in winter — which is why it’s one of the most commonly supplemented nutrients. For the short dietary version, see the Vitamins page.

Read first. Vitamin D is fat-soluble and stored, so very high doses can build up and raise blood calcium dangerously. You cannot overdose from sunlight, but you can from supplements. Information, not a prescription.

Key takeaways

Choose D3 (cholecalciferol), not D2 — it raises and holds blood levels better.
Most adults: 10–20 µg (400–800 IU)/day, taken with a fatty meal; consider a winter supplement at northern latitudes.
Stay under the 100 µg (4,000 IU)/day upper limit — you can’t overdose from sunlight, but you can from supplements.

Function

Vitamin D acts more like a hormone than a classic vitamin:

Calcium & phosphate: lets the gut absorb calcium and phosphate — the foundation of strong bone.
Bone mineralisation: without it, bone softens (rickets/osteomalacia).
Muscle function: supports muscle strength and reduces falls in older adults.
Immune modulation: immune cells carry vitamin D receptors; it helps regulate immune responses.
Cell growth: involved in regulating cell division throughout the body.

Different forms — and which to choose

D3 (cholecalciferol)The form the skin makes and the preferred supplement — raises and maintains blood levels most effectively.

D2 (ergocalciferol)Plant/fungal form; works but is somewhat less effective at raising levels (an option for strict vegans, or vegan D3 from lichen).

CalcitriolThe active hormone form — prescription-only, for specific medical conditions.

Absorption in the body

Main sourceSunlight on skin makes most of our vitamin D; supplements and a little food fill the winter gap. Fat-soluble, so take with a meal.

HelpsSummer sun exposure; taking D3 with dietary fat; adequate magnesium (needed to activate it).

HindersDark skin, high latitude, winter, sunscreen, covering up, older age and obesity all lower vitamin D status.

Sunlight — can you really get enough?

The short, honest answer: often not. Sunlight can make vitamin D, but how much depends on where you live, the season, your skin, your age and how much skin you expose — and for some people, deliberately seeking sun is the wrong advice entirely.

Only the UVB band (290–315 nm) turns 7-dehydrocholesterol in the skin into pre-vitamin D3. Warmth and brightness don't count — you need genuine UVB, roughly a UV index of 3 or higher.³

Latitude & the “vitamin D winter”

Above about 35°N (and the mirror image in the south), the sun sits too low for part of the year to make any vitamin D — a “vitamin D winter” lasting ~2 months at 37°N and up to ~8 months near the Arctic.⁴ Classic measurements found no skin synthesis in Boston (42°N) from November–February, or in Edmonton (52°N) from October–March.³ In everyday terms, that means much of northern Europe, the northern US and Canada — think London, Berlin, Warsaw, Moscow, Chicago or Vancouver — makes little or no vitamin D in the skin through the winter months, whatever the weather. The same applies each Southern-Hemisphere winter (roughly June–August) in the far south, such as southern Chile and Argentina or the south of New Zealand. The nearer the poles you live, the longer that window lasts.

It's not only latitude

Time of daySynthesis peaks around midday; morning and evening light is mostly UVA, which makes no vitamin D.

Skin exposedWith steep diminishing returns — one study found exposing ~20× more skin raised output only ~3.7×.⁴

Skin toneMelanin is natural sun protection, so darker skin needs meaningfully longer exposure for the same amount.⁴

AgeOlder skin holds less 7-dehydrocholesterol and makes markedly less vitamin D than young skin.⁴

SunscreenIn theory it blocks synthesis; in practice, because people apply too little, typical use has little measurable effect on vitamin D status⁴ — and that is not a reason to skip it.

In practice, lighter skin in summer may keep levels up with roughly 20 minutes of midday sun and 40%+ of the skin bare¹ — far longer for darker skin, and effectively impossible at high latitude in winter.

When the sun is not the answer. Dermatology bodies are clear: don't use the sun or tanning beds as a vitamin D source — UV is a proven cause of skin cancer and there is no safe UV threshold that maximises vitamin D without raising cancer risk. This matters most for anyone with a history of skin cancer, photosensitivity or immunosuppression, for whom deliberate sun exposure is contraindicated — they should get vitamin D from food and supplements, not sunbathing.⁵

Why some people don't reach target levels even on supplements

First, “target” itself varies: deficiency is <30 nmol/L (12 ng/mL); the US IOM calls ≥50 nmol/L (20 ng/mL) adequate, while the Endocrine Society aims for ≥75 nmol/L (30 ng/mL).⁶ Response to a given dose then varies widely between people:

Body size & fatVitamin D distributes into body fat, so a larger body dilutes a given dose; obesity carries around 35% higher deficiency and needs higher intakes.⁶

AbsorptionCoeliac disease, IBD and bariatric surgery all reduce uptake of this fat-soluble vitamin.⁶

MetabolismPeople differ in how fast they absorb and clear it, so the same dose lands differently.

MedicinesSome anticonvulsants, glucocorticoids and rifampicin speed its breakdown.⁶

This is exactly why a fixed rule fails. A standard maintenance dose suits many people, but those not reaching target often simply need more — high-risk adults sometimes require 3,000–6,000 IU (75–150 µg)/day to get there, occasionally more under supervision.⁶ The number that matters is your blood level, not the label: test 25(OH)D, adjust the dose, and re-test.

Bottom line: in summer, safe midday sun can top you up — but for much of the year, and for many people, a supplement is the reliable route.

Deficiency symptoms

SymptomsRickets in children; in adults soft bones (osteomalacia), bone pain and muscle weakness; low mood in some.

CommonWidespread in winter at northern latitudes, and in dark-skinned, elderly, housebound or veiled people.

Who's at riskHigh-latitude winters, limited sun, dark skin, obesity, malabsorption and older age.

Recommended intake

Adults ≈ 10–20 µg (400–800 IU)/day

Reference intakes:

Adults: ~10 µg (400 IU) EFSA · 15 µg (600 IU) US · 20 µg (800 IU) for older adults
Many at northern latitudes benefit from a winter supplement.

Best sources: summer sunlight, oily fish, egg yolk and fortified foods (milk and plant drinks).¹ ²

Therapeutic use

DeficiencyCorrecting low levels — maintenance ~20–50 µg (800–2000 IU)/day; short high-dose "loading" only under medical care.

Bone & fallsWith calcium for osteoporosis; reduces falls and fractures in deficient older adults.

Therapeutic doses

Maintenance: 400–2000 IU/day

Everyday / winter400–2000 IU (10–50 µg)/day suits most people; take with a fatty meal.

Loading dosesHigh weekly/monthly loading regimens are medical, guided by blood tests.

Toxicity & upper limit

UL: 100 µg/day (4,000 IU)

Upper limit100 µg/day (4,000 IU) for adults.¹

ExcessToo much from supplements raises blood calcium → nausea, kidney stones and kidney damage.

SunlightYou cannot overdose on vitamin D from the sun — the skin self-limits.

Bottom line: a daily D3 supplement of 400–2000 IU through the darker winter months is one of the genuinely worthwhile supplements. Take it with food, pair with adequate magnesium and calcium, and stay under 4,000 IU/day unless a doctor directs otherwise.

← All supplements Short dietary version

References

Reference and upper-limit values are general adult figures from official bodies; individual needs vary. Therapeutic doses describe clinical/research use, not prescriptions.

US NIH Office of Dietary Supplements. Vitamin D — Health Professional Fact Sheet (function, forms, intakes, food sources, deficiency, therapeutic uses, upper limit, interactions). ods.od.nih.gov
EFSA. Dietary Reference Values & Tolerable Upper Intake Levels for vitamins (European reference intakes and ULs). efsa.europa.eu
Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. J Clin Endocrinol Metab 1988;67(2):373–378. pubmed.ncbi.nlm.nih.gov
Neville JJ, Palmieri T, Young AR. Physical Determinants of Vitamin D Photosynthesis: A Review. JBMR Plus 2021;5(1):e10460. doi.org/10.1002/jbm4.10460
American Academy of Dermatology. Position Statement on Vitamin D — UV exposure is not recommended as a source of vitamin D; there is no safe threshold of UV that allows for vitamin D synthesis without increasing skin cancer risk. aad.org
US NIH Office of Dietary Supplements & the Endocrine Society clinical practice guideline — serum 25(OH)D thresholds, groups at risk (obesity, malabsorption, medication interactions) and higher repletion doses. ods.od.nih.gov

This article summarizes nutrition science from official health authorities and peer-reviewed sources for education. It is not a substitute for individual medical advice, and the doses given are not prescriptions — always consult a professional before therapeutic supplement use, in pregnancy, or alongside medication.

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Bodil Isaksson, Licensed Biomedical Scientist (Sweden)

At Food Health Lab we explain what the science says in plain language, and we cite our sources so you can check for yourself.

Vitamin D — The Complete Supplement Guide