SUPPSAVERVitamin D, K2 and Magnesium: Does the AU Triad Hold Up?
Zero. That's how many times the phrase "vitamin D triad" appears in the 2024 RACGP and Healthy Bones Australia osteoporosis guideline[1]. NHMRC has no position on combining D3, K2 and magnesium. The "non-negotiable triad" framing came from Cymbiotika, AgeImmune and a chorus of US functional-medicine sites in 2024 onwards. Our shelf benchmarking across iHerb AU, Chemist Warehouse and the practitioner channel says the marketing is overcooked, but the underlying biology is more interesting than the slogan. Our verdict up front: skip the standalone triad framing, get serum 25(OH)D checked, prioritise calcium if you're a woman over 50, and use Caruso's or Ostelin combos where the combo is appropriate.
What the AU Guideline Actually Says
The 2024 RACGP and Healthy Bones Australia osteoporosis guideline is direct (Wong et al., 2025)[1]: calcium plus vitamin D is recommended only for people with documented deficiency or for frail and institutionalised individuals, with a target serum 25(OH)D above 50 nmol/L. K2 isn't in the guideline. Magnesium isn't in the guideline. The triad, as a clinical protocol, is absent.
This matters because the two largest randomised trials we have say the same thing in different words. VITAL ran 25,871 adults on 2,000 IU of D3 daily for a median 5.3 years and found no fracture reduction in vitamin-D-replete people (LeBoff et al., 2022)[2]. The Australian D-Health trial dosed 21,315 adults at roughly 2,000 IU equivalent and saw no benefit on falls or fractures either (Neale et al., 2023). A 2025 follow-up suggested possible cardiovascular benefit in those with baseline CV risk (Waterhouse et al., 2025)[3], but the fracture story is settled. If you're replete, more D doesn't buy you bones.
Where the Triad Logic Has Real Teeth
The strongest mechanistic argument for stacking D with magnesium is unambiguous. Every step of vitamin D activation, from DBP binding through hepatic 25-hydroxylation to renal 1α-hydroxylation, requires magnesium as a cofactor (Eur J Nutr, 2024)[6]. The 2024 European Journal of Nutrition analysis found high Mg intake independently lowered 25(OH)D deficiency risk and interacted positively with vitamin D intake. About 33% of Australian adults under-consume magnesium against the NHMRC RDI. If you're in that third, supplementing D without addressing Mg is genuinely suboptimal.
K2 is where the case gets thinner. A 2024 meta-analysis (Zhang et al., 2024)[4] reported a modest +0.01 g/cm² lumbar-spine BMD improvement from K2 supplementation, with the result flagged sensitivity-unstable. Femoral neck and total hip endpoints were null. The 2025 Frontiers meta confirmed biomarker effects (better osteocalcin carboxylation, lower ucOC and CTX) but flagged the absence of long-term BMD evidence. So K2 does measurable things to bone biochemistry. Whether that prevents a fracture is unproven.
MK-7 vs MK-4, Get This One Right
If you do buy a K2 product, the form matters more than the brand. MK-4 has a serum half-life of one to four hours. The Japanese osteoporosis protocol uses 45mg per day in split doses to keep blood levels up. That's a pharmaceutical dose, not a supplement comparison.
MK-7 has a half-life around 72 hours. It incorporates into LDL, reaches extrahepatic tissues, and is effective at 90–180 mcg once daily (Sato et al., 2012)[5]. In the Sato healthy-women bioavailability study, MK-4 was undetectable in serum at every time point. MK-7 hit peak at six hours and was still measurable at 48.
Our verdict: if a K2 product lists MK-4 at typical supplement doses (a few hundred micrograms), it's the wrong form at the wrong dose. Thorne's D + K2 Liquid uses MK-4, which is why we'd swap it for an MK-7 combo despite the strong brand reputation. Caruso's, Ostelin, Bioglan and BioCeuticals all use MK-7.
The TGA Has Quietly Reshaped the Shelf
In August 2025 the TGA reviewed 10 vitamin-D-only products carrying bone-strength claims (TGA, 2025)[8]. Four were cancelled. Six sponsors had to rewrite their labels. Caruso's Vitamin D3 1000IU copped an infringement notice for failing to hold evidence supporting bone-strength claims on a D-alone product. The message to industry was clear. Standalone D3 with bone-strength language is dead. D3 with calcium, or the full combo, survives.
That's why every major AU sponsor has launched a combo SKU in the last 18 months. New ARTG listings include Microgenics K2+D3 (476402), Swisse Ultiboost K2 + Calcium + D3 (487529), Nuviskin D3+K2 (513827) and UltraLife Mg + Ca + D3 + K2 (515348). The triad isn't endorsed by NHMRC, but it is now the dominant retail format because it's the only format where bone-strength claims survive regulatory scrutiny.
There's a labelling quirk too. Menaquinone-7 isn't listed in Schedule 2 Part 3 of the Permitted Ingredients Determination as a vitamin form. Sponsors using MK-7 face restrictions on calling it "Vitamin K2" in some claim contexts, which is why you'll see "menaquinone-7 (commonly known as vitamin K2)" on the back of more compliant labels.
Calcium Is the Real Story Nobody Tells
The ABS National Nutrition Survey released in March 2026 should have been front-page news (ABS, 2026)[7]. It wasn't. 63.7% of Australians aged 2 and over had inadequate calcium intake from diet in 2023. The female figure was 75.7%. Among women aged 50 to 64, the inadequacy rate was 90.7%. Women 65 to 74 sat at 87.6%. Women 75 plus at 87.2%.
Those numbers reframe the triad. The NHMRC calcium RDI is 1,300 mg/day for women 51 plus and men 71 plus[9]. If nine in ten Australian women over 50 aren't getting it, then the supplement that matters first is calcium, and D3 is the absorption enabler at 800 to 1,000 IU/day once serum 25(OH)D drops below 50 nmol/L. K2 and magnesium are useful adjuncts. They're not the headliners.
What This Costs in Australia
In our tracking, Chemist Warehouse is the anchor and the practitioner channel sits 50–80% above it. Caruso's K2 + D3 + Magnesium 30 tablets is $19.97 at CW, working out to $0.67/day for 180 mcg MK-7, 1,000 IU D3 and 300 mg Mg. Ostelin Calcium & Vitamin D3 + K2 60 tablets is $23.49 (down from RRP $46.99) at $0.39/day with the calcium piece included. Bioglan Vitamin K2 + D3 60 caps is $21.49, working out to $0.36/day.
BioCeuticals D3 + K2 Spray 50 mL is $43.99 practitioner-channel at CW. That's roughly 277 sprays for $0.16/day at one spray, which looks cheap until you account for the distribution-channel premium baked in. Thorne D + K2 Liquid on iHerb AU is $49.26 for 30 mL, and uses MK-4 rather than MK-7, which we'd skip.
Our verdict on value: if you want the full triad in one SKU, Caruso's at $19.97 is the floor. If you need calcium in the mix (and per the ABS data, most women over 50 do), Ostelin Cal+D3+K2 at $23.49 is the better buy. Both undercut anything on iHerb once you factor in shipping.
Dosing, Without the Hype
NHMRC Adequate Intakes for D3 are 200 IU/day under 50, 400 IU/day for 51 to 70, and 600 IU/day over 70[9]. The NHMRC upper limit is 3,200 IU/day, versus the US IOM ceiling at 4,000 IU/day. Clinical trial doses sit at 2,000 IU/day. Going above 4,000 IU/day without serum monitoring isn't supported by current evidence and the J-shaped CV-mortality curve in the 2025 PMC review is reason for caution at the top end.
For K2, MK-7 at 90 to 180 mcg once daily is what the trials used. The three-year RCT that produced the most-cited BMD result ran 180 mcg. Magnesium supplemental UL is 350 mg/day, separate from food, per the NHMRC. Men's RDI is 420 mg/day; women's is 320 mg/day.
One absorption tip worth the mention. Both D3 and MK-7 are fat-soluble. Multiple absorption studies put the uplift from fat co-ingestion at around 32% versus fasted. Take your softgel with a meal that contains some fat. It's the single highest-leverage piece of advice in this guide.
The Verdict
The triad as marketed (D3 + K2 + Mg as a non-negotiable stack): no NHMRC endorsement, no formal clinical-society backing, and the K2 evidence is sensitivity-unstable at lumbar spine and null at hip. Don't buy the framing.
The triad as a sensible combo (D3 + Mg with K2 as a low-cost adjunct, calcium where intake is inadequate): defensible. The magnesium-D activation link is well-evidenced. The MK-7 biomarker effects are real. Calcium is the actually-deficient ingredient for most Australian women over 50.
Best AU buy: Ostelin Calcium & Vitamin D3 + K2 at $23.49 for 60 tablets if you're over 50 or female. Caruso's K2 + D3 + Magnesium at $19.97 if you're younger and just want the combo. Skip MK-4 products. Take with food. Get serum 25(OH)D checked before you self-prescribe at the upper end of the dose range.
- Cross-referenced the 2024 RACGP and Healthy Bones Australia osteoporosis guideline against the actual prose of the "vitamin D triad" wellness framing.
- Pulled VITAL (n=25,871) and the Australian D-Health trial (n=21,315) primary outcomes plus the 2025 cardiovascular follow-up.
- Compared MK-4 and MK-7 form choices against the Sato 2012 bioavailability data and current Japanese osteoporosis protocol dosing.
- Walked the TGA August 2025 bone-strength review, then mapped the surviving combo SKUs in the ARTG.
- Surveyed AU pricing in May 2026 across Chemist Warehouse, iHerb AU and BioCeuticals practitioner channel for D3+K2 and D3+K2+Ca combos.
- Cross-checked the March 2026 ABS National Nutrition Survey calcium-intake data against NHMRC RDIs by age and sex band.
References
- Wong et al., MJA 2025. 2024 RACGP and Healthy Bones Australia guideline for osteoporosis management and fracture prevention
- LeBoff et al., NEJM 2022. Supplemental Vitamin D and Incident Fractures (VITAL)
- Waterhouse et al., 2025. D-Health Trial Cardiometabolic Outcomes
- Zhang et al., 2024. Effects of vitamin K supplementation on BMD: meta-analysis of RCTs
- Sato et al., Nutr J 2012. MK-4 vs MK-7 bioavailability in healthy women
- Eur J Nutr 2024. Magnesium and vitamin D status
- ABS, March 2026. ABS, Australians continue to need more calcium
- TGA, August 2025. TGA, Efficacy of Vitamin D for bone strength
- NHMRC Nutrient Reference Values for Australia and New Zealand