A new systematic review published in The BMJ analysed 69 randomised controlled trials involving 153,902 participants. The conclusion: calcium supplements, vitamin D supplements, and the combination of the two provide little to no clinically meaningful benefit for preventing fractures or falls in most older adults.

For a category of supplements widely recommended by doctors and used by millions, that's significant. The conclusion sounds unambiguous, but the study doesn’t highlight two important things. First, it did not assess participants' digestive capacity, which is known to decline with age and which directly affects how well calcium can be absorbed. Second, it did not separately analyse the form of calcium used in each trial. Not all calcium sources are equally absorbent, and the trials reviewed used conventional salt forms (mostly calcium carbonate and calcium citrate) — not newer, more bioavailable forms such as ionic calcium.

But the study also contains important nuances that have been largely missed in the headlines, and those nuances matter if you take calcium for bone health, or are thinking about it.

REVIEW FINDINGS

What the study actually showed

For each intervention, the review found:

01 — CALCIUM ALONE

Calcium alone

Calcium alone (11 trials, 9,067 participants): A small directional trend toward fewer fractures (risk ratio 0.91), but the confidence interval crossed 1.00: meaning the result was not statistically meaningful. Effects on hip fracture, falls, and vertebral fracture were similarly inconclusive.

02 — VITAMIN D ALONE

Vitamin D alone

Vitamin D alone (36 trials, 92,045 participants): Essentially no effect on any fracture outcome (risk ratio 1.00). This was rated high-certainty evidence and the most confident result in the entire review.

03 — CALCIUM AND VITAMIN D COMBINED

Calcium and vitamin D combined

Calcium and vitamin D combined (15 trials, 51,126 participants): A statistically significant 9% reduction in any fracture (risk ratio 0.91, 95% CI 0.84 to 0.99). In absolute terms, however, this worked out to roughly 1% fewer fractures, which is below the threshold the authors had pre-defined as clinically meaningful. The signal also weakened substantially when one outlier trial (Chapuy 1992, conducted in very high-risk French nursing-home residents with severe vitamin D deficiency) was removed from the analysis.

04 — AUTHOR CONCLUSION

The authors concluded that the evidence "does not support routine supplementation with calcium, vitamin D, or combined supplementation to prevent fractures or falls" and called for clinical guidelines and regulatory agencies to revisit their recommendations.

THE LIMITATIONS

The limitations the authors themselves flagged

What's striking about the paper, and what tends to get lost in summary coverage, is how candidly the authors discuss the constraints of the underlying trials. Several limitations are worth understanding before drawing broader conclusions about calcium itself, as opposed to calcium as it was tested.

01

Most trials did not study high-risk people

Of the 69 trials, 87% enrolled community-dwelling adults, and 73% did not target people at high risk of fractures or falls. Only 3% of trials reported mean baseline vitamin D levels below 25 nmol/L — the threshold at which calcium absorption becomes seriously impaired. The biggest historical benefit ever shown (Chapuy 1992) was in women averaging 84 years old, severely vitamin D deficient, with dietary calcium intakes around 513 mg/day. That population is not who is typically buying calcium tablets at a pharmacy.

02

Control-group contamination was widespread

The authors note that "almost all studies either allowed participants to take non-trial supplements or did not clearly report instructions for avoiding non-trial supplementation." If placebo-group participants were quietly taking their own calcium and vitamin D — which is plausible given how widely these are recommended — the trials would systematically underestimate any real effect.

03

Tolerance was a real-world problem

The discussion section is blunt: calcium supplements "are often difficult to swallow and poorly tolerated in older adults, commonly causing gastrointestinal adverse effects such as constipation, bloating, abdominal pain, or cramps." A previous analysis cited in the paper suggested possible increases in gastrointestinal-related hospital admissions linked to calcium supplements. The Women's Health Initiative also found increased kidney stone incidence with combined supplementation, and a 10–20% relative increase in myocardial infarction has been reported in some calcium-supplement meta-analyses (though others have not replicated this).

04

The form of calcium varied enormously and was not analysed separately

The review applied "no restrictions on type or dose of calcium." The included trials used calcium carbonate, calcium citrate, and other conventional calcium salt forms. The review did not, and was not designed to, compare absorption differences between calcium sources, or to test whether more bioavailable forms produced different outcomes.

This last point is, in some ways, the most important for consumers.

THE ABSORPTION QUESTION

The absorption question the study didn't ask

Calcium carbonate, the most commonly studied form, requires stomach acid to dissociate before any calcium becomes available for absorption. Older adults, the population most often targeted for bone-health supplementation, frequently have reduced gastric acid production (aka reduced digestion). Many also take acid-suppressing medications such as proton pump inhibitors or H2 blockers, which further impair absorption. Calcium carbonate then needs adequate vitamin D status to be efficiently transported across the intestinal wall via the active calbindin pathway.

TRIAL MODEL

Calcium Salt
↓
Stomach Acid
↓
Vitamin D Transport
↓
Bone

FAILURE POINTS

In other words, the trials in the BMJ review tested a model that goes roughly like this: take a calcium salt, hope the stomach acid breaks it down, hope vitamin D status is sufficient to transport it, hope it reaches bone. There are multiple failure points in that chain.

THE QUESTION

This is a separate question from whether calcium itself matters for bone, which observational data consistently suggest it does. The question is whether the delivery system used in most trials is fit for purpose.

IONIC CALCIUM

Ca²⁺
↓
Passive Absorption
↓
Bone

WHY THIS MATTERS

Some calcium sources bypass parts of this chain. Ionic calcium: calcium already in its dissociated, charged form (Ca²⁺), does not require gastric acid to become bioavailable. Absorption research suggests ionic forms can use passive (paracellular) absorption pathways that are less dependent on vitamin D status than the active (transcellular) pathway. Proprietary ionic calcium formulations such as the SAC® formula, used in Marah Natural, are built around this principle.

IMPORTANT CAVEAT

To be clear: no large randomised trial has tested whether ionic calcium changes fracture outcomes, and the BMJ review does not speak to that question either way. But it does underscore that "calcium supplementation" as a category is not one thing and that the trials we have should not be read as a verdict on every calcium source.

The Silent Crisis Inside Your Bones: A Global Health Issue That Needs Answers

The Silent Crisis Inside Your Bones: A Global Health Issue That Needs Answers

Postmenopausal osteoporosis affects hundreds of millions of women globally. When estrogen levels drop during menopause, the body’s bone-remodeling system falls out of balance: bone-destroying cells (osteoclasts) begin outpacing bone-building cells (osteoblasts). The result is a progressive, silent architectural collapse within bone tissue.

Standard calcium supplementation has long been considered foundational to bone health. But the SKKU research team asked a harder question: does the form of calcium, how it ionizes, how it signals, how cells respond to it, actually change outcomes at the molecular level?

To answer this, they studied Sigma Anti-Bonding Calcium Carbonate (SACx®), a formulation engineered to enhance the release of freely ionized calcium (Ca²⁺) in aqueous environments. SAC® was tested in the most rigorous available preclinical framework: an ovariectomized mouse model combined with direct cellular mechanistic analysis. The researchers didn’t just measure bone density. They looked inside the bone, at its microscopic scaffold, and what they found changes the conversation about what a calcium supplement should actually do.

Bone strength depends not only on mineral quantity but also on bone quality, which encompasses microarchitecture, matrix composition, mineralization heterogeneity, and structural connectivity.

— Seeman & Delmas, New England Journal of Medicine, 2006

What Is SAC® (Sigma Anti-Bonding Calcium Carbonate)?

What Is SAC® (Sigma Anti-Bonding Calcium Carbonate)?

SAC® is not your standard calcium. It is a proprietary formulation engineered at the molecular level to enhance ionization and solubility in aqueous (watery) environments. The name refers to its structural design: by leveraging sigma anti-bonding molecular principles, SAC® weakens the intermolecular ionic interactions that normally hold calcium atoms together in tight crystalline formations.

The practical result? A greater proportion of freely dissociable calcium ions, the biologically active form, available for cellular uptake and signaling. This matters because calcium in the body is not just a building block. It is a signaling molecule that influences how bone cells behave, how they grow, and crucially, how and when they destroy bone.

Calcium exists in multiple states in the body — bound to proteins, complexed with phosphate, or as free ionized Ca²⁺. Only the ionized fraction is biologically active for cellular signaling. Conventional calcium supplements vary considerably in how much ionized calcium they release, which is influenced by gastric acidity, formulation type, and individual absorption capacity. SAC®’s design targets this critical ionization step directly.

— SCIENCE NOTE — WHY IONIC FORM MATTERS

How the Study Was Conducted

The SKKU research team ran a 16-week study. Female C57BL/6 mice underwent bilateral ovariectomy to induce estrogen deficiency. Following a 3-week surgical recovery, SAC® was administered orally at 100 or 200 mg/kg/day for 13 continuous weeks. A sham-operated group served as the healthy control baseline, and an untreated OVX group served as the disease model comparator.

At study completion, the team assessed outcomes across three levels: whole-body tolerability (body and organ weights), structural bone architecture (micro-computed tomography and histology), and blood biomarkers of bone metabolism (CTX, BALP, P1NP, osteocalcin).

In parallel, they conducted in vitro experiments using RAW264.7 cells, a validated macrophage precursor cell line, stimulated with RANKL to induce osteoclast differentiation. This allowed the team to directly observe how SAC interacts with the molecular machinery of bone destruction, gene by gene, protein by protein.

 

THE FINDINGS — WHAT SUNGKYUNKWAN UNIVERSITY DISCOVERED

Six Results That Change the Conversation About Calcium

These are not preliminary signals or marginal effects. The SKKU team found consistent, dose-dependent, statistically significant results across structural, biochemical, and molecular measures.

STRUCTURAL & BIOCHEMICAL FINDINGS

Bone Density, Resorption, and Architecture

BMD ↑

Bone Density Preserved

Significant attenuation of mineral density loss at both doses tested.

CTX ↓

Bone Resorption Reduced

Serum CTX, the gold-standard resorption biomarker, significantly lowered.

Tb.N ↑

Trabecular Architecture Saved

Trabecular number partially restored; bone structure shifted toward healthier plate-like form.

MOLECULAR & SAFETY FINDINGS

Pathway Suppression and 13-Week Safety

p38 ↓

Destruction Pathway Blocked

p38 MAPK phosphorylation suppressed, reducing the upstream trigger for osteoclast creation.

NFATc1 ↓

Master Gene Silenced

NFATc1, the master osteoclast transcription factor, reduced dose-dependently.

SAFE

13-Week Safety Confirmed

No changes to body weight or any major organ across 13 weeks of daily oral use.

STRUCTURAL BONE PARAMETERS

Bone Architecture Findings

Swipe sideways to view all columns →

Bone Parameter Effect of OVX Effect of SAC® Clinical Significance
Bone Mineral Density
(BMD)		 Significantly decreased Significantly improved Core protection of mineral density
Bone Volume Fraction (BV/TV) Substantially reduced Modest recovery More bone tissue per volume
Trabecular Number
(Tb.N)		 Significantly decreased Partially restored Network connectivity preserved
Structure Model Index (SMI) Elevated (rod-like degradation) Reduced toward plate-like Healthier architecture maintained
CTX (bone resorption marker) Markedly elevated Significantly reduced Less active bone breakdown

CLINICAL INTERPRETATION

Why SMI Matters

Of particular note is the Structure Model Index (SMI), a measure that distinguishes healthy plate-like trabecular bone from weakened rod-like bone. When bone shifts from plate-like to rod-like structure, it becomes dramatically weaker — like replacing the planks in a floor with matchsticks. SAC® treatment shifted this metric back toward healthier architecture, a finding that goes well beyond what BMD alone would reveal.

The Results: SAC® Preserves the Bone’s Inner Architecture

SAC®-mediated preservation of trabecular number is likely biomechanically meaningful. The primary lesion of postmenopausal bone loss is network disruption rather than uniform thinning.

— Sungkyunkwan University Research Team

The micro-computed tomography (micro-CT) findings were visually and quantitatively striking. Ovariectomized mice showed severe deterioration of their trabecular bone network, a sparse, disconnected scaffold. But mice given SAC® showed meaningful structural preservation in a dose-dependent manner.

The Molecular Story: SAC® Silences Bone Destroyers

The Molecular Story: SAC® Silences Bone Destroyers

The cellular experiments revealed something even more remarkable: SAC® doesn’t just supply minerals, it appears to directly interfere with the molecular machinery that creates osteoclasts, the cells responsible for bone destruction.

When RANKL binds to its receptor RANK on bone marrow precursor cells, it activates p38 MAPK (a protein kinase), which in turn activates NFATc1 — the “master switch” for osteoclast creation. NFATc1 then turns on genes like TRAP, Cathepsin K, and Atp6v0d2 that give osteoclasts their destructive capacity. SAC® was shown to interrupt this cascade at multiple points: suppressing p38 phosphorylation, reducing NFATc1 protein levels, and decreasing expression of all downstream osteoclastic genes — dose-dependently, and without any toxicity to healthy cells.

— SCIENCE NOTE — THE RANKL PATHWAY EXPLAINED

Swipe sideways to view →

Molecular Target Role in Bone Loss SAC Effect
p38 MAPK (phosphorylated) Signals osteoclast formation downstream of RANKL Reduced phosphorylation
NFATc1 protein Master transcription factor, the “on switch” for osteoclasts Dose-dependent reduction
MITF protein Co-regulator of osteoclast gene expression program Reduced expression
TRAP mRNA Osteoclast activity marker, marks actively resorbing bone Suppressed
Cathepsin K (Ctsk) mRNA Protease that physically dissolves bone collagen matrix Suppressed
Atp6v0d2 mRNA Ion pump enabling acid secretion for mineral dissolution Suppressed

What the Research Team Concluded

The SKKU authors concluded that SAC® may mitigate estrogen deficiency–associated trabecular bone deterioration through suppression of osteoclast differentiation and modulation of the p38–NFATc1/MITF signaling axis.

Modulation of calcium ion availability may therefore influence bone remodeling beyond conventional mineral supplementation and represents a potential strategy for managing postmenopausal osteoporosis.

— Oh Y, Yoo BC et al. — Dept. of Integrative Biotechnology, Sungkyunkwan University

These are preclinical findings. Human clinical trials are the next step. But when a 626-year-old institution produces results this consistent across structural imaging, blood biomarkers, and molecular signaling, the foundation for confidence is real.

Why This Research Matters for You Right Now

Menopause is not a distant event for millions of women worldwide — it is happening right now, or will happen soon. In the year following menopause, women can lose up to 3–5% of total bone mass. Within the first decade after menopause, trabecular bone loss can be even more dramatic.
 
The Sungkyunkwan University data position SAC® as a compelling option: a formulation that demonstrated structural bone protection, a meaningful reduction in the active bone resorption marker CTX, and a strong mechanistic rationale—all with a 13-week safety profile showing no organ toxicity.

Conventional Calcium: The Difference That Matters

Conventional Calcium: The Difference That Matters

The distinction between SAC® and ordinary calcium is not marketing language — it is structural chemistry with measurable biological consequences:

When RANKL binds to its receptor RANK on bone marrow precursor cells, it activates p38 MAPK (a protein kinase), which in turn activates NFATc1 — the “master switch” for osteoclast creation. NFATc1 then turns on genes like TRAP, Cathepsin K, and Atp6v0d2 that give osteoclasts their destructive capacity. SAC® was shown to interrupt this cascade at multiple points: suppressing p38 phosphorylation, reducing NFATc1 protein levels, and decreasing expression of all downstream osteoclastic genes — dose-dependently, and without any toxicity to healthy cells.

— SCIENCE NOTE — THE RANKL PATHWAY EXPLAINED

Swipe sideways to view full comparison →

Standard Calcium
Sigma Anti-Bonding Calcium Carbonate (SAC®)
— Requires stomach acid for dissolution
Engineered for enhanced ionization in aqueous environments
— Absorption efficiency declines with age
Higher proportion of freely dissociable Ca2+ ions
— Acts primarily as a mineral building block
Demonstrated direct suppression of osteoclast signaling
— Limited direct signaling modulation shown
Targets p38 MAPK → NFATc1 → MITF cascade
— Often needs large doses for effect
Safe across 13 weeks of daily oral administration

Frequently Asked Questions

Research Context and Interpretation

The study was conducted by researchers at Sungkyunkwan University’s Department of Integrative Biotechnology in Suwon, South Korea, one of Asia’s oldest and most respected research universities, founded in 1398. The research was supported by the National Research Foundation of Korea (NRF) through the Ministry of Education, following IACUC-approved protocols.

Preclinical research is the essential scientific foundation that informs what is worth testing in human clinical trials. The ovariectomized mouse model used in this study is the regulatory gold standard for evaluating interventions targeting postmenopausal bone loss. While human clinical trials remain the final confirmation, findings this consistent across both a validated animal model and cellular mechanistic assays represent a very strong evidence foundation.

CTX, C-terminal telopeptide of type I collagen, is the most widely used biomarker of bone resorption in both clinical and research settings. Elevated CTX levels directly reflect increased osteoclast activity and accelerated bone loss. The significant reduction in CTX in SAC-treated mice, alongside preserved bone architecture on micro-CT, provides convergent evidence that SAC genuinely reduces active bone destruction.

No. At both doses tested, 100 and 200 mg/kg daily for 13 weeks, SAC® produced no significant changes in body weight or in the weight of any major organ including the liver, kidney, lung, spleen, brain, stomach, and intestine. In cell viability assays, SAC showed no cytotoxicity at any dose tested.

Disclaimer

This article is based on preclinical research and is intended for educational and informational purposes only. It does not constitute medical advice. The findings described represent results from animal and cell-based studies; human clinical outcomes may differ. Consult a qualified healthcare professional before beginning any supplement regimen. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Scientific References

  1. Oh Y, Yoo BC, Moon S, et al. Sigma Anti-Bonding Calcium Attenuates Ovariectomy-Induced Bone Loss by Preserving Trabecular Microarchitecture and Suppressing Osteoclastogenesis. Sungkyunkwan University, Dept. of Integrative Biotechnology, Suwon 16419, Republic of Korea. Supported by NRF Korea (2017R1A6A1A03015642).
  2. Compston JE, McClung MR, Leslie WD. Osteoporosis. Lancet. 2019;393:364–376.
  3. Seeman E, Delmas PD. Bone quality, the material and structural basis of bone strength and fragility. N Engl J Med. 2006;354:2250–2261.
  4. Bouxsein ML, et al. Guidelines for assessment of bone microstructure in rodents using micro-computed tomography. J Bone Miner Res. 2010;25:1468–1486.
  5. Takayanagi H, et al. Induction and activation of NFATc1 integrate RANKL signaling in terminal differentiation of osteoclasts. Dev Cell. 2002;3:889–901.
  6. Negishi-Koga T, Takayanagi H. Ca2+–NFATc1 signaling is an essential axis of osteoclast differentiation. Immunol Rev. 2009;231:241–256.
  7. Matsumoto M, et al. Involvement of p38 MAPK signaling pathway in osteoclastogenesis mediated by RANKL. J Biol Chem. 2000;275:31155–31161.
  8. Choi SY, Park D, Yang G, et al. Effects of Sigma Anti-bonding Molecule Calcium Carbonate on bone turnover and calcium balance in ovariectomized rats. Lab Anim Res. 2011;27:301–307.
  9. Riggs BL, Khosla S, Melton LJ. A unitary model for involutional osteoporosis. J Bone Miner Res. 1998;13:763–773.
  10. Lu SY, Li M, Lin YL. Mitf induction by RANKL is critical for osteoclastogenesis. Mol Biol Cell. 2010;21:1763–1771.