Vitamin D in winter: who in Ireland should take a supplement

Yes. The HSE’s advice is that “everyone in Ireland needs to take a vitamin D supplement” (HSE). Between October and early March, sunlight here is not strong enough for skin to make vitamin D (HSE), and the handful of foods that contain it do not close the gap. For most adults that means a 15 microgram tablet a day from Halloween to St Patrick’s Day, and every day of the year if you are over 65, pregnant, have black or brown skin, or are rarely outdoors (HSE). A bottle at that strength is cheap and lasts months.

What it delivers is narrower than the label suggests. Vitamin D helps the body absorb calcium, so a low dose protects bones in people who would otherwise run short. Moderate-quality trial evidence says it probably cuts the rate of falls among older people in care homes who start with low levels. The claim that it wards off colds and flu looked promising in 2017 and has weakened with every larger update since. Nothing in the trials suggests that taking more than the standard dose does more, and the HSE says adults and children age 11 and older should not take more than 100 micrograms a day (HSE).

Why can’t people in Ireland make enough vitamin D from sunlight in winter?

Skin makes vitamin D from a band of ultraviolet light called UVB. In winter the sun sits so low in the sky over Ireland that almost none of that UVB reaches the ground, however bright and clear the day. Glass blocks UVB too, so sitting by a sunny window does nothing.

The HSE puts it plainly: “Between October and early March, sunlight is not strong enough to make vitamin D” (HSE). The NHS gives the same window for Britain (NHS). Oily fish, red meat, liver, egg yolks and fortified spreads and cereals contain some vitamin D, but the HSE’s line is that everyone should take a supplement even if they eat those foods (HSE).

Who does the HSE say should take a vitamin D supplement, and who is at highest risk of deficiency?

The HSE’s answer is everyone: “Everyone in Ireland is at risk of vitamin D deficiency in winter” (HSE). The dose is banded by age:

• Babies under 12 months: 5 micrograms every day from birth, all year, if breastfed. A baby taking more than 300ml of infant formula a day does not need a supplement, because formula now has vitamin D added (HSE).
• Children age 1 to 4: 5 micrograms a day from Halloween (31 October) to St Patrick’s Day (17 March) (HSE).
• Children age 5 to 12: 10 micrograms a day over the same winter months (HSE).
• People age 13 to 64: 15 micrograms a day from Halloween to St Patrick’s Day (HSE).
• Adults age 65 and older: 15 micrograms every day, all year round (HSE).

Some people should take it all year round whatever their age. The HSE names people who are not often outdoors, such as the housebound, people who usually wear clothes that cover up most of their skin outdoors, people with black or brown skin, anyone over 65, anyone who is pregnant, and babies (HSE). Darker skin makes less vitamin D from the same sunlight, and the HSE notes that people of African, African-Caribbean or south Asian background may not make enough even in summer (HSE).

The NHS advice for Britain is different in the detail. It gives adults a 10 microgram daily dose, says everyone should consider it in autumn and winter, and makes it year-round for the at-risk groups and all children aged 1 to 4 (NHS). If you have read the NHS figures online, use the HSE ones here.

What does a supplement actually do: bones, falls, colds and flu, mood?

Bones. This is the settled part. Vitamin D lets the gut absorb calcium. Prolonged shortage softens bone, which shows up as rickets in children and as bone pain caused by osteomalacia in adults (NHS). A standard daily dose in someone running short prevents that. It is the reason the supplement is in public health advice at all.

Falls. The evidence is real but specific. A Cochrane review of falls prevention in care facilities and hospitals found moderate-quality evidence, from 4 trials with 4512 participants, that vitamin D probably reduces the rate of falls, but probably makes little or no difference to the number of people who fall at all (Cochrane). The review is explicit that the people in those trials had low vitamin D levels to begin with. That is a care-home population running short, not healthy retired people living at home.

For older people living at home, the evidence is thinner. A separate Cochrane review of community-wide falls programmes found one trial that offered older residents free daily calcium and vitamin D3, and rated the evidence for it as very low certainty (Cochrane). A systematic review of 15 clinical guidelines on falls in older adults found that most strongly recommended exercise, medication review, and vision and footwear correction, while their recommendations on vitamin D were inconsistent (JAMA Network Open). Around one-third of adults aged 65 or older living in the community fall each year (Cochrane), so this matters. For an older person living at home, the guidelines agree far more firmly on exercise than on a vitamin D tablet.

Colds, flu and chest infections. This is where the claims have run ahead of the trials. A 2017 meta-analysis pooled individual data from 25 randomised trials with 11,321 participants and found that vitamin D reduced the risk of acute respiratory infection overall (BMJ). In that analysis the benefit looked strongest in people who were very deficient, with blood levels below 25 nmol/L, and in people taking a daily or weekly dose rather than occasional large “bolus” doses (BMJ).

Those subgroup findings did not hold up. A 2021 update pooled 37 studies and still found a protective effect, but described the risk reduction as small and found no significant effect in any subgroup defined by starting vitamin D level (Lancet Diabetes & Endocrinology). The 2025 update identified six new trials, including one large one, and pooled 40 studies with 61,589 participants. The point estimate barely moved, but the confidence interval now includes no effect, meaning no statistically significant protection, and the subgroup analysis found no effect modification by age, baseline vitamin D status, dosing frequency or dose size (Lancet Diabetes & Endocrinology). Further caveats push the reading towards “no reliable effect”: the funnel plot was asymmetric, which suggests small trials with negative results may be missing from the published record, and only three of the six new trials supplied data (Lancet Diabetes & Endocrinology).

The honest summary is that a supplement might slightly reduce your chance of a chest infection, but the best current evidence cannot show that it does. Nor is it a treatment: a review of nutritional treatments in children, which pooled the vitamin D trials, found no effect in pneumonia (Allergy).

Each of the big respiratory reviews checked for harm and found vitamin D did not raise the rate of serious adverse events. Modest doses are safe.

Mood. None of the reviews this article draws on looked at mood or depression, so no claim is made here either way. Treat “winter blues” marketing on the bottle as marketing.

How much should you take, and how much is too much?

For adults in Ireland the HSE dose is 15 micrograms a day (HSE). Labels often give the dose in international units, or IU, instead. The NHS conversion is 1 microgram to 40 IU, so 10 micrograms is 400 IU (NHS), and the Irish adult dose of 15 micrograms works out at 600 IU.

Take it every day rather than as an occasional large dose. That is how the HSE advice is written for every age band, and for babies it is explicit: one dose per day, because very large doses can make a baby ill (HSE). The 2017 review’s finding that daily or weekly dosing beat bolus doses did not survive the 2025 update, so do not take that as trial evidence for daily dosing. A small daily tablet is simply the form the public health advice is built around, and the easiest to keep up.

More is not better. The HSE ceilings are clear: adults and children age 11 and older should not take more than 100 micrograms a day, children age 1 to 10 no more than 50 micrograms, and babies under 12 months no more than 25 micrograms (HSE). The NHS gives the same adult ceiling and expresses it as 4,000 IU (NHS). Vitamin D is stored in the body, and the harm from overdoing it over months is a build-up of calcium, called hypercalcaemia, which the NHS says can weaken the bones and damage the kidneys and the heart (NHS). High-dose “winter strength” products are not a shortcut to anything the trials measured.

Should you get your vitamin D level tested first, and what are the signs of deficiency?

For a healthy adult taking the HSE dose, no. Neither the HSE nor the NHS page asks you to be tested before starting, and 15 micrograms is well under the ceiling, so there is nothing to prove first. The public health approach is to assume everyone in Ireland runs short in winter and to supplement on that basis.

Mild deficiency usually has no symptoms, which is why it is easy to have and why the blanket advice exists. The signs the NHS names are the serious ones: rickets in children and bone pain from osteomalacia in adults (NHS). If you have those, or you are in one of the year-round groups and have never taken a supplement, ask a GP. A GP may prescribe a higher corrective dose for a period, which is a different thing from over-the-counter supplementing and should not be copied without a test.

Tiredness and vague aches are common complaints with many causes, so a low level found on a test is not automatically the explanation for how you feel.

Does it matter which vitamin D you buy in an Irish pharmacy or supermarket?

Not much, provided the label matches the HSE dose for your age. The trials in the respiratory reviews used both forms, vitamin D3 and vitamin D2 (BMJ), and the supermarket own-brand is the same molecule as the branded bottle beside it. Tablets, capsules, sprays and drops all work if you take them.

What to check before buying:

• The dose per day on the label, not the number of tablets in the tub. Ignore anything that pushes you towards the ceiling.
• Whether you actually need the added calcium in a combined product. Most adults do not, unless a doctor has said so.
• For children age 1 to 4, the HSE says liquid drops of a vitamin D-only supplement are usually best, and not to give chewable or tablet-type supplements because they could be a choking risk (HSE).
• For babies, a product made for infants with a measured dropper, given directly into the mouth, once a day (HSE).

A cheap, plain bottle at the HSE dose, taken every day from Halloween to St Patrick’s Day, covers what the evidence supports.