Mechanism of Action
Vitamin D3 follows a two-step activation pathway before reaching its biologically active form. The liver converts it to calcidiol (25-hydroxyvitamin D3), the circulating form measured in blood tests. The kidneys then transform it to calcitriol (1,25-dihydroxyvitamin D3), the active form.
Calcitriol acts at the intestinal level by stimulating the production of calcium transport proteins (calbindins), increasing calcium absorption by 30 to 40%. In bone tissue, it orchestrates remodeling by coordinating osteoblast (bone-building cells) and osteoclast (bone-resorbing cells) activity. On the immune front, it promotes the production of antimicrobial peptides (cathelicidins and defensins) by innate immune cells. Monocytes and macrophages possess their own hydroxylation enzyme, capable of producing calcitriol locally in response to a threat.
Magnesium serves as an obligatory cofactor for the hydroxylation enzymes. Without adequate magnesium intake, conversion to the active form is compromised.
Key Benefits
- Strong
Vitamin D contributes to the maintenance of normal bones, an effect supported by data showing a 15% fracture risk reduction in adults over 65 supplemented at doses above 20 µg per day.
- Strong
Vitamin D contributes to normal immune system function, a role confirmed by a meta-analysis of 25 randomized trials (11,321 participants) showing a 12% reduction in acute respiratory infections.
- Strong
Vitamin D contributes to the maintenance of normal muscle function. Clinical trials in older adults show improved lower limb strength and a 19% reduction in fall risk.
- Strong
Vitamin D contributes to the normal absorption of calcium and phosphorus, minerals essential for bone metabolism and cellular signaling.
- Moderate
A large-scale randomized trial (VITAL, 25,871 participants, 5 years) demonstrated a 22% reduction in autoimmune condition incidence among participants receiving 50 µg of vitamin D3 per day.
- Emerging
Observational data from large cohorts suggest an association between optimal vitamin D status and the maintenance of cellular youth markers, notably telomere length.
Dosage & Forms
Three forms of vitamin D coexist on the market. D2 (ergocalciferol), of plant origin, has a shorter plasma half-life and lower efficacy for raising serum calcidiol. D3 (cholecalciferol) is bioidentical to the form produced by human skin. It raises serum status more sustainably and more pronouncedly than D2, as demonstrated by a 2012 meta-analysis of 7 randomized trials. The third option, calcifediol (25-OH-D3), offers rapid absorption without hepatic hydroxylation, but its cost and limited availability restrict its routine use.
Singular selects D3 as plant-sourced cholecalciferol (lichen), dosed between 25 and 89 µg depending on the profile. The µg unit is preferred over IU for precision (1 µg = 40 IU). The dose is calibrated based on sex, age, phototype, season and biological results when available.
In the Singular Formula
Inclusion rationale
Vitamin D contributes to the normal functioning of the immune system, to the maintenance of normal bones, normal teeth, normal muscle function and to the normal absorption of calcium and phosphorus. Vitamin D3 (cholecalciferol) is the form naturally produced by human skin under the effect of UVB rays. From October to March above the 35th parallel, the angle of solar incidence is too low to allow significant cutaneous synthesis, creating what researchers call 'vitamin D winter.' Vitamin D functions as a pro-hormone: once hydroxylated to calcitriol (1,25-dihydroxyvitamin D3) by the liver then the kidneys, it binds to the VDR (Vitamin D Receptor), a transcription factor expressed in over 30 cell types, regulating the expression of more than 1,000 genes involved in immunity, cell differentiation and phosphocalcic metabolism. D3 is part of the formula's bone axis, where it works in synergy with vitamin K2-MK7 (which directs calcium toward bones rather than arteries), magnesium (cofactor of vitamin D hydroxylation enzymes), boron and calcium alpha-ketoglutarate.
Selected form
Plant-sourced cholecalciferol (lichen), encapsulated in a protective matrix of acacia gum, modified starch and natural antioxidants (vitamin E, sodium ascorbate). This microencapsulation shields vitamin D3 from light and oxidation, ensuring optimal stability. The plant source (lichen) provides a certified vegan alternative to ovine lanolin, the classic cholecalciferol source. Vitamin D contributes to the maintenance of normal bones, normal immune system function and normal absorption of calcium. Quality: vegan, non-GMO.
Formula dosage
0 to 89 µg.
Synergies in the formula
Safety & Precautions
Vitamin D3 has an extensive safety record. The European Food Safety Authority sets the tolerable upper intake level at 100 µg (4,000 IU) per day for adults. The dosages proposed by Singular (25 to 89 µg) remain below this threshold.
Prolonged excess (above 250 µg per day over several months) may lead to hypercalcemia (elevated blood calcium), with renal and cardiovascular consequences. This risk concerns very high doses, far from standard supplementation intakes. Individuals taking medication containing vitamin D should seek medical advice before combining sources. Supplementation is not recommended in cases of granulomatosis (sarcoidosis) without medical supervision, due to increased calcitriol conversion. Use during pregnancy or breastfeeding requires medical advice to adjust the dosage.
Scientific Studies
| Authors | Year | Type | Journal | |
|---|---|---|---|---|
| Martineau AR et al. | 2017 | Meta-analysis | BMJ | View on PubMed |
Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data Meta-analysis of 25 randomized trials (11,321 participants) showing an overall 12% reduction in acute respiratory infections with vitamin D supplementation, with greater benefit in subjects with low baseline status. | ||||
| Hahn J et al. | 2022 | Randomised Controlled Trial | BMJ | View on PubMed |
Vitamin D and marine omega 3 fatty acid supplementation and incident autoimmune disease: VITAL randomized controlled trial Randomized trial (25,871 participants, 5.3-year follow-up) showing a 22% reduction in confirmed autoimmune conditions in the vitamin D3 group (50 µg/day) versus placebo. | ||||
| Manson JE et al. | 2019 | Randomised Controlled Trial | New England Journal of Medicine | View on PubMed |
Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease The VITAL trial (25,871 participants, 5 years) did not show a significant reduction in invasive cancer or major cardiovascular events with 50 µg/day of vitamin D3. A trend toward reduced cancer mortality was observed after excluding the first two years. | ||||
| Bischoff-Ferrari HA et al. | 2009 | Meta-analysis | BMJ | View on PubMed |
Fall prevention with supplemental and active forms of vitamin D: a meta-analysis of randomised controlled trials Meta-analysis of 8 randomized trials showing a 19% fall risk reduction with vitamin D doses above 17.5 µg per day, primarily in older adults. | ||||
| Bolland MJ et al. | 2018 | Meta-analysis | Lancet Diabetes and Endocrinology | View on PubMed |
Effects of vitamin D supplementation on musculoskeletal health: a systematic review, meta-analysis, and trial sequential analysis Systematic review and meta-analysis of 81 randomized trials concluding that vitamin D supplementation does not significantly alter total fracture risk in the general population, but identifying benefit in at-risk populations. | ||||
| Holick MF | 2007 | Systematic Review | New England Journal of Medicine | View on PubMed |
Vitamin D Deficiency Landmark review on vitamin D metabolism, the consequences of insufficient status and supplementation recommendations. This paper helped redefine vitamin D sufficiency thresholds. | ||||