Vitamin D in the UK: winter advice, testing and forms

Why the UK is a special case

Most vitamin D comes from sunlight on skin, and at British latitudes the sunlight that does it is simply absent for months.

• The UK sits between roughly 50 and 60 degrees north. Between about October and March, UVB is too weak at these latitudes for the skin to make meaningful vitamin D.
• That is why UK advice is seasonal in a way that advice written for southern Europe or the southern United States is not.
• Skin pigmentation, covering clothing, shift work and staying indoors all reduce production further, and those effects persist through the summer months.
• Blood 25-hydroxyvitamin D falls through winter and recovers in summer, which is why the time of year matters when interpreting a test result.

What UK bodies actually advise

This is reported here as published public-health advice. It is not personal guidance, and it is not a recommendation from DoseRoutine.

• Following the Scientific Advisory Committee on Nutrition's 2016 review, the UK set a reference nutrient intake of 10 micrograms (400 international units) of vitamin D a day for everyone aged four and over.
• NHS advice is that adults and children over four should consider a daily supplement containing 10 micrograms during autumn and winter.
• Year-round supplementation is advised for people who get little or no sun exposure, and for people with dark skin from African, African-Caribbean or South Asian backgrounds.
• Separate advice covers babies, young children and pregnancy, and the Healthy Start scheme provides free vitamins to eligible families.
• The NHS also states an upper amount adults should not exceed from supplements. Where a blood test shows deficiency, treatment doses are a clinical decision, not a shelf decision.

Testing and what the numbers mean

A vitamin D test measures 25-hydroxyvitamin D in the blood, usually reported in nanomoles per litre in the UK and nanograms per millilitre in the US — the same result, two scales.

• UK thresholds derive from the SACN review and define deficiency at a level below which risk to bone health rises; laboratories report against those reference ranges.
• Convert carefully: nanograms per millilitre multiplied by 2.5 gives nanomoles per litre, which is where most cross-border confusion starts.
• Season changes the result. A test in February and a test in August on the same person are not comparable.
• GPs test where there is a clinical reason rather than routinely; private tests are widely available but interpretation still belongs with a clinician.

Forms, food and what to record

The practical questions are usually about which product, and whether it is working.

• Vitamin D3 (cholecalciferol) and D2 (ergocalciferol) both appear on UK shelves; D3 is the more common supplement form and vegan D3 from lichen is available.
• Drops, sprays, capsules and tablets all count; UK labels state the amount in micrograms, with international units often shown alongside.
• Food sources in the UK diet are limited — oily fish, egg yolks and fortified products — which is the reason supplementation is advised at all.
• Vitamin D is fat soluble, so it accumulates. Taking several products that each contain it is the usual way people end up well above what they intended.
• Record the product, the amount per dose and the date of each blood test, so the next result can be read against what you were actually taking.

References

• SACN — Vitamin D and Health report (2016)
• NHS — Vitamin D
• NICE Clinical Knowledge Summaries — vitamin D deficiency in adults
• NHS Healthy Start — free vitamins scheme
• EFSA — dietary reference values and tolerable upper intake for vitamin D