Vitamin D Deficiency in Ireland — Patient Guide

Why is vitamin D deficiency common in Ireland?

Ireland sits at 51–55° north latitude — far enough from the equator that for roughly half the year, sunlight is too weak (the UVB component does not have enough intensity) for the skin to synthesise vitamin D. Per FSAI and HSE data, around 1 in 5 Irish adults have low vitamin D levels, with significantly higher rates in winter, in darker-skinned populations, in older adults, and in those who cover their skin or spend little time outdoors.

What does vitamin D actually do?

Vitamin D is essential for calcium absorption and bone health. Severe deficiency causes rickets in children and osteomalacia in adults. It is also involved in immune function, muscle strength, and possibly mood. Strongest evidence is for bone-health benefits — claims of dramatic immune or mental-health effects have been more variable in trials than in early observational studies.

Who is at higher risk?

Per NICE CKS and FSAI:

• People with darker skin pigmentation living in Ireland
• Older adults — especially those living indoors or in care
• People who cover most of their skin when outdoors
• Pregnant and breastfeeding women
• Infants and young children (specific dosing guidance)
• Obese individuals (vitamin D is fat-soluble and sequestered)
• People with malabsorption conditions (coeliac disease, IBD, post-bariatric surgery)
• Those on certain medications affecting vitamin D metabolism (anticonvulsants, glucocorticoids)

Recommended supplementation

Per FSAI 2023 recommendations and HSE National Maternity Guidelines:

• Infants 0–12 months: 5 µg (200 IU) daily, regardless of feeding method
• Children 1–4 years: 5 µg daily October to March (and year-round in at-risk groups)
• Adults under 65, low-risk: 10 µg daily October to March
• Adults over 65 OR at-risk groups: 15 µg daily year-round
• Pregnancy and breastfeeding: 10 µg daily year-round

Higher doses (e.g. 25–50 µg / 1000–2000 IU) are commonly recommended by clinicians and are well-tolerated. Doses above 100 µg / 4000 IU daily long-term should be discussed with a doctor.

Testing — when is it actually useful?

Routine population-level testing of vitamin D is not recommended per NICE and ICGP — because most people in Ireland are presumed-low in winter and the recommendation is to supplement regardless. Testing is appropriate for:

• Symptomatic patients (bone pain, muscle weakness)
• Suspected severe deficiency or rickets/osteomalacia
• Conditions where treatment depends on level (chronic kidney disease, hyperparathyroidism, malabsorption)
• Follow-up of confirmed deficiency on treatment

Levels are usually measured as serum 25-hydroxyvitamin D. Thresholds vary slightly between guidelines but commonly: deficient <30 nmol/L, insufficient 30–50 nmol/L, sufficient 50–125 nmol/L.

What if I am deficient?

Confirmed deficiency typically warrants a loading regimen (high doses over 6–10 weeks to replete stores) followed by ongoing maintenance dosing. An Online Doctor can review test results and prescribe an appropriate regimen.

Food sources

Vitamin D content in food is modest — supplementation matters more than dietary changes for most people. Notable sources: oily fish (salmon, mackerel, sardines), egg yolks, fortified milk, fortified breakfast cereals, and certain mushrooms exposed to UV.

Online Doctor consultation

An Online Doctor can: advise on appropriate supplementation for your situation, order or review vitamin D testing where indicated, prescribe loading or maintenance regimens for confirmed deficiency, and integrate vitamin D into broader bone-health and chronic-disease care.