Vitamin D and Your Immune System: What Does the Research Say?
Updated: Oct 13
Written by Rosy Allan, BSc (Hons), ANutr
Registered Associate Nutritionist | Level 3 Personal Trainer | Level 4 Cancer Exercise Rehabilitation Specialist
Vitamin D is often described as an essential nutrient for immune health. During the colder months, supplements promising to support immunity become particularly popular, especially in countries such as the UK where sunlight exposure is limited during winter.
But does taking vitamin D actually help prevent infections? Can it improve immune function? And should everyone in the UK be taking a supplement?
Vitamin D has several established roles in human health, but some of the claims surrounding its immune benefits go beyond what scientific research currently supports.
Let's explore the evidence and what it means for everyday nutrition.
What is vitamin D, and why do we need it?
Vitamin D is a fat-soluble vitamin involved in several important physiological processes.
One of its best-established functions is helping the body regulate calcium and phosphate, which are essential for maintaining healthy bones, teeth and muscles.
Insufficient vitamin D can contribute to musculoskeletal problems, including osteomalacia in adults and rickets in children (SACN, 2016).
Vitamin D also participates in the regulation of immune function. Receptors for vitamin D are found in several types of immune cells, suggesting that it has roles beyond bone health (Bishop et al., 2020).
However, the fact that a nutrient contributes to immune function doesn't necessarily mean taking more of it will improve immunity or prevent illness.
This distinction is important when interpreting supplement claims.
How does vitamin D influence the immune system?
The immune system is a complex network of cells, tissues and chemical signals that helps protect the body from infections and other threats.
It includes two interacting components:
Innate immunity: The body's immediate, relatively non-specific response to potential threats.
Adaptive immunity: A more targeted response involving cells such as T lymphocytes and B lymphocytes.
Research suggests that vitamin D participates in the regulation of both systems (Bishop et al., 2020).
Vitamin D and innate immunity
Laboratory research has demonstrated that vitamin D signalling can influence how certain immune cells respond to microorganisms.
For example, Liu et al. (2006) identified a mechanism through which vitamin D contributes to the production of antimicrobial substances involved in the body's defence responses.
This helps explain why scientists have been interested in its relationship with infection.
However, findings from cell-based experiments do not automatically demonstrate that taking vitamin D supplements reduces infection rates in people.
Vitamin D and inflammation
Vitamin D also influences signalling pathways involved in immune regulation and inflammation.
Experimental research indicates that active vitamin D can affect certain T-cell populations and the production of inflammatory signalling molecules (Bishop et al., 2020).
These findings are biologically interesting, but immune function isn't simply a question of increasing or decreasing inflammation.
A healthy immune response needs to be appropriately regulated.
For this reason, describing vitamin D as an immune booster can be misleading. Its physiological role is considerably more complex.
Does vitamin D supplementation prevent infections?
This is one of the most researched questions surrounding vitamin D.
Earlier systematic reviews suggested that supplementation might provide modest protection against acute respiratory infections under certain circumstances.
However, subsequent clinical trials have produced mixed findings.
A 2025 systematic review and meta-analysis published in The Lancet Diabetes & Endocrinology examined evidence from randomised controlled trials of vitamin D supplementation.
The primary analysis included 40 trials involving 61,589 participants.
Researchers found that vitamin D supplementation did not produce a statistically significant overall reduction in acute respiratory infection risk compared with placebo (Jolliffe et al., 2025).
The estimated effect was small, and the confidence interval included the possibility of no effect. Although some subgroup findings suggested possible benefits from particular supplementation approaches, these results should be interpreted cautiously and do not establish a benefit for everyone.
This doesn't mean vitamin D is unimportant for immune health. It means the current trial evidence does not establish that routine supplementation prevents respiratory infections in the general population.
Correcting vitamin D deficiency remains important for health, regardless of whether supplementation prevents infections.
What about vitamin D and cancer immunity?
Research into vitamin D and cancer has attracted considerable attention.
A 2024 study published in Science investigated how vitamin D might influence immune responses to cancer through changes in the gut microbiome.
In experiments involving mice, researchers found that greater vitamin D availability could alter intestinal microbial communities and enhance certain immune responses against tumours.
The study also identified associations between vitamin D-related biological pathways and cancer outcomes in human datasets (Giampazolias et al., 2024).
These findings are scientifically interesting, particularly because they suggest possible interactions between nutrition, gut microorganisms and immune regulation.
However, much of the mechanistic evidence came from animal experiments, while the human findings were observational.
We cannot conclude from this research that taking vitamin D supplements prevents cancer, reduces recurrence or improves the effectiveness of cancer treatment in humans.
Further clinical trials would be needed to establish whether these mechanisms lead to meaningful treatment benefits.
For people living with or recovering from cancer, vitamin D supplementation should therefore be guided by their individual nutritional and medical needs rather than unproven claims about cancer immunity.
How much vitamin D do we need in the UK?
The Scientific Advisory Committee on Nutrition (SACN) recommends a reference nutrient intake of 10 micrograms (400 IU) of vitamin D daily for people aged four years and older.
This recommendation is based primarily on maintaining musculoskeletal health and applies to total vitamin D intake from food and supplements (SACN, 2016).
Importantly, this is a dietary reference intake rather than an instruction that everybody must take a supplement throughout the entire year. The NHS provides additional guidance on when vitamin D supplementation should be considered (NHS, 2020).
October to early March
During autumn and winter, sunlight in the UK is generally insufficient for the body to produce adequate vitamin D through the skin.
Because obtaining enough vitamin D from food alone can be difficult, the NHS advises that adults and children over four years old should consider taking a daily supplement containing 10 micrograms (400 IU) during these months (NHS, 2020). This includes pregnant and breastfeeding women.
Spring and summer
Between approximately late March or early April and the end of September, most people in the UK can produce sufficient vitamin D through sunlight exposure alongside a balanced diet (NHS, 2020).
However, vitamin D production varies according to factors such as skin pigmentation, time spent outdoors and how much skin is exposed. There is no single safe duration of sun exposure that applies to everybody. Protecting the skin against sun damage remains important, and excessive sun exposure should not be encouraged as a strategy for increasing vitamin D levels (NHS, 2020).
Who may need supplements all year?
Some people have an increased risk of vitamin D deficiency because they produce less vitamin D through sunlight exposure.
The NHS advises year-round supplementation for certain groups, including people who:
Spend very little time outdoors
Are housebound or live in care settings
Usually cover most of their skin when outside
People with darker skin, including those from African, African-Caribbean and South Asian backgrounds, should also consider taking a daily 10-microgram supplement throughout the year (NHS, 2020).
Individual medical conditions may affect vitamin D requirements and should be assessed by an appropriate healthcare professional.
Can we get enough vitamin D from food?
Vitamin D occurs naturally in relatively few foods.
Useful dietary sources include:
Oily fish, such as salmon, sardines and mackerel
Egg yolks
Small amounts in red meat and some other animal products
Fortified breakfast cereals
Fortified fat spreads
Some fortified plant-based milk alternatives
The amount of vitamin D varies considerably between foods and brands.
Unlike in some other countries, ordinary cow's milk in the UK is not routinely fortified with vitamin D (NHS, 2020).
Including vitamin D-containing foods can contribute to nutritional adequacy, but meeting the recommended intake through food alone can be difficult. This is one reason why seasonal supplementation is advised.
Should you have your vitamin D levels tested?
Not everyone needs a vitamin D blood test.
NICE recommends that vitamin D status should not be routinely tested unless someone:
Has symptoms suggesting deficiency
Is considered to be at particularly high risk of deficiency
Has another clinical reason for testing, such as suspected osteomalacia
These recommendations are outlined in NICE public health guideline PH56 (NICE, 2014, updated 2017).
Symptoms such as persistent bone pain or unexplained muscle weakness may warrant medical investigation.
However, symptoms such as tiredness are non-specific and can have many causes.
Rather than assuming that fatigue means vitamin D deficiency, it's sensible to discuss persistent symptoms with your GP.
Can you take too much vitamin D?
Yes. Although vitamin D is essential, excessive supplementation can be harmful.
Because vitamin D is fat-soluble, prolonged high intake can lead to excessive calcium levels in the blood, a condition known as hypercalcaemia.
This can weaken bones and damage the kidneys and heart (NHS, 2020).
The NHS advises adults not to exceed 100 micrograms (4,000 IU) of vitamin D daily unless following an appropriately supervised medical treatment regimen (NHS, 2020).
This is an upper safety limit, not a recommended daily target.
For most adults following standard UK public health guidance, a supplement containing 10 micrograms daily is sufficient. It's also worth checking whether vitamin D is already included in a multivitamin or another supplement to avoid unintentionally combining several products.
People receiving treatment for vitamin D deficiency or managing certain medical conditions may require different advice. The NHS also provides separate safety limits and supplementation guidance for infants and children (NHS, 2020).
Final thoughts
Vitamin D is an essential nutrient with well-established roles in maintaining bone and muscle health.
It also contributes to the normal functioning of the immune system, although this does not mean higher supplement doses necessarily improve immunity.
Recent randomised trial evidence has not established a statistically significant overall benefit of vitamin D supplementation for preventing acute respiratory infections (Jolliffe et al., 2025).
Similarly, research exploring vitamin D and cancer immunity remains scientifically interesting but does not justify claims that supplementation can prevent or treat cancer (Giampazolias et al., 2024).
For most adults in the UK, the practical advice is straightforward: consider a daily 10-microgram vitamin D supplement during autumn and winter, and throughout the year if you're at increased risk of deficiency (NHS, 2020).
Good nutrition is about meeting the body's requirements, not automatically assuming that more is better.
At ROSY, we provide evidence-based online nutrition consultations throughout the UK, helping clients understand their nutritional needs and make informed decisions about their diet and health.
Disclaimer: This article is intended for general nutrition education and does not replace personalised medical or dietetic advice. Vitamin D requirements may differ according to medical conditions, medication and individual circumstances. If you suspect a deficiency, experience persistent symptoms or are undergoing medical treatment, consult your GP or relevant healthcare professional before taking high-dose supplements.
References
Bishop, E.L. et al. (2020) 'Vitamin D and immune regulation: Antibacterial, antiviral, anti-inflammatory', JBMR Plus, 5(1), e10405. Available at: https://doi.org/10.1002/jbm4.10405
Giampazolias, E. et al. (2024) 'Vitamin D regulates microbiome-dependent cancer immunity', Science, 384(6694), pp. 428–437. Available at: https://doi.org/10.1126/science.adh7954
Jolliffe, D.A. et al. (2025) 'Vitamin D supplementation to prevent acute respiratory infections: Systematic review and meta-analysis of stratified aggregate data', The Lancet Diabetes & Endocrinology, 13(4), pp. 307–320. Available at: https://doi.org/10.1016/S2213-8587(24)00348-6
Liu, P.T. et al. (2006) 'Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response', Science, 311(5768), pp. 1770–1773. Available at: https://doi.org/10.1126/science.1123933
National Health Service (NHS) (2020) Vitamin D. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/ (Accessed: 13 October 2026).
National Institute for Health and Care Excellence (NICE) (2014, updated 2017) Vitamin D: Supplement use in specific population groups. Public health guideline PH56. Available at: https://www.nice.org.uk/guidance/ph56 (Accessed: 13 October 2026).
Scientific Advisory Committee on Nutrition (SACN) (2016) Vitamin D and health. London: Public Health England. Available at: https://www.gov.uk/government/publications/sacn-vitamin-d-and-health-report (Accessed: 13 October 2026).
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