Vitamin D From Sun and Food vs Supplements

Vitamin D is unusual among nutrients: the body makes it in skin exposed to UVB, and comparatively few foods contain much of it. That combination is why supplement advice for vitamin D depends on where you live and what time of year it is, in a way it does not for other vitamins.

Sunlight, latitude and season

UVB intensity varies with latitude, season and time of day. At higher latitudes there is a period of the year during which sunlight is not sufficiently intense for meaningful synthesis, which is the basis for the seasonal supplementation advice issued by health authorities in the UK and Ireland. The NHS and the HSE both publish guidance of this kind, and it is worth reading the one for where you live rather than generalising from US sources.

Skin pigmentation, coverage by clothing, sunscreen use, age and time spent indoors all affect synthesis. None of this is a reason to seek unprotected sun exposure — skin cancer risk is the reason public health advice frames supplements as the safer route in months when synthesis is limited.

Food sources

The commonly cited dietary sources are oily fish, egg yolks, liver, and foods fortified with vitamin D — which in many countries includes some milks, plant drinks, breakfast cereals and spreads. Fortification levels vary by country and by product, so the label is the only reliable guide.

Cod liver oil is a notable case: it carries vitamin D and vitamin A together, so the panel has more than one figure to read. See our omega-3 guide.

Where supplements fit

Authorities generally frame supplements as a way to cover a shortfall rather than as a replacement for diet, with the seasonal case being the clearest exception. If you are wondering whether you need one, a blood test and a conversation with your doctor answer that question properly; a website cannot.

If you do supplement

Read the dose in a consistent unit — see IU vs mcg — and check the form, since D3 and D2 differ. Our vitamin D comparison filters by strength, form and brand.

References

• NIH Office of Dietary Supplements — Vitamin D (Health Professional Fact Sheet)
• NHS — Vitamin D
• Health Service Executive (Ireland) — Vitamin D