Vitamin D in winter: who needs a supplement and how much

Vitamin D in winter: who needs a supplement and how much

From October to early March, the sun in the UK simply is not strong enough for your skin to make vitamin D, no matter how long you spend outside at lunchtime. That is not a wellness-industry line; it is the reason NHS guidance tells everyone to consider a daily supplement for these months.

The good news is that this is one of the rare areas of nutrition where the advice is short, cheap and fairly settled. Here is what it actually says.

Why winter is the problem

Most of your vitamin D comes not from food but from your skin, which makes it when exposed to UVB light. In the UK, NHS guidance says most people can make all they need from sunlight between late March or early April and the end of September. Outside that window, the sun sits too low in the sky for enough UVB to get through.

Summer stores run down fast. The National Diet and Nutrition Survey for 2019 to 2023, published by the government in June 2025, found that 18% of adults aged 19 to 64 had blood levels below 25 nmol/L, the threshold used in the UK to define low status. Among teenagers aged 11 to 18 it was 23%. The seasonal pattern is stark: in groups aged 11 and over, between 21% and 38% had low levels in January to March, compared with 0% to 10% in July to September.

Food helps, but not much. Oily fish, egg yolks, red meat and some fortified cereals and fat spreads contain vitamin D, but it is hard to reach 10 micrograms a day from diet alone, and UK cows’ milk is not routinely fortified.

Who should take vitamin D and how much

The UK figures come from the Scientific Advisory Committee on Nutrition (SACN), whose 2016 report set a reference intake of 10 micrograms a day for everyone aged four and over, designed to protect bone and muscle health across the whole population. NHS advice builds on that:

• Adults and children over 4, October to March: consider 10 micrograms (400 IU) a day
• Adults and children over 4 who are at higher risk: 10 micrograms a day all year round. This means people who are not often outdoors (for example, frail or housebound), live in a care home or similar setting, or usually wear clothes that cover most of their skin when outside
• People with dark skin, such as those of African, African-Caribbean or South Asian heritage: NHS advice says to consider a supplement all year, because darker skin makes vitamin D less efficiently from the same sunlight
• Pregnant and breastfeeding women: 10 micrograms a day, as for other adults
• Children aged 1 to 4: 10 micrograms a day, all year round
• Babies under 1: 8.5 to 10 micrograms a day if breastfed, or if they have less than 500ml of infant formula a day. Babies drinking more formula than that do not need extra, because formula is already fortified

On units: one microgram equals 40 IU, so 10 micrograms is 400 IU.

Families on the Healthy Start scheme can get free vitamins for pregnant women and young children; ask your health visitor, midwife or pharmacist.

What vitamin D does, and what it probably does not

The job vitamin D is undeniably good at is helping your body absorb and regulate calcium and phosphate, which keep bones, teeth and muscles healthy. Severe, long-term deficiency causes rickets in children and osteomalacia, a painful softening of the bones, in adults. Preventing those is the whole basis of the 10 microgram advice.

Vitamin D has also been linked with heart disease, cancer, depression and immunity, mostly in observational studies. But people who are unwell or less active also spend less time outside, so low vitamin D may be a marker of poor health rather than its cause.

Large randomised trials have since tested this directly, with humbling results. In the US VITAL trial, an analysis published in the New England Journal of Medicine in 2022 followed more than 25,000 generally healthy midlife and older adults for a median of 5.3 years. Taking 2,000 IU of vitamin D3 a day did not reduce total, hip or non-spinal fractures compared with placebo. The authors stressed this does not apply to people who are deficient or have osteoporosis, but it suggests topping up people who already have enough adds little.

Getting the dose right (and not overdoing it)

Because vitamin D is fat-soluble, your body stores it, which is useful in summer and a risk if you take too much for too long. NHS guidance sets these daily upper limits:

• Adults (including pregnant and breastfeeding women and older people) and children aged 11 to 17: no more than 100 micrograms (4,000 IU)
• Children aged 1 to 10: no more than 50 micrograms (2,000 IU)
• Babies under 12 months: no more than 25 micrograms (1,000 IU)

Very high doses over a long period can cause a build-up of calcium in the blood (hypercalcaemia), which can weaken bones and damage the kidneys and heart. You cannot overdose on vitamin D from sunlight.

If you already take a multivitamin, check the label before adding a separate vitamin D so you are not doubling up.

Signs you might be low, and when to get checked

Mild deficiency usually causes no obvious symptoms, which is why blanket winter advice exists. More significant deficiency can cause bone pain and weak muscles, and in children, bowed legs. Tiredness has many more common causes.

For most healthy adults, routine testing is not needed. If you have persistent bone or muscle pain, are in a high-risk group and have not been taking a supplement, or have a condition that affects absorption, such as coeliac or Crohn’s disease, it is worth speaking to your GP about whether a test makes sense.

Final thoughts

Vitamin D in winter is one of the few supplement questions with a genuinely simple answer for most people: 10 micrograms a day from October to March, and all year round if you are in one of the higher-risk groups. It is cheap, safe at that dose, and protects bones and muscles during the months when UK sunlight cannot. What it is not is a winter cure-all, and taking more than you need does not buy extra protection. This is general information, not medical advice; if you have a long-term health condition, take regular medication or think you may be deficient, speak to your GP or pharmacist, and contact NHS 111 if you are worried and unsure where to turn.

FAQs

Should I take vitamin D in winter if I feel fine?

Yes, NHS advice is for everyone to consider 10 micrograms a day from October to March. Mild deficiency rarely causes noticeable symptoms, so feeling well is not a reliable guide.

How much vitamin D should an adult take a day in the UK?

10 micrograms, which is the same as 400 IU. That is the amount recommended for adults, including pregnant and breastfeeding women, and children from the age of one.

Can I get enough vitamin D from food in winter?

It is difficult. Oily fish, egg yolks, red meat and fortified cereals and spreads all help, but most people will not reliably reach 10 micrograms a day from diet alone, especially without regular oily fish.

Is 1,000 IU of vitamin D too much?

No, not for a healthy adult. 1,000 IU is 25 micrograms, which is above the standard advice but well below the 100 microgram (4,000 IU) daily upper limit set for adults.

Why do people with darker skin need more vitamin D?

Darker skin contains more melanin, which reduces how much vitamin D the skin makes from the same amount of sunlight. NHS advice is for people of African, African-Caribbean and South Asian heritage to consider a supplement all year round.

Does vitamin D stop colds and flu?

Not reliably. A large 2021 meta-analysis found a small reduction in respiratory infections with regular daily supplements, but the effect varied between trials. It is not a substitute for flu vaccination if you are eligible.

Should children take vitamin D?

Yes. Children aged one to four should have 10 micrograms a day all year, and older children should follow the adult autumn and winter advice. Babies have their own guidance depending on how much formula they drink.

Do I need a vitamin D blood test?

Most healthy adults do not. Following the standard advice is usually enough. Ask your GP about testing if you have symptoms such as bone pain, a condition that affects absorption, or are in a high-risk group.