Educational guide · 14 min read

Vitamin D, explained properly.

One in five Australians is clinically deficient in winter. Most supplements on the shelf are the wrong form, the wrong dose, or both. This guide covers the 5 forms, the 25(OH)D blood-test numbers, the D3+K2 pairing, and the latitude rule that explains why your friends in Helsinki need more than you do.

Last updated: August 23, 2026 · Sources: Holick 2011 (Endocrine Society), Tripkovic 2012 (BMJ), Vieth 1999 (JACN), Bischoff-Ferrari 2005 (JAMA), NIH ODS, RACGP

What vitamin D actually is

Vitamin D is a fat-soluble secosteroid that your skin synthesises from 7-dehydrocholesterol when ultraviolet B radiation (UVB, 290–315 nm) hits it. The molecule that comes out of your skin is cholecalciferol — vitamin D3. Plants and fungi make a slightly different version, ergocalciferol — vitamin D2 — when UV hits ergosterol. From there, both forms follow the same two-step activation pathway: liver → 25-hydroxyvitamin D (calcifediol, 25(OH)D) → kidney → 1,25-dihydroxyvitamin D (calcitriol, the active hormone).

Calling vitamin D a "vitamin" is a misnomer. It's really a prohormone — your body makes the bulk of what it needs from sunlight, and the active form (calcitriol) is a steroid hormone that turns on hundreds of genes via the vitamin D receptor (VDR). It regulates calcium and phosphate absorption in the gut, bone mineralisation, immune-cell signalling, and (per Vieth 1999) probably also neuromuscular function. A deficiency is associated with rickets in children, osteomalacia in adults, and a long list of secondary effects ranging from mood to infection risk.

The bottom line: vitamin D is a hormone, not a vitamin in the classic sense. The reason deficiency is so common — especially in Melbourne, Adelaide, Hobart, Perth, London, and any city above ~35° latitude — is that your skin makes essentially zero D3 between May and August. You need either summer sun exposure or a supplement. Most mass-market brands ignore the dose, the form, and the fat-soluble absorption problem. This guide fixes that.

How vitamin D works (in plain language)

Vitamin D's job is to keep blood calcium in a tight range (~2.1–2.6 mmol/L or 8.5–10.5 mg/dL). When blood calcium drops, the active form calcitriol is secreted and does four main things:

Increases calcium absorption in the gut. Calcitriol upregulates the TRPV6 and calbindin transporters in the duodenum, raising calcium absorption efficiency from ~30% (no D) to ~60–80% (adequate D).
Increases phosphate absorption. Phosphate is required for ATP, DNA, and bone mineralisation. Without enough D, phosphate drops and bone matrix weakens even if calcium is adequate.
Works with PTH to mobilise calcium from bone. Parathyroid hormone and calcitriol together resorb calcium from the skeleton when dietary intake falls short. This is the chronic-deficiency pathway that causes osteomalacia in adults and rickets in children.
Modulates the immune system. Calcitriol increases antimicrobial peptide production (cathelicidin, LL-37) in macrophages and shifts T-cell balance. This is the suspected mechanism behind winter colds, flu season, and the early COVID-era observational data.

The crucial detail most supplements ignore: D3 is inert until your liver hydroxylates it to 25(OH)D. If you have liver disease, take certain medications (phenytoin, rifampin), or have severe obesity (vitamin D gets sequestered in fat tissue), you may take 5,000 IU/day of D3 and still test deficient. That's when calcifediol (already-hydroxylated D) becomes the right choice.

The 5 vitamin D forms, compared

"Vitamin D" is one vitamin, but the marketplace sells it in 5 different molecules. They differ in origin, conversion steps, and clinical use case.

1. D3 (cholecalciferol) — the workhorse

Most evidence

The form your skin makes from sunlight. Most supplements are cholecalciferol from lanolin (sheep wool grease). Requires liver + kidney hydroxylation. Tripkovic 2012 (BMJ meta-analysis, 94 RCTs) settled the D2-vs-D3 debate: D3 raises serum 25(OH)D significantly more than D2.

Best for: the vast majority of healthy adults and children at higher latitudes. The default first choice.

2. D2 (ergocalciferol)

Second-line

The plant and fungal form (UV-exposed mushrooms, yeast). Once considered interchangeable with D3. The Tripkovic 2012 meta-analysis showed D2 raises 25(OH)D less efficiently than D3 and may even accelerate the breakdown of the body's own D stores when given in high bolus doses.

Best for: prescription-only situations (some US prescriptions still use D2 for insurance reasons) or strict vegan formulations where lichen D3 is not available. Most people should avoid.

3. Calcifediol (25(OH)D)

Bypasses liver

Already hydroxylated once — this is what the liver produces from D3 before the kidney activates it. ~3× more potent per microgram than cholecalciferol per the Endocrine Society 2011 guideline (Holick). Used clinically for liver disease, obesity, malabsorption, and people whose D3 blood tests do not respond despite months of supplementation.

Best for: severe deficiency (<30 nmol/L), fatty liver disease, BMI >30, post-bariatric surgery, IBD / Crohn's. Now available OTC in Australia as Swisse Ultiboost Fast-Acting.

4. D3 from vegan sources (lichen)

Vegan

Cholecalciferol extracted from UV-exposed lichen (a slow-growing northern-hemisphere algae-fungus symbiosis). The molecule is identical to animal-sourced D3 and absorbs the same way per NIH ODS, but supply is limited. Expect to pay 1.5–2× per IU for the vegan sourcing.

Best for: vegans, strict vegetarians, anyone avoiding animal-derived ingredients for religious or ethical reasons. Herbs of Gold D3 1000 Liquid is the main AU option.

5. D3 + K2 (MK-7) combination

Synergy

Vitamins D and K2 work synergistically: D upregulates calcium absorption, K2 (specifically the menaquinone-7 form, MK-7) activates the matrix Gla-protein and osteocalcin that route that calcium into bone instead of soft tissue / arteries. The mass-market Swisse / Blackmores D3 tablets do not include K2 — you have to buy a combo or stack separately.

Best for: adults over 40, anyone supplementing above 2,000 IU/day long-term, anyone with family history of osteoporosis or vascular calcification.

The latitude-aware start protocol

Vitamin D dosing is one of the few areas where the protocol genuinely depends on where you live, your skin colour, and your recent blood test. The Endocrine Society 2011 guideline (Holick) recommends 1,000–2,000 IU/day for maintenance; the Australian NHMRC recommends 400 IU/day as the AI. Most practising clinicians consider 1,000 IU/day a sensible default for adults at temperate latitudes, with higher doses only after a confirmed 25(OH)D blood test.

Step 1: Get a baseline 25(OH)D blood test

Ask your GP for a 25-hydroxyvitamin D test (the correct marker — not 1,25(OH)₂D, which is rarely useful). Medicare rebates the test in Australia if your doctor codes it as "vitamin D deficiency" or you are in a high-risk group (dark skin, veiled, osteoporotic, malabsorption). Cost: ~AUD $30 out-of-pocket without a rebate. Result interpretation:

<30 nmol/L (12 ng/mL): severe deficiency. Prescriptive high-dose regimen (often 5,000 IU/day × 8 weeks, then re-test).
30–50 nmol/L (12–20 ng/mL): deficient. Supplement 2,000–4,000 IU/day × 8–12 weeks.
50–75 nmol/L (20–30 ng/mL): insufficient. Supplement 1,000–2,000 IU/day for maintenance.
>75 nmol/L (30 ng/mL): optimal per Endocrine Society. 1,000 IU/day for maintenance is plenty.
>150 nmol/L (60 ng/mL): too high. Stop supplementing; re-test in 3 months.

Step 2: Pick the right form for your situation

Healthy adult at temperate latitude with no liver / kidney / gut issues → standard D3 (cholecalciferol) 1,000 IU/day. Liver disease, BMI >30, severe deficiency, post-bariatric, or non-responder → calcifediol 25(OH)D. Vegan / vegetarian → lichen-sourced D3. Anyone over 40 on >2,000 IU/day long-term → D3+K2 (MK-7) combination.

Step 3: Take it with breakfast fat

Vitamin D is fat-soluble. Without dietary fat at the same meal, absorption drops 30–50% per Vieth 1999. Take your D3 with the largest meal of the day that contains fat — typically breakfast (eggs, avocado, nut butter) or dinner. If you are taking a D3-only tablet in the morning with black coffee and toast, you are wasting most of it.

Step 4: Re-test in 3 months

25(OH)D has a half-life of ~3 weeks. After 3 months of supplementation, re-test and adjust. If you are still <50 nmol/L on 2,000 IU/day of D3, that is a strong signal that calcifediol or a higher dose is needed.

Maintenance (ongoing)

Once you are in the optimal 75–125 nmol/L range, 1,000 IU/day is enough for most adults at temperate latitudes. Summer sun exposure (15 min midday, arms and legs, no sunscreen) can replace the supplement. In winter (May–August in the southern hemisphere), keep supplementing — your skin makes essentially no D3 from May to August at latitudes above ~35° (Melbourne, Hobart, Adelaide, Perth, Buenos Aires, all of Europe, northern US, Canada).

Choosing by population & use case

Dark-skinned individuals at high latitudes

Melanin is a natural sunscreen — Fitzpatrick V/VI skin produces D3 at roughly 1/4 the rate of Fitzpatrick II skin. A study of Melbourne residents of African and South Asian descent found >50% were clinically deficient (<50 nmol/L) by end of winter. Higher maintenance dose: 2,000–4,000 IU/day year-round is reasonable. Get tested.

Veiled women & modest dress codes

Clothing blocks UVB. Women who cover most of their skin for religious or cultural reasons are at very high risk of deficiency regardless of latitude. The Endocrine Society 2011 guideline explicitly calls this group out as needing 1,500–2,000 IU/day minimum, with a baseline blood test strongly recommended.

Elderly (65+)

Skin's 7-dehydrocholesterol content drops ~50% between age 20 and 70, and the kidney's ability to hydroxylate 25(OH)D to calcitriol declines too. Combined with reduced outdoor time, >60% of Australian adults over 65 are insufficient. Bischoff-Ferrari 2005 (JAMA meta-analysis) showed 700–800 IU/day reduces hip fracture risk by ~26% and non-vertebral fracture by ~23%. Pair with calcium (1,000–1,200 mg/day from diet) per RACGP osteoporosis guideline.

Obesity (BMI >30)

Vitamin D is fat-soluble and gets sequestered in adipose tissue. Obese individuals need 2–3× higher doses to achieve the same serum 25(OH)D as normal-weight peers. Calcifediol is often a better choice than D3 here because it bypasses the liver's first-pass sequestration issue.

IBD, Crohn's, coeliac, post-bariatric surgery

Malabsorption of fat-soluble vitamins is the issue. Standard D3 tablets may not raise 25(OH)D adequately. Calcifediol, sublingual D3 liquid, or intramuscular injections (D3 or calcifediol) are typically used. Work with a gastroenterologist; doses of 5,000–10,000 IU/day may be prescribed.

Liver disease (NAFLD, hepatitis, cirrhosis)

The liver performs the first hydroxylation (D3 → 25(OH)D). In significant liver impairment, this step is rate-limiting and people can be deficient despite adequate D3 intake. Calcifediol is the appropriate form — it bypasses the liver step entirely.

What to pair with vitamin D (and what to avoid)

Pairs well

Vitamin K2 (MK-7 form) — routes absorbed calcium into bone, not arteries. Critical if you are on >2,000 IU/day long-term. 100–200 mcg MK-7/day is the typical pairing dose.
Magnesium — required cofactor for the liver and kidney hydroxylation steps. People on diuretics, with low magnesium diets, or taking PPIs are often magnesium-deficient and will not convert D3 → 25(OH)D efficiently without it.
Calcium (from food, not supplements) — 1,000–1,200 mg/day from dairy, leafy greens, or fortified foods. Avoid taking >500 mg calcium supplement at the same time as D — they compete for absorption.
Dietary fat at the same meal — improves absorption 30–50%. Eggs, avocado, olive oil, nuts, fatty fish.

Avoid or separate

Thiazide diuretics (HCTZ) — raise serum calcium. Combined with high-dose D, risk of hypercalcemia. Monitor blood calcium.
Corticosteroids (prednisone) — accelerate D breakdown. Higher D dose may be needed, but coordinate with prescriber.
Orlistat / Alli — block fat-soluble vitamin absorption. Take D at least 2 hours apart from orlistat dose.
Cholestyramine / colestipol — bile-acid sequestrants bind fat-soluble vitamins. Take D ≥4 hours apart.
Anticonvulsants (phenytoin, phenobarbital, carbamazepine) — induce CYP24A1, which breaks down 25(OH)D. May need 2–3× normal D dose.

Pregnancy & breastfeeding

Vitamin D in pregnancy is not optional — it is essential. The WHO and the Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) recommend routine supplementation during pregnancy. Deficiency is associated with increased risk of pre-eclampsia, gestational diabetes, low birth weight, and neonatal hypocalcaemia. A Cochrane review supports 600–800 IU/day as a baseline; many clinicians prescribe 1,000–2,000 IU/day.

The upper limit during pregnancy is 4,000 IU/day per NIH ODS. Do not exceed this without blood-test monitoring. Breastfeeding mothers should continue 1,000–2,000 IU/day because breast milk is naturally low in D (especially if the mother is deficient) and the infant relies on stores built during pregnancy plus their own supplementation (400 IU/day for babies <12 months per NHMRC).

Calcifediol is increasingly used in pregnancy for women with severe deficiency or malabsorption. Vegan D3 from lichen is also safe in pregnancy. D2 (ergocalciferol) is considered second-line per Tripkovic 2012 but is not contraindicated.

Not medical advice. Always consult your obstetrician and a registered dietitian before starting, stopping, or changing dose of any supplement during pregnancy or breastfeeding. The exact dose should be individualised based on a 25(OH)D blood test.

Frequently asked questions

▶ Why is vitamin D deficiency so common in Australia?

Two reasons. First, the sun-protection messaging after the 1980s slip-slop-slap campaign reduced Australian UVB exposure dramatically — and we now have one of the highest melanoma rates in the world, so the message was necessary. Second, Australia's southern cities (Melbourne, Hobart, Adelaide) are above 35°S latitude, which means skin makes essentially no vitamin D between May and August. By end of winter, 1 in 5 Australian adults is deficient (<50 nmol/L). Indoor work, sunscreen use, and darker skin make it worse.

▶ D2 vs D3 — which one should I take?

D3 (cholecalciferol) for nearly everyone. The Tripkovic 2012 BMJ meta-analysis (94 studies) showed D3 raises serum 25(OH)D more efficiently than D2. D2 is plant-sourced and still used in some prescription supplements (especially in the US), but D3 is the default in Australia and has more evidence behind it. The only reason to choose D2 is if you are vegan and the only vegan D3 option available to you is in a formulation you cannot tolerate.

▶ What is calcifediol and when do I need it instead of D3?

Calcifediol (25(OH)D) is D3 that has already been hydroxylated once by the liver. It is the form your blood test actually measures. You would choose calcifediol over standard D3 if you have liver disease, are obese (BMI >30), have IBD or coeliac, are post-bariatric surgery, or have been taking D3 for months and your 25(OH)D blood test has not moved. Endorsed by the Endocrine Society 2011 guideline for these specific indications.

▶ Why do I need to take vitamin D with food?

Vitamin D is fat-soluble, which means it is absorbed alongside dietary fat in your gut. Without fat at the same meal, absorption drops 30–50%. Take your D tablet with the largest meal of the day that contains fat (eggs, avocado, olive oil, nuts, dairy). If you take it on an empty stomach with just water, you are losing half the dose to poor absorption. This is the single most common mistake people make with vitamin D.

▶ Do I need K2 with my vitamin D?

Not strictly required, but strongly recommended if you are supplementing above 2,000 IU/day long-term. Vitamin D increases calcium absorption; vitamin K2 (specifically the MK-7 form) activates the proteins (matrix Gla-protein, osteocalcin) that route that calcium into bone rather than soft tissue / arteries. The MK-7 form has a long half-life and 100–200 mcg once daily is the standard pair dose. Mass-market D3 brands (Swisse, Blackmores) do not include K2 — you would stack separately.

▶ Can I take too much vitamin D?

Yes, but it is rare with oral supplementation at typical doses. Hypervitaminosis D requires sustained serum 25(OH)D >375 nmol/L (150 ng/mL), which usually only happens with prolonged megadosing (50,000+ IU/day for months) or manufacturing errors. The symptoms are caused by hypercalcemia: nausea, vomiting, constipation, excessive thirst, confusion, and in severe cases kidney stones or kidney failure. The NIH ODS sets the tolerable upper limit at 4,000 IU/day for adults — staying at or below that is safe indefinitely per Vieth 1999. Above 4,000 IU/day requires blood-test monitoring every 3 months.

References & sources

Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. (2011). Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology & Metabolism 96(7):1911-1930. PMID: 21646368
Tripkovic L, Lambert H, Hart K, et al. (2012). Comparison of vitamin D2 and vitamin D3 supplementation: a systematic review and meta-analysis. BMJ 344:e5702. PMID: 22071892
Vieth R (1999). Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. Journal of the American College of Nutrition 18(5):463-472. PMID: 10539756
Bischoff-Ferrari HA, Willett WC, Wong JB, et al. (2005). Fracture prevention with vitamin D supplementation: a meta-analysis of randomized controlled trials. JAMA 293(18):2257-2264. PMID: 15687326
NIH Office of Dietary Supplements — Vitamin D Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
Examine.com — Vitamin D Research Review. https://examine.com/supplements/vitamin-d/
Royal Australian College of General Practitioners — Osteoporosis prevention and vitamin D / calcium (RACGP guideline). RACGP osteoporosis guidelines
Chemist Warehouse Australia, iHerb, and Amazon product listings — July 2026 snapshot price references for Swisse, Blackmores, BioCeuticals, Ethical Nutrients, Herbs of Gold, Nature's Own, Mega We Care, and Thorne vitamin D SKUs.

Trusted brands we rate

These 10 brands are featured on every vitfacts.com comparison page. We track them for ingredient transparency, third-party testing, and Thailand availability.

Swisse

Since 1969

MID

One of the most globally recognised Australian brands, dominating the market with comprehensive multivitamins and targeted wellness blends.

Available in TH Search “vitamin d” ↗

Blackmores

Since 1932

MID

A heritage naturopathic brand spanning over 90 years, known for rigorous ingredient sourcing and massive pharmacy presence throughout the Asia-Pacific region.

Available in TH Search “vitamin d” ↗

BioCeuticals

Since 1993

PREMIUM

A premium, practitioner-only brand (owned by Blackmores) offering highly concentrated, clinical-strength formulas that are frequently recommended by healthcare professionals.

TH: limited Search “vitamin d” ↗

Ethical Nutrients

Since 1986

MID

A high-quality pharmacy brand heavily focused on evidence-based ingredients, widely known for its "Inner Health" probiotic range and highly absorbable magnesium powders.

TH: limited Search “vitamin d” ↗

Herbs of Gold

Since 1980s

MID

Born in Australia, this brand formulates therapeutic herbal and nutritional supplements with a strict focus on purity, offering many vegan-friendly options.

TH: limited Search “vitamin d” ↗

Nature's Own

Since 1970s

BUDGET

One of Australia's longest-running and most accessible vitamin brands, providing a massive, budget-friendly range found in most major supermarkets and pharmacies.

TH: limited Search “vitamin d” ↗

Mega We Care

Since 1982

MID

A leading pharmaceutical-grade supplement brand in Southeast Asia, widely trusted by Thai consumers for its fish oil and targeted clinical nutrition products.

Available in TH Search “vitamin d” ↗

Thorne Research

Since 1984

PREMIUM

While based in the US, Thorne is heavily imported and highly sought after by functional medicine practitioners globally—including in both Australia and Thailand. It is considered a gold standard due to its strict four-round testing protocol and complete absence of unnecessary fillers or binders.

TH: limited Search “vitamin d” ↗

Healthy Care

Since 2006

BUDGET

An Australian-made value brand founded in 2006, sold through Chemist Warehouse and major AU pharmacies. Part of the Nature's Care family (est. 1990), offering a wide mass-market supplement range covering fish oil, vitamins C/D, magnesium, probiotics, zinc, iron, ashwagandha, collagen, and CoQ10.

TH: limited Search “vitamin d” ↗

Why these 10? See the VitFacts editorial policy page (forthcoming) for the criteria — minimum 10 years of documented safety record, public certificate-of-analysis policy, and verifiable Thailand distribution.

Not medical advice. This article is for educational purposes only and does not constitute medical advice. Consult a qualified healthcare practitioner before starting, changing, or stopping any vitamin D supplement, especially if you are pregnant, breastfeeding, taking prescription medications, or have a pre-existing liver, kidney, or bone condition. A 25(OH)D blood test is the appropriate starting point before high-dose supplementation.