Vitamin ·Strong evidence ·Reviewed May 2026
The sunshine vitamin, essential for calcium absorption, bone health, immune function, and mood regulation. Most people are deficient, especially in northern latitudes.
The bottom line
Evidence rating strong. Most-documented uses: bone health, immune support, mood regulation (primarily in deficient individuals). 21 sources indexed (2017–2024), with 55 interaction records on file.
Core mechanism
Converted to calcidiol (25-OH-D) in the liver, then to calcitriol (1,25-OH2-D) in the kidneys. Calcitriol acts as a hormone, binding to vitamin D receptors (VDR) in nearly every cell, regulating calcium/phosphorus absorption and immune cell function.
Ranked by evidence and value.
Real-world pricing across three quality tiers. Assumes Softgel / Oil-Based D3.
Assumes a 1,000-4,000 IU daily maintenance dose. Vitamin D3 remains one of the cheapest supplements in the database on a per-effective-dose basis. Updated 2026-04-02.
How much you'd eat to match a supplemental dose.
Fortified foods help, but oily fish or sunlight usually matter more.
Food alone rarely covers therapeutic vitamin D targets.
Therapeutic-style vitamin D intakes are difficult to get from food alone.
Dose: 1,000-2,000 IU daily
Timing: With a fat-containing meal
Adjust to lab values rather than using one fixed dose year-round.
Dose: 1,000-4,000 IU daily (higher only clinician-directed short term)9
Timing: With a fat-containing meal
Recheck 25-OH vitamin D and calcium before using doses above 4,000 IU/day or staying at the high end long term.
What to test, the optimal window inside the conventional range, and how long a response takes.
Effective vitamin D3 supplementation should raise serum 25(OH)D; the target range should be individualized.2,21
Late winter is useful for a baseline. Recheck after dose changes and pair with calcium review if using higher doses.
Repleting low vitamin D tends to lower PTH back toward the lower part of the normal range, an effect that is typically modest, dose-dependent, and clearest when baseline vitamin D is deficient and PTH is elevated.1,2
Order PTH alongside 25-hydroxyvitamin D and serum calcium so results can be interpreted together. PTH has a mild diurnal pattern, so try to draw at a consistent time of day, and retest after about 3 months of consistent dosing.
Serum calcium usually stays stable, but high-dose vitamin D3 can raise it, so the aim is to keep it within normal range; any rise is typically modest and dose-dependent, and is most relevant at high intakes or with impaired regulation.2,5
Draw fasting if possible and consider albumin to allow a corrected-calcium calculation, since low albumin can mask a true rise. Recheck within 1 to 3 months when starting high-dose D3, and pair with 25-hydroxyvitamin D.
Where this appears in the symptom-to-supplement map, ranked by relevance.
Fat malabsorption in active celiac lowers vitamin D, which alongside calcium loss contributes to the low bone density seen at diagnosis, so repletion supports bone recovery.1,10
Adjunctive; ask your clinician for a 25-hydroxyvitamin D level to set the dose rather than guessing, especially before high-dose repletion.
Vitamin D is essential for calcium absorption and bone mineralization; deficiency increases fracture risk.12,5
Test 25(OH)D; target 30 to 50 ng/mL.
Vitamin D drives intestinal calcium absorption and bone mineralization, and low status is a recognized risk factor for stress fractures in athletes and military recruits.12,7
Recurrent stress fractures need medical workup for bone density, energy availability, and hormonal causes; supplements are supportive only.
Vitamin D supports antimicrobial peptide production and balanced respiratory mucosal immunity, and correcting low levels reduces acute respiratory infection risk.13,20
Daily or weekly dosing outperforms infrequent megadoses for infection prevention; test your level to set the dose.
Reduced winter sunlight lowers vitamin D synthesis, and because D receptors are present in mood-related brain regions, low status may be associated with seasonal low mood, though a causal role is not established.15,19
Check a 25-hydroxyvitamin D blood level before high-dose supplementing and aim to keep it in the sufficient range.
Vitamin D modulates both innate and adaptive immune responses, and correcting the deficiency common in older adults may support immune defense against respiratory infections.5,13
Frequent infections in older adults warrant a clinician review; supplements are supportive only and ideally dosed to a measured blood level.
Vitamin D supports innate and adaptive immune regulation, especially when status is low.1,2
Check labs if infections are recurrent.
Vitamin D is necessary for efficient calcium absorption and bone turnover regulation.7,12
Vitamin D status should be measured, not guessed.
Vitamin D regulates innate and adaptive immune function, and correcting deficiency is associated with fewer upper respiratory infections in athletes.13,20
Check blood levels before high-dose use; deficiency correction matters most in low-sun seasons.
Vitamin D regulates keratinocyte proliferation and immune signaling, and low status is common in psoriasis, making repletion a reasonable adjunct.1,2
Check 25-OH vitamin D first and dose to target rather than empirically at high amounts.
Vitamin D status correlates with testosterone levels in men, and correcting a deficiency modestly supports the broader endocrine and musculoskeletal decline of aging.
The clearest benefit comes from restoring a low level toward normal rather than pushing high; testosterone gains are modest and not a replacement for clinical evaluation.
Vitamin D modulates regulatory immune function and is frequently low in people with autoimmune thyroid disease, so correcting deficiency supports immune balance.20
Strongest when correcting a measured deficiency; check 25-hydroxyvitamin D rather than dosing blindly, and it does not replace thyroid hormone therapy.
Evidence-based stacks that include it, with the exact dose and timing each one uses.
Supports vitamin D status when intake or labs are low; dose should be adjusted to 25-OH vitamin D and calcium context.3,18
Supports immune-response markers when vitamin D status is low; stay within routine upper-limit guidance unless clinician-directed.13,20
Commonly used to support vitamin D status; routine dosing should be adjusted to labs, sun exposure, diet, and calcium status.13,21
Supports vitamin D status, which is associated with some hormone markers; dose to labs rather than assuming testosterone or thyroid effects.3,10
Vitamin D modulates Th1/Th17 balance and reduces inflammatory cytokines; deficiency exacerbates chronic inflammation.1,2
Vitamin D participates in calcium homeostasis and contributes to steroid hormone and neurotransmitter regulation, and observational and some interventional data link adequate status to a lower premenstrual symptom burden. Dosing toward the upper end is reasonable when blood levels are low, ideally guided by testing.5,17
The androgen-producing Leydig cells of the testes express vitamin D receptors, and observational data link low vitamin D status to lower testosterone. Repletion is most relevant for men who are deficient, while trials in vitamin D replete men have shown little to no testosterone benefit.
Vitamin D3 helps modulate immune balance, including regulatory T cell activity, and low vitamin D status has been associated with greater allergic sensitization in observational studies. Correcting a deficiency may support a calmer allergic baseline, and dosing is ideally guided by blood 25-hydroxyvitamin D levels.13,20
Vitamin D receptors are expressed in brain regions involved in mood, and correcting insufficiency is associated with a modest mood benefit. The signal is strongest in people who start deficient rather than those already replete.1,2
The vitamin D receptor is expressed in the hair follicle and participates in normal follicle cycling, and low vitamin D status has been linked to several patterns of hair loss; correcting a deficiency is reasonable, though regrowth benefit from supplementing replete people is unproven.1,2
Common vitamin D receptor polymorphisms are being studied, but routine VDR-guided supplement dosing is not clinically established.
Recommendation: Dose to measured 25(OH)D response rather than genotype alone; recheck labs 8-12 weeks after dose changes.
Vitamin D3 increases calcium absorption, while K2 activates osteocalcin and matrix GLA protein to direct calcium into bones and away from arteries.
Recommendation: Take together. K2 (MK-7 100-200mcg) is recommended whenever supplementing D3 above 2000 IU.
Vitamin D3 is fat-soluble and absorption improves when taken with omega-3 fatty acids. Both support cardiovascular and immune health.
Recommendation: Take D3 with fish oil or a meal containing fat for optimal absorption.
Magnesium is essential for vitamin D metabolism. It's required for the enzymes that convert D3 to its active form calcitriol.
Recommendation: Ensure adequate magnesium when supplementing D3. Magnesium deficiency can impair D3 activation.
Vitamin D3 is essential for calcium absorption from the gut. Without adequate D3, only 10-15% of dietary calcium is absorbed.
Recommendation: Take D3 to optimize calcium absorption. D3 can increase calcium absorption to 30-40%.
High-dose vitamin A (retinol) can antagonize vitamin D activity by competing for the shared RXR nuclear receptor.
Recommendation: If supplementing both, keep vitamin A under 10,000 IU and ensure adequate D3 (2000+ IU). Balanced ratios are key.
Boron supports vitamin D metabolism and may increase the half-life of 25(OH)D in the body.
Recommendation: 3-6mg boron may enhance vitamin D status, particularly when D levels are suboptimal.
Both support musculoskeletal function. Vitamin D3 enhances muscle protein synthesis, and creatine increases phosphocreatine stores for energy.
Recommendation: Effective combination for strength and muscle health, especially in those over 50.
Adequate zinc status supports normal zinc-finger transcription biology, including vitamin D receptor DNA-binding domains. Evidence does not show that extra zinc improves vitamin D signaling in zinc-replete people.
Recommendation: Use zinc to meet normal intake or correct deficiency; do not add high-dose zinc solely to activate vitamin D.
Same synergy as fish oil + D3. Krill oil phospholipid-bound omega-3s provide dietary fat for D3 absorption.
Recommendation: Take D3 with krill oil. Phospholipid-bound omega-3s in krill may offer slightly better absorption than triglyceride-form fish oil.
At balanced physiological ratios, vitamins A and D work synergistically on immune regulation and gene expression.
Recommendation: Balanced supplementation (e.g., cod liver oil ratios) supports immune function. Avoid mega-dosing either one alone.
Vitamin D supports the gut immune barrier. Probiotics modulate the microbiome. Together they support gut-immune axis health.
Recommendation: Complementary gut health support. Vitamin D strengthens gut barrier; probiotics diversify beneficial flora.
Vitamin D3 increases calcium absorption, while K2 activates osteocalcin and matrix GLA protein to direct calcium into bones and away from arteries.
Recommendation: Take together. K2 (MK-7 100-200mcg) is recommended whenever supplementing D3 above 2000 IU.
Numbered references. Citations throughout the page link here.
Sobczak M, Pawliczak R. Effect of Vitamin D3 Supplementation on Severe COVID-19: A Meta-Analysis of Randomized Clinical Trials. Nutrients. 2024
van den Heuvel EG, Lips P, Schoonmade LJ et al.. Comparison of the Effect of Daily Vitamin D2 and Vitamin D3 Supplementation on Serum 25-Hydroxyvitamin D Concentration (Total 25(OH)D, 25(OH)D2, and 25(OH)D3) and Importance of Body Mass Index: A Systematic Review and Meta-Analysis. Advances in nutrition (Bethesda, Md.). 2024
Vitamin D3 at 3200-4000 IU/day appears to increase the risk of hypercalcemia (4 per 1000) and some adverse events in a small proportion of individuals
Combined supplementation with whey protein, leucine, and vitamin D improved muscle mass and physical performance in patients with sarcopenia.
Manoj P, Derwin R, George S. What is the impact of daily oral supplementation of vitamin D3 (cholecalciferol) plus calcium on the incidence of hip fracture in older people? A systematic review and meta-analysis. International journal of older people nursing. 2023
Petrelli F, Oldani S, Borgonovo K et al.. Vitamin D3 and COVID-19 Outcomes: An Umbrella Review of Systematic Reviews and Meta-Analyses. Antioxidants (Basel, Switzerland). 2023
Systematic review assessing the effect of vitamin D3 supplementation on bone mineral density across multiple RCTs
Vitamin D supplementation reduced depressive symptoms more in those with existing depression (SMD: -0.57) than in the general population
Significantly higher prevalence of vitamin D deficiency in children and adolescents with obesity compared to non-obese peers.
No significant reduction in major adverse cardiovascular events or cancer incidence; cancer mortality reduced in some subgroups with longer follow-up
Vitamin D supplementation was associated with a statistically significant reduction in all-cause mortality in a meta-analysis of randomized controlled trials.
Vitamin D supplementation did not prevent fractures or falls or have clinically meaningful effects on bone mineral density in unselected community-dwelling adults
Vitamin D supplementation reduced the risk of acute respiratory tract infection (OR 0.88); greatest benefit in those with baseline 25-OHD <25 nmol/L and with daily/weekly dosing
Putranto R, Setiati S, Nasrun MW et al.. Effects of cholecalciferol supplementation on depressive symptoms, C-peptide, serotonin, and neurotrophin-3 in type 2 diabetes mellitus: A double-blind, randomized, placebo-controlled trial. Narra J. 2024
Among 18,353 adults aged 50+, vitamin D3 vs placebo did not result in statistically significant difference in incidence of depression or mood score changes over 5.3 years
Vitamin D exerts immunomodulatory activities on innate and adaptive immune systems; low 25-OHD associated with increased risk of autoimmune diseases and infections
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