Vitamin D gets more attention than almost any other nutrient, and for good reason. It’s involved in bone health, immune function, mood regulation, and dozens of other processes. It’s also the nutrient most commonly deficient in modern populations, with estimates suggesting that roughly a billion people worldwide have inadequate levels.

I’ve spent time going through the clinical literature on vitamin D. What emerges is a complicated picture. Some benefits are solidly established through rigorous randomised controlled trials. Others rest on observational data that show associations but can’t prove causation. And several popular claims simply don’t hold up when properly tested.

This guide covers what we actually know, where the evidence is uncertain, and what precautions to consider before supplementing.

What is vitamin D?

Vitamin D isn’t technically a vitamin in the classical sense. It’s a hormone precursor that your body converts through several steps into calcitriol, its biologically active form. From there, it binds to vitamin D receptors found throughout the body, including in the brain, blood vessels, immune cells, and bones, directly or indirectly influencing over 2,000 genes [1].

The two main forms you’ll encounter are:

Vitamin D3 (cholecalciferol): Made in human skin when exposed to UVB light, and found in animal products like oily fish, egg yolks, and fortified foods
Vitamin D2 (ergocalciferol): Derived from fungi and plants, often used in supplements and fortified foods

Most research uses D3, and there’s reasonable evidence that D3 raises blood levels more effectively than D2 at equivalent doses.

Your vitamin D status is measured by blood levels of 25-hydroxyvitamin D (written as 25(OH)D). 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), though where exactly to draw the line remains debated [2].

Who’s at risk of deficiency?

Several groups face higher risk of inadequate vitamin D:

Older adults: As skin ages, it produces vitamin D less efficiently when exposed to sunlight. Many older people also spend less time outdoors due to mobility limitations or institutional living.

People with darker skin: Higher melanin concentrations protect against UV damage but also reduce the skin’s ability to synthesise vitamin D from sunlight.

Those living far from the equator: In the UK and similar latitudes, UV levels during winter months are too low for meaningful vitamin D production in the skin.

People with fat malabsorption conditions: Since vitamin D is fat-soluble, conditions like coeliac disease, Crohn’s disease, liver disease, or previous bariatric surgery can impair absorption.

Those who cover their skin: Whether for religious, cultural, or sun-protection reasons, covering most skin limits sun-driven vitamin D production.

Benefits with strong evidence

Bone health and fracture prevention

This is vitamin D’s most established function. It promotes calcium absorption in the intestines and maintains the serum calcium levels needed for proper bone mineralisation. Without adequate vitamin D, bones become soft and weak.

A systematic review and meta-analysis published in JAMA examined 32 randomised controlled trials and found that daily vitamin D3 supplementation (800-1000 IU) reduced hip fracture risk by 30% and non-vertebral fractures by 14% in older adults [3]. The key word is “daily”. Intermittent high-dose supplementation, such as 50,000 IU monthly, doesn’t appear to work as well.

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

Rickets and osteomalacia prevention

These classical vitamin D deficiency diseases cause bone pain, muscle weakness, and skeletal deformities. Rickets affects children; osteomalacia affects adults. Both are preventable and treatable with adequate vitamin D. This isn’t controversial; it’s why vitamin D fortification programmes exist.

Reducing respiratory infections

A Cochrane-quality individual participant data meta-analysis of 25 randomised controlled trials found that vitamin D supplementation reduced the risk of acute respiratory tract infections [4]. 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 was moderate. You’d need to supplement roughly 33 people to prevent one respiratory infection. But given vitamin D’s low cost and good safety profile, this represents a meaningful benefit for those at risk.

Pregnancy outcomes

Vitamin D supplementation during pregnancy appears to reduce pre-eclampsia risk by approximately 60% according to meta-analyses [5]. There’s also evidence suggesting reduced risk of preterm birth and low birth weight, though effect sizes vary between studies.

The NHS already recommends pregnant women take 10 micrograms (400 IU) of vitamin D daily, particularly during autumn and winter.

Benefits with moderate evidence

Non-alcoholic fatty liver disease

A meta-analysis of 16 randomised controlled trials found that vitamin D supplementation in people with fatty liver disease improved several markers: weight, BMI, liver enzyme levels (ALT), fasting glucose, and insulin resistance [6].

My honest take: the improvements were modest, and vitamin D certainly isn’t a substitute for the lifestyle changes (weight loss, dietary modification, exercise) that remain primary treatments. But for those with fatty liver who are also vitamin D deficient, correcting that deficiency seems reasonable.

Type 2 diabetes prevention

Observational studies consistently link vitamin D deficiency with higher diabetes risk. Intervention trials show more mixed results, but a meta-analysis of 28 randomised controlled trials found improvements in fasting glucose and insulin resistance in people with prediabetes or elevated diabetes risk [7].

The catch: benefits were most pronounced in people who were actually vitamin D deficient at baseline. 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 [8]. Research quality varies considerably, and I wouldn’t rely on vitamin D alone to treat clinical depression. But for people with depression who also have low vitamin D, correcting that deficiency as part of broader treatment seems reasonable.

Sleep quality

A meta-analysis examining 25 studies found that vitamin D deficiency was associated with poorer sleep quality, and supplementation improved scores on the Pittsburgh Sleep Quality Index [9].

The evidence base isn’t robust enough to recommend vitamin D specifically as a sleep aid. But if you have sleep problems and happen to be deficient, correcting the deficiency might help.

Chronic urticaria (hives)

A systematic review of 17 observational studies and 5 intervention trials found that adults with chronic urticaria often have lower serum vitamin D levels, and supplementation reduced symptom severity [10].

Sample sizes were small and heterogeneity was high. I’d treat this as preliminary evidence that warrants further investigation rather than a definitive treatment recommendation.

Benign paroxysmal positional vertigo

This condition causes brief episodes of dizziness triggered by head position changes. A meta-analysis of 7 controlled trials found that vitamin D supplementation reduced recurrence rates in affected patients [11].

Again, sample sizes were limited. If you have recurring BPPV and vitamin D deficiency, supplementation seems worth trying alongside standard repositioning manoeuvres.

Knee osteoarthritis

A systematic review and meta-analysis of 6 randomised controlled trials (1,599 patients total) found that vitamin D supplementation improved pain, function, and stiffness scores on the WOMAC scale for osteoarthritis patients [12].

The improvements in joint structure (cartilage volume, joint space width) weren’t statistically significant. So vitamin D might help symptoms without slowing structural disease progression.

Pneumonia prevention

A meta-analysis of 13 randomised controlled trials found that vitamin D supplementation reduced the chance of pneumonia recurrence, particularly in children and when using higher doses [13].

This aligns with the broader evidence that vitamin D supports immune function against respiratory pathogens.

Sarcopenia

Age-related muscle loss affects quality of life and increases fall risk. A meta-analysis found that combining vitamin D with protein supplementation and exercise improved grip strength in people with sarcopenia [14].

Vitamin D alone probably isn’t enough. But as part of a comprehensive approach including resistance exercise and adequate protein, it may contribute to functional recovery.

Chronic obstructive pulmonary disease

A meta-analysis of 25 trials in people with COPD found modest improvements in lung function measures and reduced exacerbations with vitamin D treatment [15]. 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. Some pooled analyses suggest substantial reductions at levels above 40 ng/mL.

But here’s the problem: randomised trials of vitamin D supplementation haven’t shown the same cancer prevention benefit. A major meta-analysis found no reduction in cancer incidence, though it did find a modest reduction in cancer mortality [16].

My interpretation: correcting deficiency probably matters, but pushing levels higher through supplements doesn’t appear to prevent cancer.

Cardiovascular disease

Same pattern as cancer. Deficiency correlates with higher cardiovascular risk in observational studies, but randomised supplementation trials haven’t shown cardiovascular benefits. A meta-analysis of 21 trials found no reduction in heart attacks, strokes, or cardiovascular death [17].

Cognitive decline and dementia

Low vitamin D levels are associated with higher dementia risk in observational studies. However, we lack good randomised trial data showing that supplementation prevents cognitive decline.

Multiple sclerosis and autoimmune conditions

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 MS disease activity [18].

COVID-19

Early in the pandemic, vitamin D received considerable attention. A meta-analysis found reduced ICU admissions with supplementation [19]. Study quality was variable, and I’d treat these findings as hypothesis-generating rather than definitive. What seems clear is that deficiency is associated with worse outcomes, so correcting deficiency in at-risk populations makes sense.

What vitamin D probably doesn’t help

Lower back pain

A meta-analysis of 8 clinical trials found no benefit of vitamin D supplementation for chronic lower back pain [20]. If you have back pain and vitamin D deficiency, correcting the deficiency is reasonable, but don’t expect it to fix the pain.

Male testosterone

Despite marketing claims, a meta-analysis found no effect of vitamin D supplementation on testosterone levels in adult men [21].

Weight loss

Vitamin D deficiency correlates with obesity, but supplementation doesn’t appear to cause weight loss. The relationship is probably reverse causation or confounding rather than direct effect.

How much vitamin D do you need?

The NHS recommends 10 micrograms (400 IU) daily for adults during autumn and winter, when UV levels are insufficient for skin synthesis [22]. People at higher risk of deficiency (those who rarely go outside, cover their skin, or have dark skin) should consider supplementing year-round.

The Endocrine Society and some researchers suggest higher intakes of 1000-2000 IU daily may be needed to achieve blood levels above 50 nmol/L, particularly for people with limited sun exposure or higher body weight.

Blood levels above 50 nmol/L (20 ng/mL) prevent deficiency. Whether aiming for 75 nmol/L (30 ng/mL) or higher provides additional benefits remains debated.

Timing and absorption

Vitamin D is fat-soluble, so taking it with a meal containing some fat improves absorption [23]. Taking it with your largest meal of the day is a practical approach.

Safety and side effects

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

Toxicity requires extremely high doses (40,000-100,000 IU daily for months) and causes hypercalcaemia, meaning too much calcium in the blood. Symptoms include nausea, vomiting, weakness, and kidney problems. This is rare and almost always results from taking massive supplement doses rather than sun exposure or food.

Common side effects at therapeutic doses are unusual, though some people report nausea, constipation, or dry mouth with higher doses.

Five important precautions

1. Primary hyperparathyroidism and granulomatous diseases

Conditions including primary hyperparathyroidism, sarcoidosis, and tuberculosis can cause your body to convert vitamin D to its active form in an uncontrolled manner, raising calcium levels dangerously. If you have these conditions, check with your doctor before supplementing.

2. Kidney disease and kidney stones

People with kidney stones or chronic kidney disease need medical supervision for vitamin D supplementation, as it affects calcium handling. High-dose vitamin D may increase stone risk in susceptible individuals.

3. Medication interactions

Vitamin D can interact with several medications:

Thiazide diuretics: May increase calcium levels when combined with vitamin D
Digoxin: Hypercalcaemia from excessive vitamin D can trigger dangerous heart rhythms
Corticosteroids: Long-term use reduces vitamin D absorption
Some cholesterol medications: Cholestyramine and orlistat reduce vitamin D absorption
Anticonvulsants: May increase vitamin D metabolism, requiring higher doses

Discuss vitamin D supplementation with your pharmacist or doctor if you take these medications.

4. Pregnancy considerations

Pregnant women should take 400 IU daily as recommended by the NHS. Higher doses require medical supervision, as excessive intake may affect foetal development.

5. Individual variation in response

Some people are “hyper-responders” who achieve high blood levels with modest supplementation, while others require higher doses to reach adequate levels. If you’re taking high-dose vitamin D, periodic blood testing helps ensure you stay within the safe range.

Who should consider supplementation?

Based on current evidence, vitamin D supplementation makes most sense for:

People over 65
Those with limited sun exposure or who cover their skin
People with darker skin living in northern latitudes
Pregnant and breastfeeding women
Those with osteoporosis or high fracture risk
People with conditions affecting fat absorption (coeliac disease, Crohn’s disease, inflammatory bowel disease)
Those with documented vitamin D deficiency

For healthy adults with regular sun exposure who eat a varied diet, routine supplementation is less clearly beneficial during summer months. The NHS recommends everyone in the UK consider supplementation during winter.

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, fatty liver, 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 might mean that correcting deficiency matters, but pushing levels higher through supplements doesn’t add much.

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

References

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