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Mechanism of ActionKey BenefitsDosage & FormsIn the Singular FormulaSafety & PrecautionsScientific StudiesFrequently Asked Questions
Vitamin D3

Vitamin D3

Cholécalciférol · Cholecalciferol · Vitamine du soleil · Sunshine vitamin

VitaminsGenomic stability

Vitamin D3 (cholecalciferol) is the form of vitamin D that human skin produces under UVB light. Its best-established roles concern bone and muscle: it contributes to the maintenance of normal bones and to the maintenance of normal muscle function. It also contributes to normal absorption/utilisation of calcium and phosphorus, the two minerals that feed bone renewal. It further contributes to the normal function of the immune system, to normal blood calcium levels and to the maintenance of normal teeth. It also has a role in the process of cell division. These functions matter all the more with age, precisely when vitamin D status itself tends to fall: cutaneous synthesis declines and sun exposure becomes rarer. Maintaining that status supports biological capital where time weakens it earliest.

Last updated: August 13, 2026

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, the calbindins, which carry the mineral from the intestinal lumen into the blood. In bone tissue, it orchestrates remodeling by coordinating two cell families: osteoblasts, which build bone, and osteoclasts, which resorb it.

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

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    Bone upkeep, year after year: vitamin D contributes to the maintenance of normal bones.

  • Strong

    Steadier footing after 60. Vitamin D helps to reduce the risk of falling associated with postural instability and muscle weakness. Falling is a risk factor for bone fractures among men and women 60 years of age and older. The beneficial effect is obtained with a daily intake of 20 µg of vitamin D from all sources.

  • Strong

    Muscles that answer the demand: vitamin D contributes to the maintenance of normal muscle function.

  • Strong

    Defences that stay operational all year: vitamin D contributes to the normal function of the immune system.

  • Strong

    Two structural minerals put to better use: vitamin D contributes to normal absorption/utilisation of calcium and phosphorus.

  • Strong

    Blood calcium held within range. Vitamin D contributes to normal blood calcium levels, the circulating calcium that the body regulates continuously.

  • Strong

    Teeth maintained by the same mineral mechanism as bone: vitamin D contributes to the maintenance of normal teeth.

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    Present down to tissue renewal: vitamin D has a role in the process of cell division.

Dosage & Forms

Three forms of vitamin D coexist on the market. D2 (ergocalciferol), of plant origin, raises serum calcidiol less effectively; calcidiol is the circulating form measured in blood tests. D3 (cholecalciferol) is bioidentical to the form human skin produces. A 2024 meta-analysis of twenty direct human comparisons places it above D2, including under daily dosing, with a mean difference of 10.4 nmol/L on calcidiol. The third option, calcifediol, is absorbed without hepatic hydroxylation; its cost and availability limit routine use.

Clinical trials have explored a wide range, from 10 µg (400 IU) to 100 µg (4,000 IU) per day. The condition of use attached to the falling-risk claim sets the reference intake at 20 µg per day from all sources.

Ceilings differ by market. The European tolerable upper intake level for adults is 100 µg per day. France allows at most 89 µg per day in food supplements, Switzerland 70 µg. Intestinal absorption rises when the intake accompanies a meal containing fat.

In the Singular Formula

Inclusion rationale

Vitamin D contributes to the normal function of the immune system, to the maintenance of normal bones, normal teeth and normal muscle function, to normal blood calcium levels and to normal absorption/utilisation of calcium and phosphorus. Vitamin D3 (cholecalciferol) is the form naturally produced by human skin under the effect of UVB rays. Above the 42nd parallel, the angle of solar incidence stays too low from November to February to allow cutaneous synthesis, and that window lengthens as latitude rises: this is 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 many cell types, where it governs the expression of 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 helps a protein in bone bind calcium there) and magnesium, cofactor of vitamin D hydroxylation enzymes.

Selected form

L-VITAL® D3 PW25 Fiber is a liposomal cholecalciferol powder derived from lanolin and deposited on an insoluble plant-fibre matrix. This animal origin is not suitable for vegan diets. The finished ingredient is manufactured in France, and its matrix combines pea fibre, sunflower lecithin, inulin and coconut MCT oil. Vitamin D contributes to the maintenance of normal bones, to the normal function of the immune system and to normal absorption/utilisation of calcium and phosphorus.

Formula dosage

0 to 70 µg.

Dose expressed as active substance, excluding excipients and carriers of the raw material.

Synergies in the formula

Vitamin D3 holds a central position in the bone axis of the Singular formula. It facilitates intestinal calcium absorption. Vitamin K2-MK7 helps a protein in bone bind calcium there. The two bioactives act at complementary stages of the bone axis. Magnesium acts upstream, as a cofactor of the enzymes that convert vitamin D3 into its active form. Insufficient magnesium intake limits that conversion, whatever serum calcidiol is reached. The complementarity is physiological, and it is why the three sit on the same axis. Beyond the bone axis, vitamin D3 shares an immune support role with zinc. Both contribute to the normal function of the immune system, through distinct and complementary pathways.

Linked Biomarkers

Vitamin D (25-OH)

Safety & Precautions

Vitamin D3 has an extensive record of use. The European tolerable upper intake level for adults, including pregnant and breastfeeding women, was re-examined in 2023 and kept at 100 µg (4,000 IU) per day. The endpoint used to set it is persistent hypercalciuria, a sustained rise in urinary calcium output, considered an earlier sign than raised blood calcium.

An intake sustained above 250 µg per day is the level at which that effect has been observed in humans. A synthesis of twenty-two trials also reports, between 80 and 100 µg per day over six months or more, a higher frequency of raised blood calcium and of falls. The intakes offered stay below that range.

People already taking a medicine that contains vitamin D should seek medical advice before combining sources. Supplementation is not advised without medical supervision in cases of granulomatosis, including sarcoidosis, where conversion to calcitriol runs high. It calls for close monitoring in cases of impaired kidney function, since renal activation of vitamin D is altered. During pregnancy or breastfeeding, medical advice helps adjust the intake.

Scientific Studies

AuthorsYearTypeJournal

Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data

Individual participant data meta-analysis of 25 randomized trials (11,321 participants): supplementation is associated with a 12 % lower risk of acute respiratory infection. The effect is larger in subjects with the lowest baseline status, and appears only with daily or weekly dosing.

Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials

Update of the previous analysis by the same team, extended to 43 studies and 48,488 participants. The risk reduction falls to 8 % and the authors describe it as small. It appears only in daily-dosing trials, at 400 to 1,000 IU, over durations of twelve months or less, and in participants enrolled between 1 and 16 years of age.

Comparison of the Effect of Daily Vitamin D2 and Vitamin D3 Supplementation on Serum 25-Hydroxyvitamin D Concentration and Importance of Body Mass Index: A Systematic Review and Meta-Analysis

Head-to-head comparison of vitamin D2 and D3 restricted to daily or weekly dosing, across twenty studies. D3 raises serum calcidiol more, with a mean difference of 10.4 nmol/L across twelve daily comparisons analysed by mass spectrometry. Body mass index is the strongest modifier of the response.

Fall prevention with supplemental and active forms of vitamin D: a meta-analysis of randomised controlled trials

Meta-analysis of 8 randomized trials (2,426 participants, mean age 65 years or older). The 19 % lower fall risk applies to the high-dose stratum, that is 7 trials and 1,921 participants receiving 700 to 1,000 IU per day; doses below 700 IU make no difference. The sample is modest compared with syntheses published since.

Effects of vitamin D supplementation on musculoskeletal health: a systematic review, meta-analysis, and trial sequential analysis

Systematic review of 81 randomized trials (53,537 participants). Supplementation alters neither total fracture risk (36 trials, 44,790 participants), nor fall risk (37 trials, 34,144 participants), nor bone mineral density. The authors find no difference between higher and lower doses, including above 800 IU per day.

Supplemental Vitamin D and Incident Fractures in Midlife and Older Adults

The largest and longest trial devoted to fractures: 25,871 adults followed for 5.3 years on 2,000 IU per day, not selected on vitamin D status. No effect on total, non-vertebral or hip fractures. No substantial between-group difference in adverse events.

Vitamin D and marine omega 3 fatty acid supplementation and incident autoimmune disease: VITAL randomized controlled trial

Autoimmune arm of the VITAL trial: 25,871 participants, 5.3 years median follow-up, 2,000 IU per day. The vitamin D group had 123 confirmed autoimmune conditions versus 155 on placebo, a 22 % reduction, at the exact threshold of significance.

Vitamin D and Marine n-3 Fatty Acids for Autoimmune Disease Prevention: Outcomes Two Years After Completion of a Double-Blind, Placebo-Controlled Trial

Two-year extension of the same trial, in 21,592 participants. The effect observed during randomization is not sustained after supplementation stops. Adding 65 cases with onset during the randomized period but confirmed later brings that period below statistical significance.

Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease

The VITAL trial (25,871 participants, 5.3 years median follow-up) showed no significant reduction in invasive cancer incidence (hazard ratio 0.96) or major cardiovascular events (0.97) with 50 µg per day of vitamin D3. Cancer mortality was lower without reaching significance (0.83). No excess of hypercalcemia or other adverse events was identified.

Vitamin D Deficiency

Landmark review of vitamin D metabolism and of the consequences of insufficient intake. It helped set the serum status thresholds still in use today. The safety limits it discusses have since been re-examined by the European authorities.

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