Vitamin D3
The sunshine vitamin, and the one the NHS suggests everyone thinks about in winter.
- Bones and muscles
- Colds and chest infections
- Heart health
- Diabetes risk
- Living longer
A plain-English overview, not advice. Written from published reviews and trials and from what people say in public forums.
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On this page
What is vitamin D3?
Vitamin D3 (cholecalciferol) is the vitamin your skin makes when sunlight hits it, and the one that is hard to get enough of in a British winter, when the sun is too low for skin to make any. Food gives a little: oily fish, egg yolks, and fortified spreads and cereals. Supplements are sold as tablets, capsules, drops and sprays, on their own or paired with vitamin K2, alongside vitamin D2, the plant-derived form. It is the supplement the NHS suggests everyone in the UK considers through autumn and winter, and one of the most widely taken of all. Many people take it simply for winter tiredness and low mood, which is also what the forums talk about most.
Why do people take vitamin D3?
Bones and muscles
The thinking. Vitamin D is what lets the gut absorb calcium, and without enough of it bones soften and muscles weaken; a severe shortage causes rickets in children and bone pain in adults. Keeping bones strong is the classic reason for taking it, especially later in life.
What research has found. The largest and most recent reviews of trials in generally healthy adults found no clear difference in fractures, falls or bone density with vitamin D on its own; putting right a real shortage is a different question from what extra does in people who already have enough. A meta-analysis of trials measuring muscle strength found small gains, mostly in people who started with low levels. This is one of the best-studied uses, and several reviews of trials broadly agree.
Colds and chest infections
The thinking. Immune cells carry vitamin D receptors and use it to switch on their own antimicrobial defences, and infections peak in the months when levels are lowest, so a daily dose through winter is one of the commonest reasons people take it.
What research has found. Two large meta-analyses of randomised trials, pooling data from tens of thousands of people, found a small reduction in the number of acute respiratory infections with vitamin D, clearest with a daily or weekly dose rather than occasional large ones. A small trial in people with COVID-19 found faster clearance of the virus. Several reviews of trials have measured this, and they broadly agree.
Heart health
The thinking. Low vitamin D levels go together with higher rates of heart disease in population studies, and the vitamin acts on blood pressure and blood vessel walls in the laboratory, so heart health is a reason some people take it.
What research has found. A meta-analysis of 21 trials in more than 83,000 people found no reduction in heart attacks, strokes or deaths from heart disease with vitamin D, and two further large trials found the same; the link seen in population studies has not so far turned into a benefit in trials. A fair body of trials has looked at this, and most point the same way.
Diabetes risk
The thinking. Vitamin D plays a part in how the pancreas releases insulin and how the body responds to it, and people with prediabetes are more likely to run low, so it has been tested as a way of holding off type 2 diabetes.
What research has found. A pooled analysis of three trials in adults with prediabetes found fewer of them going on to develop diabetes with vitamin D than with placebo, a modest reduction; the largest of those trials, reported on its own, found a similar-sized difference that did not reach statistical significance. A reasonable number of trials have measured this, and most of them agree.
Living longer
The thinking. People with low vitamin D die younger on average in population studies, which is a big part of why it is taken as a general insurance supplement.
What research has found. One meta-analysis found a small reduction in deaths from any cause with vitamin D3 specifically; a broader review and a trial in critically ill adults found no clear effect. For cancer, a meta-analysis found no fewer diagnoses but fewer cancer deaths, and a nationwide trial found no difference. Studies here point in different directions, so this one is still being worked out; the dose, how long people took it and who took part all seem to matter.
What does vitamin D3 do in the body?
Vitamin D is really a hormone in waiting. Skin makes it from a relative of cholesterol when ultraviolet light hits it; the liver turns it into the storage form that blood tests measure, and the kidneys turn that into the active hormone as needed. The active form’s best-known job is to switch on calcium absorption in the gut and keep blood calcium steady, which is what bones and muscles depend on.
The receptor it acts through is found in most tissues, not just bone, including immune cells, muscle, the pancreas and blood vessels, which is why so many different benefits have been proposed. The pattern in the research is that a real shortage has clear effects and putting it right matters, while extra on top of an adequate level has so far been harder to show a benefit for in trials.
What do people say about taking vitamin D3?
These are anecdotes people have shared, not evidence.
What people like
- Very commonly, people who had a blood test showing a low level say the bone-deep tiredness lifted once they started taking it. A few felt it within a week or two; most describe it building over one to three months.
- Often, aches easing, particularly a general all-over ache or sore joints and bones, most of all from those whose test result was very low.
- Often, a lift in mood and feeling brighter or less anxious; several only made the connection when they ran out, felt flat again a couple of weeks later, and restarted.
- Some say they slept better, felt clearer-headed or noticed their skin looked better a month or two in, and a minority describe a big turnaround and wish they had asked for a test years earlier.
Common complaints
- Often, people say they noticed nothing at all. The common explanation is that the standard shop doses of 400 to 800 IU are too small to feel, and that people who were not actually low to begin with rarely notice a change.
- Some found it slow going, with no change for weeks and energy only returning after around three months, and a few gave up before then.
- Worries about high doses come up in every thread, with calcium build-up the main fear: one person described going over the top after six months at 7,000 IU a day, and there is a running argument between the take-much-more camp and the stick-to-the-guidance camp.
- Whether to take K2 or magnesium alongside it splits opinion and confuses newcomers, and a recurring frustration is with GPs who were reluctant to test or dismissive of a low result.
How people take it
- For everyday winter use the most common self-chosen doses are 1,000 to 4,000 IU (25 to 100 micrograms) a day, with a smaller group at 5,000 IU alongside K2. Some double their dose from October to March. Large loading doses only appear where a doctor prescribed them after a very low test.
- Blood tests guide the keener people: several paid around thirty pounds for a private finger-prick test, tracked their number from below 30 up to around 100 nmol/L over months, and retested after roughly three months.
- Tablets and capsules are the default because they are cheap; drops and sprays are chosen for children and for people who dislike swallowing tablets; combined D3 and K2 products have become common in the more recent threads. Most take it once a day with breakfast.
- On timing of results, the pattern is a week or two for the lucky few, about a month for many, and two to three months for those who started out very low. Several say the effect fades within a few weeks of stopping, which is what convinced them it was real.
Read from these public discussions:
- Vitimin D, can it have this effect? (Mumsnet, 2015)
- If you are vitamin D deficient, what have you been prescribed? (Mumsnet, 2017)
- How much vitamin d are you taking? (Mumsnet, 2020)
- Vitamin D (Mumsnet, 2020)
- Vitamins (B, Ds & Cs) for mental health issues (anxiety/depression) (Mumsnet, 2023)
What forms of vitamin D3 are there?
| Form | What’s different | Per serving, on products we list |
|---|---|---|
| Vitamin D3 | Made in skin from sunlight, or from lanolin or lichen; the form most trials use. | 400 IU to 5,000 IU |
| Vitamin D2 | Ergocalciferol, made from a plant or fungal source rather than sunlight or lanolin. | 4,000 IU |
What else is worth knowing about vitamin D3?
- The NHS advises everyone in the UK to consider a daily 10 microgram (400 IU) supplement through autumn and winter, and says not to take more than 100 micrograms (4,000 IU) a day without medical advice (NHS: vitamin D).
- Labels give the amount in micrograms (µg) or international units (IU), sometimes both: 10 µg is 400 IU, 25 µg is 1,000 IU, and a serving may be more than one capsule or drop.
- Large trials mostly used 2,000 to 4,000 IU a day and generally excluded people with high blood calcium or kidney stones; one trial found slightly lower bone density after three years on 4,000 or 10,000 IU a day than on 400 IU.
- Vitamin D is fat-soluble and builds up, so the harm at very high doses is too much calcium in the blood.
If you take medication, are pregnant, or have a health condition, check with a pharmacist or GP first.
The research behind this page (26 studies)
- Vitamin D metabolism, mechanism of action, and clinical applications. (2014, review)
- Vitamin D: Metabolism, Molecular Mechanism of Action, and Pleiotropic Effects. (2016, review)
- Vitamin D deficiency 2.0: an update on the current status worldwide. (2020, review)
- Skeletal and Extraskeletal Actions of Vitamin D: Current Evidence and Outstanding Questions. (2019, review)
- Vitamin D supplementation and fracture risk: an updated systematic review and meta-analysis of randomised controlled trials. (2026, meta-analysis)
- Effects of vitamin D supplementation on musculoskeletal health: a systematic review, meta-analysis, and trial sequential analysis. (2018, meta-analysis)
- Association Between Calcium or Vitamin D Supplementation and Fracture Incidence in Community-Dwelling Older Adults: A Systematic Review and Meta-analysis. (2017, meta-analysis)
- Effects of vitamin D supplements on bone mineral density: a systematic review and meta-analysis. (2014, meta-analysis)
- The effect of vitamin D on falls and fractures: a systematic review. (2014, systematic review)
- A pooled analysis of vitamin D dose requirements for fracture prevention. (2012, meta-analysis)
- Supplemental Vitamin D and Incident Fractures in Midlife and Older Adults. (2022, randomised trial)
- Effect of High-Dose Vitamin D Supplementation on Volumetric Bone Density and Bone Strength: A Randomized Clinical Trial. (2019, randomised trial)
- The effects of vitamin D on skeletal muscle strength, muscle mass, and muscle power: a systematic review and meta-analysis of randomized controlled trials. (2014, meta-analysis)
- Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. (2017, meta-analysis)
- Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. (2021, meta-analysis)
- Short term, high-dose vitamin D supplementation for COVID-19 disease: a randomised, placebo-controlled, study (SHADE study). (2022, randomised trial)
- Vitamin D Supplementation and Cardiovascular Disease Risks in More Than 83 000 Individuals in 21 Randomized Clinical Trials: A Meta-analysis. (2019, meta-analysis)
- Effect of Vitamin D and Omega-3 Fatty Acid Supplementation on Cognitive Function in Older Adults. (2020, randomised trial)
- Effect of Monthly High-Dose Vitamin D Supplementation on Cardiovascular Disease in the Vitamin D Assessment Study: A Randomized Clinical Trial. (2017, randomised trial)
- Vitamin D and Risk for Type 2 Diabetes in People With Prediabetes: A Systematic Review and Meta-analysis of Individual Participant Data From 3 Randomized Clinical Trials. (2023, meta-analysis)
- Vitamin D Supplementation and Prevention of Type 2 Diabetes. (2019, randomised trial)
- Vitamin D supplementation to prevent mortality in adults. (2014, meta-analysis)
- Vitamin D and multiple health outcomes: umbrella review of systematic reviews and meta-analyses of observational studies and randomised trials. (2014, systematic review)
- Early High-Dose Vitamin D3 for Critically Ill, Vitamin D-Deficient Patients. (2019, randomised trial)
- Vitamin D supplementation and total cancer incidence and mortality: a meta-analysis of randomized controlled trials. (2019, meta-analysis)
- Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. (2019, randomised trial)
How much does vitamin D3 cost?
That depends on the form and the pack, so we work out the price of the same dose of every vitamin D3 product we can find and rank them.