VITAMIN D - MORE HORMONE THAN VITAMIN
VITAMIN D - THE IMMUNE SYSTEM’S TRAFFIC CONTROLLER.
Why this "sunshine hormone" matters so much for a calm, well-regulated immune system that responds appropriately.
As the clocks go back and the days shorten, vitamin D is one of the first things I check with clients. Most of us still think of it as the bone vitamin, and it is certainly essential for healthy bones and teeth. But some of its most fascinating work happens in the immune system, where it acts less like a nutrient and more like a messenger, helping to keep our defences measured, appropriate and calm.
WHY IT ISN'T REALLY A VITAMIN
Strictly speaking, a vitamin is something the body can't make and must get from food. Vitamin D breaks that rule. We make most of ours in the skin when it is exposed to UVB light from the sun, and relatively little comes from food.
It is also built differently. Vitamin D belongs to the same broad family as steroid hormones such as oestrogen and cortisol. The form made in the skin, or taken as a supplement, is inactive. The liver first converts it into 25-hydroxyvitamin D, the storage form measured in a blood test. It is then converted again, mainly in the kidneys, into calcitriol, the active hormone.
Calcitriol works by binding to the vitamin D receptor inside cells, which then switches a wide range of genes on or off. That is classic hormone behaviour, and it is why many researchers prefer to describe vitamin D as a hormone rather than a vitamin.
A HORMONE THE IMMUNE SYSTEM MAKES FOR ITSELF
Here is the part I find most remarkable. Many immune cells, including macrophages, dendritic cells and T cells, carry vitamin D receptors. Some of them can also activate vitamin D themselves, converting the storage form into the active hormone right where it is needed, at the site of an infection or inflammation.
In other words, the immune system has its own local vitamin D supply chain. But it can only use what is available in the bloodstream, which is one reason low levels matter.
HOW VITAMIN D HELPS THE IMMUNE SYSTEM GET IT RIGHT
In my article on immune resilience this month, I explain that the goal isn't to "boost" the immune system but to help it respond appropriately: acting quickly against genuine threats, then standing down. Vitamin D supports both sides of that balance, which is why I think of it as the immune system's traffic control.
It helps you respond to genuine threats. When immune cells detect an invader, vitamin D helps them produce natural antimicrobial proteins, such as cathelicidin, which can damage bacteria and some viruses. This is part of our first line of defence in the airways and the gut lining.
It helps stop the immune system overreacting. This is where vitamin D's role becomes particularly important for anyone with allergies, inflammation or an autoimmune condition. Vitamin D:
Encourages the development of regulatory T cells, the immune system's peacekeepers, which help switch off a response once it has done its job.
Tempers the more inflammatory branches of the immune response and reduces the production of some pro-inflammatory signalling molecules.
Helps dendritic cells, the immune system's scouts, adopt a more tolerant stance, so they are less likely to raise the alarm over harmless triggers or the body's own tissues.
Put simply, vitamin D helps the immune system tell the difference between a real threat and a false alarm, and helps it calm down again once the danger has passed.
WHAT THE RESEARCH SHOWS
Low vitamin D levels have been linked with more frequent respiratory infections, asthma, eczema and a greater likelihood of several autoimmune conditions, including multiple sclerosis, type 1 diabetes, rheumatoid arthritis and autoimmune thyroid disease. Association doesn't prove cause, of course: being unwell can itself lower vitamin D levels.
That's why the VITAL trial, published in the BMJ in 2022, attracted so much interest. Nearly 26,000 adults in the US, with an average age of 67, took either 2,000 IU of vitamin D a day or a placebo for around five years. Those taking vitamin D had a 22% lower rate of new autoimmune disease, such as rheumatoid arthritis, polymyalgia rheumatica and autoimmune thyroid disease.
It's worth keeping this in perspective: autoimmune diagnoses were uncommon in both groups, at around 1 in 100 people. And the participants weren't, on the whole, deficient to begin with. But it was the first large randomised trial to suggest that vitamin D may help prevent autoimmune disease, rather than simply being associated with it.
For colds and other respiratory infections, reviews of the trials suggest a modest protective effect, seen mainly with regular daily doses rather than large occasional ones, and most clearly in people who start out low.
ARE YOU AT RISK OF LOW VITAMIN D?
In the UK, the sun is too low between roughly October and March for our skin to make meaningful amounts of vitamin D, so levels tend to fall through the winter. You may be more likely to run low if you:
Have darker skin, which needs more sun exposure to make the same amount of vitamin D.
Spend most of your time indoors, or cover most of your skin when outside.
Are older, as the skin makes vitamin D less efficiently with age.
Carry more body fat, which can hold on to vitamin D and reduce circulating levels.
Have a condition affecting fat absorption, such as coeliac disease or inflammatory bowel disease.
Always wear a high-factor sunscreen when outdoors in summer.
TEST, DON'T GUESS
A simple blood test for 25-hydroxyvitamin D will tell you where you are. In the UK, levels below 25 nmol/L are classed as deficient, and between 25 and 50 nmol/L may be inadequate for some people. In clinic, I generally aim for around 90 to 125 nmol/L for immune resilience, which is higher than many conventional reference ranges treat as adequate.
Retesting after three months or so shows whether your approach is working, as people respond very differently to the same dose.
GETTING THE DOSE RIGHT
The NHS recommends that everyone in the UK considers 10 micrograms (400 IU) a day through autumn and winter. It's a useful baseline, but it was set to keep most people above the level linked with bone problems such as rickets and osteomalacia, not to reach the levels I aim for in clinic for immune health. In fact, I'm not alone in thinking this: in 2026 a group of leading UK vitamin D researchers published an urgent call for the UK's definition of deficiency to be raised. In my experience, many people need considerably more to get there, which is exactly why testing matters. The right dose for you will depend on your starting level, skin tone, body size and time spent outdoors, and is best guided by a test and a retest after 3 months.
Because vitamin D is stored in the body, more isn't always better. Too much can raise calcium levels in the blood, which can be harmful. Adults shouldn't take more than 100 micrograms (4,000 IU) a day unless advised by a practitioner who is monitoring their levels. If you have kidney disease, sarcoidosis or another condition that affects calcium levels, please take advice before supplementing.
HELPING VITAMIN D DO ITS JOB
Choose vitamin D3 (cholecalciferol) where possible, as it is generally more effective at raising levels than D2. Vegan D3 made from lichen is widely available.
Take it with a meal containing some fat, as vitamin D is fat-soluble.
Make sure you have enough magnesium, which the body needs to activate vitamin D. Leafy greens, nuts, seeds and wholegrains are good sources.
Consider vitamin K2 alongside it. K2 helps direct the calcium that vitamin D helps you absorb towards bones and teeth. If you take warfarin or another blood thinner, speak to your GP before taking K2.
An easy way to remember it: D boosts calcium absorption, K2 tells calcium where to go, and magnesium is the ignition that lets D do its job.
FOOD AND SUNLIGHT STILL COUNT
Food alone rarely provides enough vitamin D, but it all adds up. Oily fish such as salmon, mackerel and sardines, egg yolks, fortified foods and mushrooms that have been exposed to UV light all contribute.
And from spring onwards, short spells of sun make a real difference. Around 10 to 20 minutes outdoors with forearms or lower legs uncovered, without sunscreen for that short window and always without burning, helps your skin make its own. Even in winter, getting outside in daylight supports your body clock and your mood.
THE BOTTOM LINE
Vitamin D is far more than a bone nutrient. It behaves like a hormone, and it helps the immune system strike that all-important balance: ready to act when needed, but not wasting energy on false alarms or turning on the body itself. Through the UK winter, many of us simply don't have enough.
If you'd like to know your level and what it means for you, this is something we can look at together.
ACADEMIC REFERENCES: