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Vitamin D in winter: who needs a supplement in Belgium?

Vitamin D in winter in Belgium: why your skin makes so little, who needs a supplement, which dose, when to test and how to avoid an overdose.

17 October 20266 min readDr. Selim Zenginvitamine-d · hiver · supplement

Why your skin makes so little vitamin D in winter

Vitamin D helps the body use calcium: it is essential for bones, teeth and muscles. The skin makes it thanks to the sun's UVB rays; food provides only a small part.

In Brussels, as everywhere in Belgium, the sun stays low from October to early March and most of its UVB rays are filtered out by the atmosphere. In the United Kingdom, at a similar latitude, the health authorities state that during this period the sun is too weak for the skin to make vitamin D: production almost stops. Clouds, air pollution, covering clothes and high-factor sunscreen reduce it even further. Summer stores gradually run out, so deficiency is common at the end of winter and in early spring. The sun itself does not cause an overdose; still, protect your skin during long periods in the sun.

Belgian recommendations

Recommended intakes are set by the Superior Health Council (SHC) in its advisory report no. 9285 of September 2016, still cited in 2026 by the Belgian Centre for Pharmacotherapeutic Information (BCFI/CBIP).

  • Children up to 10 years: 10 µg (400 IU) a day.
  • Adolescents and adults up to 70 years: 10 to 15 µg (400 to 600 IU) a day.
  • After 70, during pregnancy and breastfeeding: 20 µg (800 IU) a day.

Food provides a European adult with only about 5 µg a day on average. For a healthy adult who goes outdoors regularly, opinions differ: the United Kingdom advises everyone to consider 10 µg a day in autumn and winter, while the BCFI/CBIP considers that, so far, there is no argument for routine supplementation outside the risk groups.

Who should take a supplement?

  • Babies and young children: ONE and Kind en Gezin recommend 400 IU (10 µg) a day for all children up to the age of 6, from birth and all year round according to Kind en Gezin. Breast milk and infant formula usually do not provide enough. If there is a risk factor such as dark skin, Kind en Gezin recommends 600 IU a day, possibly up to the age of 18. For a premature baby, the paediatrician sets the dose.
  • Older children and adolescents at risk: dark skin, little sun, treatment with certain epilepsy medicines.
  • Pregnant and breastfeeding women: 20 µg (800 IU) a day according to the SHC. Deficiency is more common with dark skin or in women who wear a veil.
  • Older people, especially in nursing homes, where deficiency is very common: there, 800 to 2,000 IU a day, with enough calcium, can reduce the risk of fractures due to osteoporosis.
  • Dark skin, covered skin, rarely outdoors: people of African, Caribbean or South Asian origin; people who cover most of their skin outdoors; frail or housebound people.
  • Certain diseases: malabsorption (chronic inflammatory bowel disease, coeliac disease, obesity surgery, cystic fibrosis), cholestasis, osteoporosis. The doctor then adjusts the dose.

The blood test is not useful for everyone

The blood test measures '25-OH vitamin D'. According to the BCFI/CBIP, the value of routine testing has not been established, including in older people, and there is no universally accepted level that protects against fractures.

The rules of the NIHDI (INAMI/RIZIV) provide in principle for one test per calendar year, and up to three a year in certain situations: chronic kidney disease from stage IIIb, dialysis or kidney transplant, documented malabsorption, hyper- or hypoparathyroidism, phosphate diabetes, intravenous bisphosphonates. Your doctor may suggest a test if there are signs of deficiency (bone pain, muscle cramps) or one of these conditions. The blood sample can then be taken at the practice or at home by our nursing team.

Too much vitamin D: a real risk

Vitamin D builds up in the body. At too high a dose, it raises the calcium level in the blood (hypercalcaemia): nausea, vomiting, constipation, intense thirst, frequent urination, tiredness, confusion, and later kidney stones and damage to the kidneys and heart. Unless a doctor advises otherwise, the SHC sets a maximum of 25 µg (1,000 IU) a day up to the age of 10 and 50 µg (2,000 IU) from the age of 11.

Poisonings do occur, often after a mistake. In France, a poison centre recorded 1,255 errors or accidental intakes of vitamin D drops between 2017 and 2020: faulty droppers, a dose given twice, a misunderstood prescription, a bottle drunk by the child. A 3-month-old baby was admitted to hospital after receiving a supplement bought online, dosed per drop rather than per millilitre: 40,000 IU a day instead of 1,200. In older people, highly concentrated solutions can also lead to errors, and high doses (3,000 IU a day or 60,000 IU a month and above) might increase the risk of falls.

  • Check the concentration (per drop or per millilitre) when you change product.
  • Follow the prescribed rhythm (day, week or month).
  • Do not combine several products containing vitamin D without medical advice.
  • Keep bottles out of children's reach.

If a child has swallowed a bottle or tablets, call the Poison Centre on 070 245 245. In case of confusion or serious illness, call 112. Outside our opening hours, the doctor on duty can be reached on 1733.

What large studies show (and do not show)

Observational studies link low vitamin D levels with cardiovascular disease, diabetes or certain cancers, but a low level may be a consequence rather than a cause. Large placebo-controlled trials have not shown a general benefit:

  • VITAL (25,871 participants, 2,000 IU a day for 5 years): no reduction in invasive cancers, cardiovascular events or mortality.
  • D-Health (21,310 people aged 60 to 84, 60,000 IU a month for 5 years): no effect on total, cardiovascular or cancer mortality.

A limitation: in D-Health, 80% of participants already had a sufficient level, and Belgian levels may be lower. These trials therefore do not call into question supplements for risk groups, which aim to prevent a deficiency. But taking vitamin D 'just in case', without a deficiency, to protect yourself against cancer or heart disease is not supported by evidence.

Foods that provide vitamin D

Oily fish (salmon, mackerel, herring, sardines, trout) and egg yolk are the main natural sources. In Belgium, vitamin D is added to some spreads and cooking fats, to some milks, yoghurts and soya products, and to some breakfast cereals.

Talk to your doctor

Ask for advice before taking a supplement if you have kidney disease, kidney stones, sarcoidosis, a parathyroid disorder or a high calcium level, or if you take thiazide diuretics, lithium or calcium: the risk of too much calcium is then higher. Also talk to your doctor in case of pregnancy, epilepsy treatment or use of other supplements.

At Maison Médicale Parc Astrid in Anderlecht, your general practitioner assesses with you whether a supplement is useful, at what dose and in what form, in English, French, Dutch or Turkish. Not yet registered with us? See the new patient page.

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