Vitamin D in winter: when to test and when to adjust

General information, not medical advice. If you take medication, have kidney disease or are unsure about a supplement, talk to your GP or pharmacist.

Vitamin D in winter comes up every year, because your skin makes it mainly in response to sunlight. This article explains why that barely works during a Belgian winter, what a blood test measures and which limits European experts use.

Why the sun falls short for vitamin D in winter

Your skin makes vitamin D using UVB, the part of sunlight that also causes sunburn. The lower the sun, the less of that radiation reaches the ground. A classic 1988 study by Webb and colleagues measured the effect: in Boston, no vitamin D was formed in the skin from November to February, and in Edmonton, Canada, slightly further north than Belgium, none from October to March. During those months you rely on what your body stored in summer and on your diet.

That does not make going outside in winter pointless. Daylight and movement have their own benefits, as the article on how many steps a day are worthwhile explains, but for vitamin D a winter walk adds little.

What a blood test measures

A doctor checks your vitamin D status by measuring 25-hydroxyvitamin D in your blood, the form in which vitamin D circulates in the body. The European Food Safety Authority (EFSA) uses 50 nanomoles per litre as the target for most people. A test is mainly useful if the result would change what you do, for example if you rarely go outside, always keep your skin covered or already have a condition that weakens your bones. You ask your GP for the test, and they go over the result with you. Guessing a deficiency from how you feel does not work: winter tiredness has many possible causes.

How much vitamin D you need

In its 2016 opinion, EFSA set an adequate intake of 15 micrograms a day for adults, including older adults. That figure assumes little or no production in the skin, which is exactly the winter situation. Labels sometimes list vitamin D in international units (IU); 15 micrograms equals 600 IU.

Getting that amount from ordinary food is hard. Oily fish such as salmon, herring and mackerel, eggs and foods with added vitamin D all help, but rarely enough for the whole winter. That is why many people take a supplement during the dark months. The article on vitamin D in autumn covers what sunlight, food and supplements contribute.

More is not better

Vitamin D builds up in your body, and too much can raise the calcium level in your blood. In 2023, EFSA set the safe upper limit for adults at 100 micrograms a day from all sources combined. If you take a supplement, also count what is in multivitamins or fortified foods, and do not start high doses on your own. If a deficiency is confirmed, your doctor decides the dose and duration, and a follow-up test shows whether it worked.

References

• Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3. Journal of Clinical Endocrinology and Metabolism. 1988;67(2):373-378.
• EFSA Panel on Dietetic Products, Nutrition and Allergies. Dietary reference values for vitamin D. EFSA Journal. 2016;14(10):e04547.
• EFSA Panel on Nutrition, Novel Foods and Food Allergens. Scientific opinion on the tolerable upper intake level for vitamin D, including the derivation of a conversion factor for calcidiol monohydrate. EFSA Journal. 2023;21(8):e08145.