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Vitamin D

Vitamin D

Best for

Feel less tired every day

Likely strong benefit 40–100000 IU/day 24 weeks 2 meta-analyses, n=465 #2 of 31 studied

Vitamin D offers a likely strong benefit for stamina and fatigue, especially in adults with deficiency, while broader outcomes such as bone density show less consistent gains.

Median effective dose, most-studied outcomes

  • Improve vitamin D status 5,250 IU <1–104 wk
  • Lower fracture risk 800 IU 104–276 wk
  • Reduce asthma flares 2,000 IU 24–48 wk
Full protocol, 13 outcomes

Risk, worst reported severity

  • hypercalcaemia severe
  • nephrolithiasis severe
  • phenobarbital major
  • tacrolimus major
All 18 events and interactions

What vitamin d actually does

Outcome Effect Evidence Dose Time Rank
Strong benefits 1
Feel less tired every day Likely strong benefit Everyday energy helps you get through daily tasks without feeling worn down.
large 		2 meta-analyses 3 papers · n=465 40–100000 IU 24 wk #2 /31
Clear benefits 4
Raise vitamin D levels Proven benefit Vitamin D levels show the body's supply of a nutrient tied to bone and muscle health.
moderate 		21 meta-analyses 57 papers · n=73k 3.6–600000 other 0–156 wk #1 /5
Vitamin D levels in infants Proven benefit Vitamin D levels reflect a nutrient that helps infants absorb calcium for growing bones.
moderate 		21 meta-analyses 57 papers · n=73k 3.6–600000 other 0–156 wk #1 /5
Higher vitamin D levels in pregnancy Proven benefit Vitamin D levels in pregnancy matter for maternal and fetal health.
moderate 		21 meta-analyses 57 papers · n=73k 3.6–600000 other 0–156 wk #1 /5
Relieve allergic rhinitis symptoms Likely benefit Allergic rhinitis can affect your nose, eyes, breathing, sleep, and daily comfort.
moderate 		2 meta-analyses 2 papers · n=1.3k 400–60000 IU 4–12 wk #1 /8
Modest benefits 1
Lower fracture risk Proven but unnoticeable Fractures can limit mobility and independence, especially after a fall.
trivial 		6 meta-analyses 7 papers · n=92k 300–4000 IU 104–276 wk #2 /5
Worth watching 2
Lower your risk of COVID-19 Promising early signal COVID-19 can disrupt daily life and may cause illness that needs medical care.
moderate 		2 meta-analyses 5 papers · n=10k 400–100000 IU 26–39 wk —
Balance bone turnover Promising early signal Bone turnover replaces old bone with new bone to maintain the skeleton over time.
moderate 		2 RCTs 2 papers · n=72 1000–3000 IU 12–16 wk #2 /11
Early research 1
Breathe easier with asthma Faint early signal Good asthma control means fewer symptoms and flare-ups interrupting daily life.
small 		4 meta-analyses 6 papers · n=4.6k 125–4000 mcg 12–48 wk #1 /3
Doesn't appear to help 4
Support height growth in children Doesn't appear to help Height growth reflects how children grow in length and stature as they develop.
trivial 		2 meta-analyses 2 papers · n=1.9k 200–300000 IU — —
Build bone mineral in children Probably doesn't help Bone mineral helps children build a strong skeleton as they grow.
trivial 		2 RCTs 1 papers · n=665 1000–2800 IU — —
Build bone density Probably doesn't help Bone density reflects the mineral that helps bones bear weight and resist breaking.
trivial 		2 meta-analyses 3 papers · n=610 800–10000 IU 52–156 wk —
Build stronger bones Probably doesn't help A bone's strength and shape help it handle force without breaking.
trivial 		1 RCT 1 papers · n=574 2000 IU 104 wk —

Effect tiers come from native-unit MCID where available, Cohen's d otherwise.

Protocol

Dose, timing & forms

How much

The most dependable reason to supplement is to raise a low level, and oral trials span everything from routine daily dosing to clinician-directed loading regimens. For long-term fracture outcomes, studies generally used lower daily amounts over years, and the average benefit was small rather than something you feel day to day.3 A trial in people with vitamin D deficiency also found that a single 100,000 IU dose improved self-reported fatigue over four weeks, but that is a treatment protocol, not a casual maintenance dose.2

When and how long

Vitamin D works through steady replenishment, not an instant boost, so take it consistently and judge a level-correction plan over weeks to months. Because it is fat-soluble, taking it with a meal that contains some fat is a sensible practical choice. Avoid copying intermittent high-dose protocols without clinician guidance, especially if you have kidney disease, a history of kidney stones, or high calcium levels.1

Forms & standardisation

Look for the amount listed in IU or micrograms, and check whether the label specifies vitamin D3 (cholecalciferol) or D2 (ergocalciferol). D3 has extensive clinical use and is the form used in the fatigue trial, but the key question is whether your dose corrects a documented low blood level safely.2 If you are taking vitamin D because of a low test result, repeat testing and clinician-guided adjustment beat guessing from the front-label number.1

Studied range, by outcome

Outcome Studied range Median effective Duration Evidence
Improve vitamin D status 125–600,000 IU 5,250 IU <1–104 wk 57 papers 56 sig. endpoints
Lower fracture risk 300–4,000 IU 800 IU 104–276 wk 7 papers 3 sig. endpoints
Reduce asthma flares 143–4,000 IU 2,000 IU 24–48 wk 5 papers 3 sig. endpoints
Maintain bone density 800–7,943 IU 2,000 IU 52–104 wk 3 papers 1 sig. endpoint
Prevent COVID-19 infection 400–100,000 IU 50,200 IU 26–39 wk 3 papers 4 sig. endpoints
Have less fatigue 40–100,000 IU 100,000 IU 24 wk 3 papers 3 sig. endpoints
Support steady height gain 100–300,000 IU 150,100 IU — 2 papers 1 sig. endpoint
Lighten the overall allergy load 400–60,000 IU 30,300 IU 4–12 wk 2 papers 2 sig. endpoints
Protect your spinal bones 5,000–7,943 IU — 52 wk 1 paper
Support stronger bones under load 2,000 IU — 104 wk 1 paper
Protect forearm bones 5,000–7,943 IU — 52 wk 1 paper
Build new bone more quickly 3,000 IU — 16 wk 1 paper
Build a stronger childhood skeleton 1,000 IU — — 1 paper

Adverse events and drug interactions

Safety events

hypercalcaemia severe
nephrolithiasis severe
postpartum haemorrhage severe
Serious adverse events (aggregate) severe
serious adverse events severe
pancreatitis severe
nephrocalcinosis severe
first fall with hospitalization severe

Drug interactions

phenobarbital major decreases concentration
tacrolimus major decreases concentration
sirolimus major decreases concentration
rifampin major decreases concentration
phenytoin major decreases concentration
carbamazepine major decreases concentration
hydrochlorothiazide major increases toxicity
primidone major decreases concentration
warfarin major increases effect
atorvastatin major decreases effect
Buy well

Recommended Vitamin D products

Real products that dose Vitamin D where the research does, in a clean formulation, from brands we've audited. We only list ones we'd stand behind.

Chosen for adequate Vitamin D dosing vs. the studied range, a clean formulation (grade A–C), and independent brand-quality audits. Not sponsored.

In context

What this is and how it works

  • Across 3 studies (n=465), vitamin D improved stamina for daily tasks in five measured endpoints.2
  • The review covers 139 papers and 167 outcomes, spanning deficiency, pregnancy, older women, and bone health.1
  • Benefits are most relevant when vitamin D levels are low; high-dose self-treatment requires medical guidance.1

Vitamin D is a fat-soluble nutrient your skin can make after sun exposure, and you also get smaller amounts from foods and supplements. Vitamin D2 and D3 both raise vitamin D status, while D3 is the form most commonly used in supplements. Your liver and kidneys convert it into the active hormone-like form your body uses.1

Think of vitamin D as the warehouse manager for bone minerals: it tells your gut to bring more calcium and phosphorus from food into circulation instead of leaving them behind. Its active form plugs into vitamin D receptors, which act like gene-control switches in cells throughout the body.1

Bottom line

Vitamin D makes the most sense if a blood test shows you are low, if sun exposure is limited, or if a clinician has identified a higher need. It reliably raises vitamin D status and shows a notable fatigue signal in deficient adults, but do not buy it expecting dramatic bone-density gains or a cure-all.124 Skip self-prescribed high doses if you have kidney stones, high calcium, or use medicines that affect vitamin D, and get individualized advice instead.

Frequently asked

Common questions

What does vitamin D actually do in your body?

Vitamin D helps your intestine absorb calcium and phosphorus, the raw materials your body uses to maintain mineralized bones. Its active form also switches genes on and off through vitamin D receptors in many tissues.1

Can vitamin D help with tiredness?

If you have low vitamin D, evidence shows supplementation can reduce self-reported fatigue. It is not a universal energy booster, so persistent exhaustion still deserves a medical check-in.2

Does vitamin D build bone density?

Not reliably. Trials and a major review showed little meaningful improvement in bone mineral density for most adults taking vitamin D alone, even though correcting a deficiency remains important for bone health.4

Is vitamin D important during pregnancy?

Pregnancy increases the importance of reaching an adequate vitamin D level, but the right supplement plan depends on your starting level and prenatal care. Clinical trials have tested supplementation during pregnancy, with blood-level improvement as the clearest outcome.5

Sources

Sources

  1. 1. Vitamin D deficiency (2007) ↑
  2. 2. Effect of vitamin D3 on self-perceived fatigue: A double-blind randomized placebo-controlled trial (2016) ↑
  3. 3. Effect of calcium and vitamin D supplementation on fracture incidence in older women (2005) ↑
  4. 4. Effects of vitamin D supplements on bone mineral density: a systematic review and meta-analysis (2014) ↑
  5. 5. Vitamin D supplementation for women during pregnancy (2019) ↑

Generated August 27, 2026

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