Vitamin D Benefits: What Trials Show

Quick answer

Vitamin D benefits, in short: Your body needs vitamin D to keep bones, teeth and muscles healthy, and US guidance sets a daily amount of 15 micrograms (600 IU) for most adults [10]. In places like the UK, people don’t make enough from sunlight between October and early March, and the NHS advises everyone there to consider a daily supplement in those months [1]. Beyond that basic need, the big trials in generally healthy adults have mostly found no clear extra benefit from taking vitamin D for heart disease, cancer or broken bones [2][3][4][5]. We rate the evidence for healthy-aging goals C: worthwhile if your levels are low, much less convincing if they are already fine.

What is vitamin D?

Vitamin D is a vitamin your skin makes when it is exposed to summer sunlight. You also get a little from oily fish, red meat, egg yolks and fortified foods such as some cereals [1]. Supplements usually contain vitamin D3 (cholecalciferol). A lack of it can lead to bone problems such as rickets in children and bone pain (osteomalacia) in adults [1].

Vitamin D benefits: what might it help with?

Possible benefit	Rating	In plain words
Normal bones, teeth and muscles (if you are short of it)	Strong	A basic need; official advice is based on this [1][10]
Fewer broken bones or falls in the general population	No clear benefit	Large trials and reviews found no effect [3][5]
Heart disease and cancer	No clear benefit	No meaningful difference in a trial of 25,871 adults [2]
Colds and chest infections	Mixed	Small benefit overall, mostly in people with very low levels [6][7]
Autoimmune conditions	Early	One trial reported a borderline result; it needs repeating [8]
Living longer	No clear benefit	No benefit in a trial of 21,315 adults [4]

Ratings describe how good the research is, not a promise that it will work for you.

What the research says

• VITAL trial (US, 25,871 adults). People took 2,000 IU (50 micrograms) a day or a placebo. Over several years there was no meaningful difference in cancer or cardiovascular events [2]. PubMed · DOI The same trial found no difference in fractures [3]. PubMed · DOI
• D-Health trial (Australia, 21,315 adults aged 60 to 84). A large monthly dose (60,000 IU) did not lower the risk of dying from any cause [4]. The researchers noted a possible signal for cancer deaths after two years, which they treated as exploratory. PubMed · DOI
• Falls and fractures. A 2018 review of 81 trials (53,537 people) found no effect on fractures or falls [5]. PubMed · DOI
• Colds and chest infections. A 2017 analysis pooling individual data from 25 trials (11,321 people) found a small overall reduction in the chance of having an infection. The benefit was biggest in people with very low starting levels, and in people who took it daily or weekly rather than in large occasional doses [6]. PubMed · DOI A 2021 review of 46 trials (75,541 people) found a smaller effect [7]. PubMed · DOI
• Autoimmune conditions. In a VITAL follow-up, vitamin D was linked to fewer new autoimmune diagnoses, but the result was borderline and is not yet confirmed [8]. PubMed · DOI

Vitamin D amounts: IU vs mcg, US vs UK

These are amounts used in studies or in official advice, not personal recommendations.

Vitamin D amounts are given in micrograms (mcg) or international units (IU). 1 mcg equals 40 IU.

Source	mcg a day	IU a day
US recommended amount, most adults up to age 70 [10]	15	600
US recommended amount, over 70 [10]	20	800
NHS advice for fall and winter (UK) [1]	10	400
Upper limit for adults, NHS and NIH [1][10]	100	4,000
VITAL trial dose [2]	50	2,000

• US advice (NIH Office of Dietary Supplements): the Recommended Dietary Allowance is 15 micrograms (600 IU) a day for most adults up to age 70, and 20 micrograms (800 IU) for people over 70. This is set assuming little sun exposure, and counts vitamin D from food and supplements together [10].
• NHS advice (UK): consider 10 micrograms (400 IU) a day in fall and winter (October to early March in the UK). People at higher risk of low levels, such as those with very little sun exposure or an African, African-Caribbean or south Asian background, should consider taking it all year round [1].
• Upper limits: the NHS and the NIH both set an upper limit of 100 micrograms (4,000 IU) a day for adults [1][10]. Limits for children and babies are lower, for example 50 micrograms a day for children aged 1 to 10 in the NHS advice [1].
• Trial doses were higher: 50 micrograms (2,000 IU) a day in VITAL [2] and 60,000 IU a month in D-Health [4].
• Form: vitamin D3 is the most common form.

Vitamin D3 vs D2, and D3 with K2

Vitamin D3 is the most common supplement form. We haven’t reviewed how D3 compares with D2, so we don’t make a claim either way; see vitamin D2 vs D3. Vitamin D is sometimes sold with vitamin K2, and we found no trial in our sources testing the pairing. Our vitamin K2 entry covers what the research shows for K2 itself.

Side effects, interactions and who should check first

At sensible doses side effects are uncommon. Taking too much for a long time can cause calcium to build up in the blood (hypercalcemia), which can weaken bones and damage the kidneys and heart [1]. The NIH adds that this almost always comes from supplements, not sunlight or food [10].

Who should check with a doctor or pharmacist first

Check with a doctor or pharmacist first if you:

• take “water tablets” called thiazide diuretics (such as bendroflumethiazide). They can reduce calcium loss in urine, so calcium can build up [9][10]
• take digoxin or similar heart medicines. High calcium can affect how they work, so doctors may monitor you [9]
• take epilepsy medicines such as phenytoin or barbiturates, or steroid tablets. These can reduce vitamin D’s effect [9]
• take orlistat (a weight-loss medicine). It may reduce vitamin D absorption [9][10]
• have high calcium, kidney stones, kidney problems, sarcoidosis or similar conditions. Your doctor may advise against supplements or want blood tests [9]
• are pregnant or breastfeeding, or buying for a child. Doses differ, so follow your national guidance [1][10].

Where to buy

Look for vitamin D3 with the amount clearly shown in micrograms (and IU), a dose close to official daily-intake advice rather than very high strengths, and third-party testing if offered. Our vitamin D buying guide explains how we choose products.

FAQ

What is vitamin D?

Vitamin D helps keep your bones, teeth and muscles healthy [1]. Your skin makes it from sunlight, and it is also found in some foods and supplements. In places like the UK, people don’t make enough from sunlight between October and early March, and the NHS advises considering a supplement then [1].

How much vitamin D per day?

The US recommended amount is 15 micrograms (600 IU) for most adults up to age 70 and 20 micrograms (800 IU) over 70 [10]. The NHS advises considering 10 micrograms (400 IU) a day in fall and winter (October to early March in the UK) [1]. The upper limit is 100 micrograms (4,000 IU) [1][10].

Does vitamin D work without K2?

We found no trial in our sources testing vitamin D with and without K2, so we can’t say whether K2 changes how well it works. In big trials of healthy adults, vitamin D on its own made no clear difference to heart disease, cancer or fractures [2][3][5]. See our vitamin K2 entry.

Who should not take vitamin D?

Check with a doctor or pharmacist first if you take thiazide water tablets, digoxin, epilepsy medicines, steroid tablets or orlistat, or if you have high calcium, kidney stones, kidney problems or sarcoidosis [9][10]. Also check first if you are pregnant or breastfeeding, or buying for a child [1][10].

Can you take vitamin D with iron?

We found no verified data on vitamin D and iron taken together, so we can’t say whether the combination is safe or unsafe. If you take iron or other supplements as well, ask a doctor or pharmacist, especially if you also take any of the medicines listed in the section above.

Related pages

• Vitamin K2
• Magnesium: another everyday basic
• Omega-3: tested alongside vitamin D in the VITAL trial
• Vitamin D2 vs D3
• Our vitamin D buying guide
• Longevity supplements for beginners: where this fits in a beginner’s routine
• All supplements A-Z

Talk to your doctor

This page is for general information and is not medical advice. Speak to a doctor or pharmacist before starting any supplement, especially if you are pregnant or breastfeeding, take any medicine, or have a health condition.

References

We cite only studies and official pages we checked against PubMed, the publisher or the agency’s own site. The A to E evidence grades are explained in our editorial policy. See also our medical disclaimer and affiliate disclosure.

1. NHS. Vitamin D. nhs.uk (accessed 1 November 2026)
2. Manson JE, Cook NR, Lee IM, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med. 2019. PubMed · DOI
3. LeBoff MS, Chou SH, Ratliff KA, et al. Supplemental vitamin D and incident fractures in midlife and older adults. N Engl J Med. 2022. PubMed · DOI
4. Neale RE, Baxter C, Romero BD, et al. The D-Health Trial: a randomised controlled trial of the effect of vitamin D on mortality. Lancet Diabetes Endocrinol. 2022. PubMed · DOI
5. Bolland MJ, Grey A, Avenell A. Effects of vitamin D supplementation on musculoskeletal health: a systematic review, meta-analysis, and trial sequential analysis. Lancet Diabetes Endocrinol. 2018. PubMed · DOI
6. Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017. PubMed · DOI
7. Jolliffe DA, Camargo CA Jr, Sluyter JD, et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. Lancet Diabetes Endocrinol. 2021. PubMed · DOI
8. Hahn J, Cook NR, Alexander EK, et al. Vitamin D and marine omega 3 fatty acid supplementation and incident autoimmune disease: VITAL randomized controlled trial. BMJ. 2022. PubMed · DOI
9. Adcal-D3 summary of product characteristics (sections 4.3-4.6). electronic Medicines Compendium. emc (accessed 1 November 2026)
10. National Institutes of Health, Office of Dietary Supplements. Vitamin D: health professional fact sheet. ods.od.nih.gov (accessed 1 November 2026)