Vitamin D from sunlight: the UK latitude problem

Vitamin D synthesis in skin requires UV-B radiation (290–315 nm). This sounds simple (get more sun, make more vitamin D), but the practical reality at UK latitudes is more complicated than people assume.

The biology#

UV-B converts 7-dehydrocholesterol in skin to previtamin D3, which thermally isomerises to vitamin D3, which is then hydroxylated in liver and kidney to the active form (1,25-dihydroxyvitamin D). The skin step is the rate-limiting one for most people.

The whole process requires:

• Skin exposed to UV-B
• UV-B actually reaching the skin (not blocked by clothing, glass, or sunscreen)
• Sun angle high enough that UV-B isn't filtered out by atmosphere
• Sufficient exposure duration

The latitude problem#

Above roughly 35° N latitude, the sun's angle in winter is too low for meaningful UV-B to penetrate the atmosphere. The entire UK sits well north of this threshold.

Specifically:

• April to September: sufficient UV-B reaches the UK for skin synthesis
• October to March: insufficient UV-B for synthesis in most of the UK
• Northern Scotland: the window is shorter still

This is why Public Health England recommends 10 µg (400 IU) supplementation for everyone during autumn and winter. See vitamin-d-uk-guidance.

How much summer sun is enough#

In the summer-window months, with arms and legs exposed at midday:

• Light skin: 5–15 minutes 2–3 times per week produces approximately the equivalent of 1,000–3,000 IU oral vitamin D
• Medium skin: 15–30 minutes 2–3 times per week
• Dark skin: longer durations needed; some research suggests 30–60+ minutes

These are rough estimates from skin-type research. Individual variation is large.

What blocks synthesis#

• Glass. Window glass blocks UV-B. Sitting in a sunlit room produces minimal vitamin D.
• Sunscreen. SPF 30 reduces vitamin D synthesis substantially; SPF 50 nearly completely. (For the population, the skin-cancer prevention benefit of sunscreen substantially outweighs the vitamin D synthesis reduction.)
• Clothing. Heavy fabrics block most UV-B.
• Latitude. As discussed.
• Atmospheric pollution. Modest effect.

The skin cancer trade-off#

UV-B is also what causes most skin cancers. The relationship is complex:

• Skin cancer risk rises with cumulative UV exposure
• Burning is particularly damaging
• Dark skin has substantially lower skin cancer rates but also lower vitamin D synthesis at given exposure
• BPF (British Photodermatology Group) and Cancer Research UK guidance balances both: "Little and often", short regular exposures, avoiding burning

Tanning beds#

A persistent claim that tanning beds are a vitamin D source. Caveats:

• Most tanning beds emit primarily UV-A (which tans but doesn't synthesise vitamin D), not UV-B
• WHO classifies tanning beds as Group 1 carcinogens
• The skin cancer risk of regular tanning bed use is substantial
• Vitamin D supplementation achieves the same target serum level at much lower cancer risk

We do not recommend tanning beds for vitamin D.

What we'd actually say#

In the UK summer (April–September), a few short outdoor exposures per week (arms and legs exposed during normal outdoor activities) provides meaningful vitamin D synthesis. Avoiding sunburn matters more than maximising exposure.

In the UK winter (October–March), skin synthesis is minimal regardless of how much time you spend outdoors. Supplementation per Public Health England guidance is the practical approach.

For people with darker skin, supplementation is appropriate year-round in the UK. For housebound elderly adults, supplementation is appropriate year-round.

The "get more sun" approach to vitamin D is reasonable in summer; in winter it doesn't work at our latitude regardless of intent.

When to talk to a clinician#

• Significant family history of skin cancer
• New or changing skin lesions
• Symptoms of vitamin D deficiency (bone pain, muscle weakness)
• Conditions that affect vitamin D metabolism (CKD, malabsorption, on certain anticonvulsants)
• Pregnancy: vitamin D status particularly matters