Vitamin D and sunscreen

Sunscreen blocks UVB, and UVB is what makes vitamin D in your skin. So does wearing sunscreen more often cost you vitamin D? In Britain, for half the year, no, because the sun is not making any. In summer it makes less difference than you would think.

How your skin makes vitamin D

When UVB light reaches the skin it turns a form of cholesterol in the outer layer, 7-dehydrocholesterol, into previtamin D3. Over the next few days your body heat turns that into vitamin D3, which passes into the blood. The liver converts it into 25-hydroxyvitamin D, the form a blood test measures, and the kidneys turn that into the active hormone.

Only a narrow band of light does this: wavelengths from about 295 to 315 nanometres, strongest between 295 and 300. That is UVB. UVA runs from 315 to 400 nanometres, makes up at least 95 per cent of the UV in sunlight, and makes no vitamin D at all.

The sun also stops you making too much. Stay in it and the same light breaks previtamin D3 and vitamin D3 down into forms the body does not use. As the NHS puts it, you cannot overdose on vitamin D from sunlight.

Why the sun makes no vitamin D in a British winter

UVB is the part of sunlight the atmosphere filters most. When the sun is low, its light crosses more air and more ozone, and the shortest wavelengths are lost before they reach the ground. In the original experiment, skin and its vitamin D precursor were set out in clear winter sun. At 52 degrees north, London's latitude, nothing was made from October to March.

The Scientific Advisory Committee on Nutrition, which sets the UK's intake advice, says the same: "In the UK, sunlight-induced vitamin D synthesis is only effective between late March/early April and September and not from October onwards throughout the winter months."

UVA does not fall away like that. On a clear December midday in London the UV Index is under 1, less than a tenth of June's. UVA is about a fifth of June's, and stays in our Moderate band for about three hours. So in winter, sunscreen is protecting you from UVA, and it costs you no vitamin D, because there is none to lose.

Where winter vitamin D comes from

Food, or a supplement. The NHS advises everyone over four to consider a daily 10 microgram supplement from October to March. People who are rarely outdoors, who cover their skin, or who have African, African-Caribbean or south Asian backgrounds are advised to take one all year. Do not take more than 100 micrograms a day.

Low vitamin D is common in a British winter, and it is not a women's problem. In the National Diet and Nutrition Survey for 2019 to 2023, 18 per cent of adults aged 19 to 64 were below the deficiency line over the year, and 31 per cent in January to March. Men were marginally more often deficient than women. In the UK Biobank, a study of half a million adults, being male went with higher odds of deficiency.

If you are pregnant

The NHS advises pregnant and breastfeeding women to consider a daily 10 microgram vitamin D supplement between October and March, as everyone should. Free vitamins are available through Healthy Start for those entitled to them. In a study of 99 pregnant women in Northern Ireland, 96 per cent were below 50 nanomoles per litre at 12 and 20 weeks, and their levels were lower than those of women who were not pregnant. Sunscreen is not the reason. The season and the supply are. If you are worried, ask your midwife or GP.

Summer: how much sun, when and which skin

UK guidance is that most people can make enough vitamin D by going out for short periods with forearms, hands or lower legs uncovered. The Manchester studies put numbers on it. In summer, about 13 minutes of midday sun on a third of the skin, a T-shirt and shorts, took 90 per cent of a group of 120 adults to a sufficient level. A regime worked out from that research, 9 minutes a day between 12 and 2pm with a third of the skin exposed from June to August, keeps 97.5 per cent of white adults above the deficiency line all winter, at a fraction of a sunburn dose.

Midday is the time to go out. UVB peaks sharply around noon while UVA runs on through the afternoon, so a short spell at midday gives the most vitamin D for the least UVA.

Where the light lands matters less than how much of it there is. In a trial of 92 people, the rise in vitamin D depended mainly on the UVB dose, not on the area exposed. The Manchester team's own words: "areas are interchangeable so if the face is protected then upper arms or upper chest might be exposed instead." Hands and face alone are not enough at this latitude.

Darker skin needs longer. The same course of summer light that took 90 per cent of white adults to sufficiency took none of a group of South Asian adults there. That is why the NHS advises a supplement all year.

Does sunscreen in summer stop it?

It reduces it. It does not stop it.

Under laboratory lamps, a full 2 milligrams per square centimetre of sunscreen, the thickness the SPF is tested at, can stop the rise in blood vitamin D. In real sun the picture is different. In an Australian trial, 113 adults wore SPF 17 or a placebo cream every day for a summer, and vitamin D rose by the same amount in both groups. In the Nambour trial in Queensland, four and a half years of daily SPF 16 made no difference. On a week's holiday in Tenerife with a UV Index of 9, people who applied SPF 15 properly did not burn, and their vitamin D still rose strongly, by two thirds as much as those who used sunscreen as they pleased and did burn.

There are two reasons. Most people apply a fifth to two fifths of the tested thickness, and the protection falls fast as the layer thins: an SPF 50+ at a typical thickness measured as SPF 21. And the dose of UVB that makes vitamin D is tiny, well under a sunburn's.

The newest and largest trial adds a caution. In the Sun-D trial, 639 Australian adults applied SPF 50+ every morning on days the UV Index was forecast to reach 3, for a year. Their vitamin D ended about 5 nanomoles per litre lower than that of the group who used sunscreen as they chose, and 46 per cent were below Australia's 50 nanomole line, against 37 per cent. The researchers' advice was to keep wearing the sunscreen and consider a supplement.

If you wear a high SPF properly every day in summer, a 10 microgram supplement covers it. In Britain the NHS advises one from October to March anyway.

Behind glass

Window glass filters out UVB and lets UVA and visible light through. Every type of commercial and car glass blocks most UVB, so a sunny desk or a long drive makes no vitamin D. UVA is another matter: car windscreens block about 96 per cent of it, but side windows only 71 per cent on average, and some as little as 44.

In short

• From October to March in Britain the sun makes no vitamin D, so sunscreen costs you none. Take 10 micrograms a day.
• From late March to September a few minutes of midday sun with arms or legs bare makes vitamin D. Your face can stay covered.
• Sunscreen reduces vitamin D production but does not stop it. If you wear a high SPF properly every day, add a supplement.
• If you are pregnant, take the supplement, and ask your midwife if you are worried.
• Behind glass you make no vitamin D, and UVA may still reach you.

Sources

Webb, Kline and Holick 1988; MacLaughlin, Anderson and Holick 1982; Holick et al. 1980; Webb, DeCosta and Holick 1989; Norval, Björn and de Gruijl 2010; SACN, Vitamin D and Health, 2016; NHS, Vitamin D; NHS, Vitamins and supplements in pregnancy; National Diet and Nutrition Survey 2019 to 2023; UK Biobank, BMJ Open 2021; Holmes et al. 2009; Rhodes et al. 2010; Webb et al. 2010; Webb et al. 2018 (two papers); Farrar et al. 2011; Bogh et al. 2011; Webb and Engelsen 2006; Moan et al. 2008; Faurschou et al. 2012; Faurschou and Wulf 2007; Autier et al. 2001; Neale et al. 2002; Marks et al. 1995; Jayaratne et al. 2012; Young et al. 2019; Neale et al. 2019; Passeron et al. 2019; Tran et al. 2025; Tuchinda et al. 2006; Almutawa et al. 2013; Boxer Wachler 2016.