Vitamin D in the UK: Why Autumn and Winter Matter

From October to March, UK sunlight can't make vitamin D in your skin. Here's who needs a supplement, how much, and how to choose one well.

The clocks have gone back, the evenings are dark by five, and for the next six months most of us in the UK can't make enough vitamin D from sunlight. That's why the NHS advises everyone to consider a daily 10 µg (400 IU) vitamin D supplement from October to March.

It's one of the simplest nutrition habits you can build, yet national data show that almost a third of working-age adults are deficient by late winter. Here's why it happens, who needs to pay most attention, and how to choose a supplement that does the job.

Why the UK sun stops working in October

From October to March, the sun sits too low in the UK sky for its UVB rays to reach us in useful amounts. Without UVB, skin can't make vitamin D.

Your skin makes vitamin D when UVB light converts a cholesterol-like compound (7-dehydrocholesterol) into vitamin D3. London sits at about 51°N, and Edinburgh at 56°N. At these latitudes, winter sunlight passes through so much atmosphere that most UVB is filtered out before it reaches you, however bright the day looks.

A few other things make winter harder:

• Glass blocks UVB. Sitting by a sunny window or in a conservatory won't raise your levels.
• Short days, long indoors. Commuting in the dark and working inside means little skin exposure, even on the rare strong-sun days.
• Summer stores run down. Vitamin D is stored in fat and the liver, but reserves built over summer gradually fall, which is why deficiency peaks between January and March.

That peak is measurable. In the latest National Diet and Nutrition Survey (2019–2023), 18% of adults aged 19–64 were deficient across the year, rising to 31% in January to March. For 11–18 year olds, the winter figure was 36%.

What vitamin D actually does

Vitamin D's best-established role is helping the body absorb calcium and phosphate, which supports healthy bones, teeth and muscles. Severe, long-term deficiency causes rickets in children and osteomalacia (soft, painful bones) in adults.

Beyond bones, the evidence is promising but more mixed. Here is an honest summary:

• Immune function (moderate evidence). Vitamin D contributes to the normal function of the immune system. A 2017 BMJ analysis of 25 randomised trials (about 11,000 people) found supplements reduced the risk of acute respiratory infections by around 12%. A 2021 update including more than 40 trials found a smaller benefit, of around 8%. In both, the effect was clearest with daily or weekly doses, not large occasional ones, and in people who started out low.
• Muscle strength and falls (supportive in older adults). Low vitamin D is linked to muscle weakness, which matters most for people over 65.
• Gut microbiome (emerging). A 2025 review of 14 randomised trials found vitamin D supplements were linked to shifts toward bacteria such as Bifidobacterium and Lactobacillus. But results vary a lot by dose and starting level, and a large Australian trial using monthly high doses saw no change in gut diversity. It's an exciting area, not yet a reason to take vitamin D on its own.
• Mood and energy (uncertain). Tiredness, aches and low mood are common signs of deficiency, but taking vitamin D hasn't been clearly shown to lift mood in people whose levels are already adequate.

The practical message: correcting low vitamin D clearly matters. Taking very large doses on top of normal levels hasn't been shown to add much.

Who is most at risk

Some people struggle to make enough vitamin D even in summer, so UK guidance advises them to supplement all year round, not just from October to March:

• People with darker skin, such as those of African, African-Caribbean or South Asian heritage. More melanin means more sun is needed to make the same amount of vitamin D. In UK Biobank, deficiency affected 54% of Asian and 35% of Black participants, compared with 12% of white participants.
• People who cover most of their skin when outdoors, for cultural, religious or medical reasons.
• People who are rarely outdoors, including those who are housebound, frail or living in a care home.
• Babies and young children. Breastfed babies need 8.5–10 µg daily from birth; formula-fed babies drinking more than 500 ml a day don't, as formula is already fortified. Children aged 1–4 need 10 µg daily all year.

Others worth extra attention in winter: pregnant and breastfeeding women (10 µg daily is advised, and the Healthy Start scheme offers free vitamins to eligible families), teenagers, people living with obesity, night-shift workers, and anyone living further north.

What UK guidance says

For most people over 4, the advice is simple: 10 µg (400 IU) a day from October to March. This comes from the Scientific Advisory Committee on Nutrition (SACN) and is endorsed by the NHS and the Department of Health and Social Care.

Who	Daily amount	When	Don't exceed (per day)
Adults and children over 4	10 µg (400 IU)	October to March	100 µg (4,000 IU) for adults and 11–17s
Pregnant or breastfeeding	10 µg (400 IU)	Autumn and winter at least	100 µg (4,000 IU)
Darker skin, covered skin, rarely outdoors	10 µg (400 IU)	All year	100 µg (4,000 IU)
Children aged 1–4	10 µg (400 IU)	All year	50 µg (2,000 IU) for ages 1–10
Babies under 1 (breastfed, or under 500 ml formula a day)	8.5–10 µg (340–400 IU)	All year	25 µg (1,000 IU)

A quick conversion: 1 µg = 40 IU. Many supplements come in 25 µg (1,000 IU) doses, which is also suitable for adults.

More is not better. Taking very high doses for a long time can cause calcium to build up in the blood (hypercalcaemia), which can damage the kidneys, heart and bones. You can't overdose on vitamin D from sunlight. If your GP has prescribed a higher dose to treat a diagnosed deficiency, follow their advice.

Can you get enough from food?

Food helps, but very few foods contain much vitamin D, so most people can't reach 10 µg a day from diet alone in winter. Approximate amounts per typical portion:

Food	Portion	Vitamin D (approx.)
Wild salmon	100 g	10–17 µg
Farmed salmon	100 g	5–9 µg
Rainbow trout	100 g	15–19 µg
Mackerel or herring	100 g	5–10 µg
UV-treated mushrooms	100 g	up to 10 µg or more (check the label)
Tinned sardines	100 g	4–5 µg
Egg	1 large	about 1–1.5 µg
Fortified cereal, spread or plant milk	1 serving	about 1–2.5 µg (check the label)

Figures vary with species, farming method and brand, so treat them as rough guides.

Two UK-specific points are worth knowing. First, unlike in the US, standard UK cow's milk is not routinely fortified with vitamin D. Second, ordinary mushrooms contain very little; only those exposed to UV light (look for "high in vitamin D" on the pack) are a meaningful source, and they provide vitamin D2.

A considered winter plate: oily fish twice a week, eggs a few times a week, and fortified foods where you can. That's a good foundation, with a supplement filling the gap.

How to choose and take a supplement

A simple vitamin D3 at 10–25 µg, taken daily with a meal, covers most people. Here is what to look for.

1. Choose D3 (cholecalciferol) where you can. A UK trial in 335 women found D3 raised winter blood levels around twice as much as D2 at the same daily dose. Most D3 comes from lanolin (sheep's wool), so it isn't vegan.
2. Vegan? Look for lichen-derived D3. Lichen is a plant-like organism that makes D3, giving vegans the more effective form. D2 (from fungi or yeast) is the alternative and is still useful.
3. Take it with food. Vitamin D is fat-soluble. Small studies suggest taking it with a meal that contains some fat, often your main meal, improves absorption. Consistency matters more than time of day.
4. Daily beats occasional mega-doses. For everyday use, a small daily dose is what the trial evidence supports best.
5. Pick a format you'll stick with. Tablets, softgels, sprays and drops all work. Sprays and drops suit people who dislike swallowing tablets.
6. Check your total. Add up the vitamin D from every product you take (multivitamins, fish oils, "immune" blends) so you stay well under the upper limit.

What about vitamin K2? D3 and K2 combinations are popular because vitamin K is involved in bone metabolism. At the standard 10–25 µg of vitamin D, there's no clear evidence you need K2 alongside it. Anyone taking warfarin or other blood thinners should avoid vitamin K supplements unless their doctor approves.

Should you get tested? Most healthy people don't need a test to follow the 10 µg advice. A blood test (25-hydroxyvitamin D) is worth discussing with your GP if you have symptoms such as persistent bone pain or muscle weakness, a condition affecting absorption (such as coeliac or Crohn's disease), or you're in a higher-risk group. In the UK, a level below 25 nmol/L counts as deficient. If you retest after changing your dose, wait 8–12 weeks for levels to settle.

Check with a professional first if you have kidney disease, sarcoidosis, hyperparathyroidism or high calcium levels, or take medicines such as thiazide diuretics, digoxin, steroids or anti-epileptics.

The bottom line

• From October to March, UK sunlight can't make vitamin D in your skin.
• Almost a third of working-age adults are deficient by late winter.
• Take 10 µg (400 IU) daily through autumn and winter, and all year if you have darker skin, cover your skin or rarely get outside.
• Choose D3 (lichen D3 if vegan), take it with a meal, and don't go above 100 µg a day without medical advice.
• Eat oily fish, eggs and fortified foods too, but don't rely on food alone.

Lifelong health is built quietly, through small choices made consistently. A daily vitamin D through the darker months is one of the simplest of them.

This article is for general information and education only. It is not medical advice and does not replace advice from your GP or another qualified healthcare professional. Please speak to your doctor before making changes to medication or starting supplements, particularly if you are pregnant, breastfeeding or managing a health condition.

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