Vitamin D contributes to normal immune function, but can taking more prevent infections or autoimmune conditions? Here's what the evidence says and how much adults need.
🕒 7 min read
Vitamin D and the immune system are closely linked. Vitamin D contributes to the normal function of the immune system, alongside its established roles in maintaining normal bones, teeth and muscle function.1
That does not mean that taking more vitamin D will necessarily prevent infections or treat an autoimmune condition. Research into these areas is ongoing, and it is important to distinguish vitamin D's established role in normal immune function from claims about preventing or treating disease.
Does vitamin D support the immune system?
Yes. Vitamin D contributes to the normal function of the immune system. Maintaining an adequate vitamin D intake is therefore one part of supporting normal immune health.
Vitamin D is perhaps best known for helping the body absorb and use calcium and phosphorus, but vitamin D receptors are also found in cells throughout the immune system.3
However, supporting the normal function of the immune system is different from claiming that a vitamin D supplement can prevent or cure an infection or immune-related disease.
How does vitamin D interact with the immune system?
The immune system has two closely connected parts: innate immunity, which provides a rapid first response to potential threats, and adaptive immunity, which develops more targeted responses.
Research shows that vitamin D signalling is involved in both. Immune cells including monocytes, macrophages and lymphocytes can respond to the active form of vitamin D through vitamin D receptors.3
This provides a biological explanation for vitamin D's role in normal immune function, but laboratory mechanisms alone do not show that taking additional vitamin D will prevent a particular illness. Clinical trials are needed to establish whether supplementation changes real-world health outcomes.
What does the evidence say about vitamin D, infections and immune conditions?
Vitamin D and respiratory infections
Researchers have investigated whether vitamin D supplementation can reduce the likelihood of respiratory infections such as colds and other acute respiratory illnesses.
Earlier studies suggested a small potential benefit in some circumstances. However, a large updated systematic review and meta-analysis published in 2025 combined data from 40 randomised trials involving more than 61,000 participants. Overall, vitamin D supplementation did not produce a statistically significant reduction in the risk of acute respiratory infections.4
This means vitamin D remains important for normal immune function, but supplements should not be presented as a way to prevent colds, flu or other respiratory infections.
For more on the nutrients involved in seasonal immune health, read our guide to vitamins C and D and colds.
Vitamin D and autoimmune conditions
Autoimmune conditions occur when the immune system mistakenly attacks the body's own cells or tissues. Scientists have therefore been interested in whether vitamin D's involvement in immune regulation could influence autoimmune disease.
One large randomised controlled trial, the VITAL study, followed 25,871 older US adults for a median of 5.3 years. Participants assigned to 2,000 IU (50 micrograms) of vitamin D per day had fewer confirmed new autoimmune diseases than those given placebo.5
This is an interesting finding, but it does not mean vitamin D should be used to prevent or treat autoimmune conditions. The study involved a particular older population and used a research dose higher than the standard UK daily recommendation. More evidence is needed before conclusions can be applied more widely.
If you have an autoimmune condition, do not alter your treatment or take high-dose vitamin D specifically for the condition without speaking to your doctor or another qualified healthcare professional.
Who is at risk of low vitamin D?
From around late March or early April until the end of September, most people in the UK can make the vitamin D they need when their skin is exposed to sunlight outdoors. Between October and early March, sunlight in the UK is not strong enough for the body to make sufficient vitamin D.2
Some people are at greater risk of not getting enough vitamin D throughout the year. The NHS advises year-round supplementation for people who:
- are not often outdoors, for example because they are frail or housebound
- live in an institution such as a care home
- usually wear clothing that covers most of their skin while outdoors.
People with darker skin, including those from African, African-Caribbean and South Asian backgrounds, may also not make enough vitamin D from sunlight and should consider taking a daily supplement throughout the year.2
If you're concerned about your vitamin D status, read our guide to vitamin D deficiency symptoms and how to get enough.
How much vitamin D should you take?
Adults need 10 micrograms (400 IU) of vitamin D a day. During autumn and winter, everyone should consider taking a daily supplement containing 10 micrograms because it is difficult to make enough vitamin D from sunlight in the UK at this time of year.2
People at greater risk of vitamin D deficiency should consider taking 10 micrograms throughout the year.
The NHS advises adults not to take more than 100 micrograms (4,000 IU) of vitamin D per day unless a healthcare professional has advised otherwise. Taking too much vitamin D over a long period can cause calcium to build up in the body and may damage the bones, kidneys and heart.2
If you use more than one supplement, check the labels so you know your total daily vitamin D intake.
You can explore Healthspan's full range of vitamin D supplements or browse our wider range of immune-support supplements.
How can you get vitamin D naturally?
Sunlight is our main natural source of vitamin D. During spring and summer, most people in the UK can make vitamin D through normal time spent outdoors.
Vitamin D is also found in a relatively small number of foods, including:
- oily fish such as salmon, sardines, trout, herring and mackerel
- egg yolks
- red meat
- some fortified foods.
Because it can be difficult to obtain sufficient vitamin D through food alone, UK guidance recommends supplementation during autumn and winter, with year-round supplementation for people at greater risk of deficiency.
