Vitamin D is one of those nutrients where the gap between what we know and what people claim is quite wide. Called the “sunshine vitamin” because our skin manufactures it when exposed to UV light, it’s also the nutrient most likely to be deficient in modern populations. Some estimates suggest that a billion people worldwide have inadequate vitamin D levels.

I’ve gone through the clinical literature on vitamin D, and the picture that emerges is more complicated than you might expect. Some benefits are well-established. Others are based on observational studies that show associations but can’t prove cause and effect. And quite a few popular claims simply don’t hold up when tested in rigorous trials.

This article covers what we actually know, where the evidence is shaky, and what precautions matter.

What is vitamin D?

Vitamin D is technically a hormone precursor rather than a vitamin in the traditional sense. Your body converts it through several steps into calcitriol, its active hormonal form. From there, it affects over 200 genes throughout the body.

The two main forms you’ll encounter are vitamin D3 (cholecalciferol, made in human skin and found in animal products) and vitamin D2 (ergocalciferol, from fungi and plants). Most research uses D3, and there’s reasonable evidence that D3 raises blood levels more effectively.

Your blood level of 25-hydroxyvitamin D (usually written as 25(OH)D) is how we measure vitamin D status. The NHS considers levels below 25 nmol/L (10 ng/mL) deficient. Many researchers argue that optimal levels should be 50-75 nmol/L (20-30 ng/mL) or higher, though this remains debated [1].

Benefits with strong evidence

Bone health and fractures

This is vitamin D’s most established function. It helps your intestines absorb calcium and maintains the calcium levels needed for normal bone mineralisation. Without adequate vitamin D, bones become soft and weak.

A meta-analysis of 32 randomised controlled trials found that daily vitamin D3 supplementation at 800-1000 IU reduced the risk of fractures by 13% and falls by 19% [2]. The key word there is “daily” - intermittent high-dose supplementation (say, 50,000 IU once monthly) doesn’t seem to work as well.

If you’re over 65, have limited sun exposure, or have osteoporosis risk factors, vitamin D supplementation makes sense for bone health.

Rickets and osteomalacia prevention

These are the classic vitamin D deficiency diseases. Rickets in children and osteomalacia in adults cause bone pain, muscle weakness, and skeletal deformities. Both are preventable and treatable with vitamin D. This isn’t controversial - it’s why vitamin D supplementation was introduced in the first place.

Reducing respiratory infections

A Cochrane-quality meta-analysis of 25 randomised controlled trials found that vitamin D supplementation reduced the risk of acute respiratory infections [3]. The protective effect was strongest in people who were deficient at baseline and who took daily or weekly doses rather than large bolus doses.

The effect size wasn’t dramatic - you’d need to supplement about 33 people to prevent one respiratory infection. But given vitamin D’s low cost and good safety profile, this is a reasonable benefit to consider.

Benefits with moderate evidence

Type 2 diabetes prevention and management

Observational studies consistently link vitamin D deficiency with higher diabetes risk and poorer glycaemic control. Intervention trials show more mixed results, but a meta-analysis of 28 randomised controlled trials found that vitamin D supplementation improved fasting glucose and insulin resistance in people with prediabetes or at high risk [4].

The effect was most pronounced in people who were actually vitamin D deficient to begin with. If your levels are already adequate, additional supplementation probably won’t help your blood sugar.

Depression

Several meta-analyses have found that vitamin D supplementation improves depression scores, with moderate effect sizes [5]. The research quality is mixed, and I wouldn’t rely on vitamin D alone to treat clinical depression. But for people with depression who are also vitamin D deficient, correcting that deficiency seems reasonable as part of broader treatment.

Non-alcoholic fatty liver disease

A meta-analysis of 16 randomised controlled trials found improvements in weight, BMI, liver enzymes, and insulin resistance with vitamin D supplementation in people with fatty liver disease [6]. The effect sizes were modest, and vitamin D certainly isn’t a substitute for dietary changes and exercise, which remain the primary treatments.

Pregnancy outcomes

Vitamin D supplementation during pregnancy appears to reduce the risk of pre-eclampsia by roughly 60% according to one meta-analysis of 27 trials [7]. There’s also evidence for reduced risk of preterm birth and low birth weight [8].

The NHS already recommends that pregnant women take 10 micrograms (400 IU) of vitamin D daily. This evidence supports that guidance.

Chronic obstructive pulmonary disease

A meta-analysis of 25 trials in people with COPD found improvements in lung function measures and reduced exacerbations with vitamin D treatment [9]. The improvements were modest but consistent. If you have COPD and low vitamin D levels, supplementation seems sensible.

Benefits with weaker or mixed evidence

Cancer

This is where vitamin D research gets complicated. Higher blood vitamin D levels are consistently associated with lower cancer risk in observational studies. The pooled analysis showing a 67% reduction in cancer risk at levels above 40 ng/mL gets cited frequently [10].

But here’s the problem: randomised trials of vitamin D supplementation haven’t shown the same cancer prevention benefit. One large meta-analysis of 10 randomised trials found no reduction in cancer incidence, though it did find a modest reduction in cancer mortality [11].

My honest interpretation: vitamin D deficiency probably isn’t good for cancer risk, and correcting deficiency makes sense. But I wouldn’t take high-dose vitamin D expecting it to prevent cancer.

Cardiovascular disease

Same story as cancer. Deficiency is associated with higher cardiovascular risk in observational studies, but randomised trials of supplementation haven’t shown cardiovascular benefits [12]. A major meta-analysis of 21 trials found no reduction in heart attacks, strokes, or cardiovascular death.

Cognitive decline and dementia

Observational studies link low vitamin D with higher dementia risk. A meta-analysis in Asian populations found that levels below 20 ng/mL increased cognitive impairment risk [13]. But we lack good randomised trial data showing that supplementation prevents cognitive decline.

Autoimmune conditions

This is a speculative but interesting area. Vitamin D has immunomodulatory effects, and deficiency has been associated with multiple sclerosis, rheumatoid arthritis, and other autoimmune conditions. However, a Cochrane review found that vitamin D supplementation had no significant effect on multiple sclerosis disease activity [14].

COVID-19

Early in the pandemic, there was substantial interest in vitamin D for COVID-19 prevention and treatment. A meta-analysis of 9 randomised trials found reduced ICU admissions with vitamin D supplementation [15]. However, study quality was variable, and I’d treat these findings as preliminary rather than definitive.

Conditions where vitamin D probably doesn’t help

Lower back pain

A meta-analysis of 8 clinical trials found no benefit of vitamin D for chronic lower back pain [16]. If someone has back pain and happens to be vitamin D deficient, correcting the deficiency is reasonable, but don’t expect it to fix the back pain.

Male testosterone

Despite marketing claims, a meta-analysis found no effect of vitamin D supplementation on testosterone levels in adult men [17].

Irritable bowel syndrome

One trial showed symptom improvements, but the evidence base is too thin to draw conclusions.

How much vitamin D do you need?

The NHS recommends 10 micrograms (400 IU) daily for most adults, particularly during autumn and winter when sun exposure is limited [18]. The Endocrine Society and many researchers suggest higher intakes of 1000-2000 IU daily may be needed to achieve optimal blood levels, particularly for people with limited sun exposure.

Blood levels of 25(OH)D above 50 nmol/L (20 ng/mL) prevent deficiency. Whether higher levels (75 nmol/L or 30 ng/mL) are better remains debated.

When to take it

Vitamin D is fat-soluble, so absorption improves when taken with a meal containing some fat [19]. Taking it with your largest meal of the day is a reasonable approach.

Safety and side effects

Vitamin D is remarkably safe at typical supplement doses. The safe upper limit is officially 4000 IU daily for adults, though research suggests that doses up to 10,000 IU daily don’t cause toxicity in healthy people [20].

Toxicity occurs with extremely high doses (40,000-100,000 IU daily for months) and causes hypercalcaemia - too much calcium in the blood. Symptoms include nausea, vomiting, weakness, and kidney problems. This is rare and almost always from taking massive supplement doses, not from sun exposure or food.

Five important precautions

Primary hyperparathyroidism: These conditions can cause hypercalcaemia even with normal vitamin D doses. Check with your doctor before supplementing.
Granulomatous diseases (sarcoidosis, tuberculosis): These can convert vitamin D to its active form uncontrollably, raising calcium levels.
Pregnancy: Stay within the 400 IU daily recommendation unless your doctor advises otherwise. Excessive doses may cause problems for the developing baby.
Drug interactions: Vitamin D may interact with calcium, digoxin, thiazide diuretics, and some cholesterol-lowering medications. If you take these, discuss vitamin D supplementation with your pharmacist or doctor.
Kidney disease: People with kidney stones or kidney disease need medical supervision for vitamin D supplementation, as it affects calcium handling.

Medications that reduce vitamin D absorption

If you take anticonvulsants, corticosteroids, orlistat (weight-loss medication), or cholestyramine, you may need higher vitamin D doses or more careful monitoring.

Who should consider vitamin D supplementation?

Based on current evidence, supplementation makes most sense for:

People over 65
Those who spend little time outdoors or cover their skin
People with darker skin living in northern latitudes
Pregnant and breastfeeding women
People with osteoporosis or at high risk of fractures
Those with conditions affecting fat absorption (coeliac disease, Crohn’s disease)
People with documented vitamin D deficiency

For healthy adults with regular sun exposure who eat a varied diet, routine supplementation is less clearly beneficial. The NHS recommends everyone consider vitamin D supplements during winter months when UV levels are too low for skin synthesis.

The bottom line

Vitamin D deficiency is common and worth correcting. The evidence for bone health, fall prevention, and reducing respiratory infections is solid. Benefits for diabetes, depression, and pregnancy outcomes are plausible but less certain.

For cardiovascular disease, cancer prevention, and cognitive decline, the association between deficiency and disease is real, but supplementation trials haven’t shown clear benefits. This doesn’t mean vitamin D is useless for these conditions - it may mean that correcting deficiency matters but pushing levels higher doesn’t help.

A daily supplement of 400-1000 IU is reasonable for most people, particularly in winter. Higher doses may be appropriate for those with documented deficiency or specific risk factors, ideally guided by blood testing.

References

NHS. Vitamin D. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
Zhao JG, Zeng XT, Wang J, Liu L. Association Between Calcium or Vitamin D Supplementation and Fracture Incidence in Community-Dwelling Older Adults: A Systematic Review and Meta-analysis. JAMA. 2017;318(24):2466-2482.
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;356:i6583.
He S, Yu S, Zhou Z, et al. Effect of vitamin D supplementation on fasting plasma glucose, insulin resistance and prevention of type 2 diabetes mellitus in non-diabetics: A systematic review and meta-analysis. Biomed Rep. 2018;8(5):475-484.
Gowda U, Mutowo MP, Smith BJ, Wluka AE, Renzaho AM. Vitamin D supplementation to reduce depression in adults: meta-analysis of randomized controlled trials. Nutrition. 2015;31(3):421-429.
Jaruvongvanich V, Ahuja W, Sanguankeo A, et al. Vitamin D and histologic severity of nonalcoholic fatty liver disease: A systematic review and meta-analysis. Dig Liver Dis. 2017;49(6):618-622.
Fogacci S, Fogacci F, Banach M, et al. Vitamin D supplementation and incident preeclampsia: A systematic review and meta-analysis of randomized clinical trials. Clin Nutr. 2020;39(6):1742-1752.
Bi WG, Nuyt AM, Bhutta ZA, et al. Effect of vitamin D supplementation during pregnancy on birth size: a systematic review and meta-analysis. J Clin Endocrinol Metab. 2018;103(5):1940-1948.
Zhu M, Wang T, Wang C, Ji Y. The association between vitamin D and COPD risk, severity, and exacerbation: an updated systematic review and meta-analysis. Int J Chron Obstruct Pulmon Dis. 2016;11:2597-2607.
McDonnell SL, Baggerly C, French CB, et al. Serum 25-Hydroxyvitamin D Concentrations ≥40 ng/ml Are Associated with >65% Lower Cancer Risk: Pooled Analysis of Randomized Trial and Prospective Cohort Study. PLoS One. 2016;11(4):e0152441.
Zhang Y, Fang F, Tang J, et al. Association between vitamin D supplementation and mortality: systematic review and meta-analysis. BMJ. 2019;366:l4673.
Barbarawi M, Kheiri B, Zayed Y, et al. Vitamin D Supplementation and Cardiovascular Disease Risks in More Than 83,000 Individuals in 21 Randomized Clinical Trials: A Meta-analysis. JAMA Cardiol. 2019;4(8):765-776.
Lv Y, Zhou C, Guo H, et al. Vitamin D deficiency and cognitive impairment in the elderly: A comprehensive systematic review and meta-analysis. Medicine (Baltimore). 2019;98(17):e15363.
Cochrane Library. Vitamin D and multiple sclerosis. 2018.
Tentolouris A, Ntanasis-Stathopoulos I, Vlachakis PK, et al. The impact of vitamin D supplementation on mortality, ICU admission, and invasive mechanical ventilation in COVID-19: A systematic review and meta-analysis of randomized controlled trials. Clin Nutr ESPEN. 2022;52:240-248.
Zadro JR, Shirley D, Ferreira M, et al. Is vitamin D supplementation effective for low back pain? A systematic review and meta-analysis. Pain Physician. 2018;21(2):121-145.
D’Andrea S, Martorella A, Coccia F, et al. Relationship of vitamin D status with testosterone levels: A systematic review and meta-analysis. Endocrine. 2021;72(1):49-61.
NHS. Vitamins, supplements and nutrition in pregnancy. https://www.nhs.uk/pregnancy/keeping-well/vitamins-supplements-and-nutrition/
Dawson-Hughes B, Harris SS, Lichtenstein AH, et al. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet. 2015;115(2):225-230.
Hathcock JN, Shao A, Vieth R, Heaney R. Risk assessment for vitamin D. Am J Clin Nutr. 2007;85(1):6-18.