Winter daylight through a window beside a vitamin D supplement

Vitamin D in winter

From late autumn to early spring, the sun over much of Europe is too low for skin to make much vitamin D.

Why winter changes vitamin D

Skin makes vitamin D3 when ultraviolet B reaches it. In the Journal of Clinical Endocrinology and Metabolism in 1988, Webb, Kline and Holick showed that at higher latitudes the winter sun is too low for that production to amount to much, even at midday. From late autumn to early spring, much of Europe is in that position. Food and food supplements are then the practical sources.

The EU nutrient reference value used on labels is 5 micrograms (200 IU) of vitamin D a day. EFSA’s 2016 dietary reference value for adults is higher, 15 micrograms under conditions of minimal cutaneous synthesis. Those two numbers answer different questions. The label percentage uses 5 micrograms. The dietary reference value is not itself a product claim.

Micrograms and IU

Labels may show vitamin D in micrograms or in international units. One microgram is 40 IU. A portion of 15 micrograms is 600 IU. A portion of 25 micrograms is 1,000 IU. Converting the units is the only way to compare two bottles that chose different conventions.

Vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) are both used in supplements. D3 is the form skin makes. The authorised claims refer to vitamin D, not to a brand of either form. The ingredient list tells you which molecule is in the capsule.

Authorised claims and the 15% line

A daily portion must provide at least 15% of the nutrient reference value before an authorised claim is allowed. Fifteen percent of 5 micrograms is 0.75 micrograms (30 IU). Almost every vitamin D supplement sold as such clears that line. The claim is still not a licence to invent a dose.

Where the condition is met, Regulation (EU) No 432/2012 allows wording of this kind: vitamin D contributes to the normal absorption and utilisation of calcium and phosphorus; to normal blood calcium levels; to the maintenance of normal bones; to the maintenance of normal muscle function; to the maintenance of normal teeth; to the normal function of the immune system; and it has a role in the process of cell division. A separate claim, with its own conditions, says vitamin D is needed for normal growth and development of bone in children.

The immune-system sentence means a normal function of the immune system. It does not mean that a capsule stops infections, changes an autoimmune condition, or replaces vaccination. EFSA’s opinions on vitamin D, including the 2016 dietary reference value (EFSA Journal 2016;14(10):4547), discuss the nutrient. They are not a product advertisement.

How to read a bottle in winter

Check the micrograms per daily portion, convert IU if you need to, and keep the label’s dose. Directive 2002/46/EC requires a warning not to exceed the stated recommended daily dose. Vitamin D is fat-soluble. Taking several products that each contain it, plus a multivitamin, can stack the same nutrient without the front of any one bottle showing the total.

The vitamin category on NovaLixir is where these labels sit next to each other. Food supplements do not replace a varied diet or a healthy lifestyle. People with a medical condition that affects calcium, including granulomatous disease or unexplained high blood calcium, need individual medical advice rather than a general winter note.

Questions

Does the winter sun in Germany or Hungary still make vitamin D?

At these latitudes, cutaneous production falls sharply in winter because UVB is weak. Webb, Kline and Holick described that seasonal and latitude effect in 1988. Diet and supplements are the remaining sources in those months.

Is 15 micrograms the same as the 5 microgram label reference?

No. Five micrograms is the nutrient reference value used to calculate the % on an EU label. Fifteen micrograms is EFSA’s 2016 dietary reference value for adults when the skin is making little vitamin D. A label percentage is based on 5 micrograms.

May a product say it stops colds or flu?

No. The authorised immune claim is that vitamin D contributes to the normal function of the immune system, and only when the portion meets the 15% rule. A statement about an infection is not an authorised claim.

Can I return an opened vitamin D bottle?

Once the seal of a food supplement is broken, the product is excluded from the right of withdrawal for health and hygiene reasons. Only an unopened product can be returned.

Sources

Food supplements do not replace a varied diet and a healthy lifestyle.

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