Nature

Sunlight and vitamin D: the balance with your skin

From about late March or early April to the end of September, most people at UK latitudes make the vitamin D they need from short spells outdoors with forearms, hands or lower legs uncovered, and staying out until the skin reddens adds nothing. Between October and early March the sun is too weak for the skin to make enough, so the NHS advises everyone to consider 10 micrograms (400 IU) a day, and some groups all year round. The UV index marks the other side of the balance: from 3 and above the WHO recommends shade, clothing and sunscreen, and it attributes about 1.2 million non-melanoma skin cancers and 325,000 melanomas in 2020 to too much UV.

What the skin makes, and when

Vitamin D is made in the skin when UVB reaches it, and the body uses it to regulate calcium and phosphate, the 2 minerals bones, teeth and muscles depend on. A lasting shortage shows up as rickets in children and as bone pain from osteomalacia in adults. UVB also burns, and at UK latitudes it reaches the ground in useful amounts for only part of the year: the NHS puts the window from about late March or early April to the end of September, and NICE, in its 2016 guideline, gives March to October with the peak in late June.

NICE states that UK sunlight is strongest between 11am and 3pm, when the skin makes vitamin D fastest and sunburn is most likely; before 11am and after 3pm it takes longer, and the burn risk is lower. Cloud reduces UV without removing it, and snow, sand, concrete and water reflect it into the shade. Window glass does not remove the risk, it changes the mix: the vitamin D-producing UVB does not pass through, so a desk by the window makes none, while UVA does and damages skin over long periods. Between October and early March the sun sits too low, whatever the weather.

How much sun is enough

No one can give a fixed number of minutes: latitude, month, hour, cloud, altitude and skin type all change the answer. NICE gives the practical version. Most people can make sufficient vitamin D by going out for short periods and leaving only areas of skin that are often exposed uncovered, such as forearms, hands or lower legs, and longer periods may be needed for those with darker skin. Skin that is rarely exposed, the back, abdomen and shoulders, is particularly likely to burn.

NICE splits it by skin type. People with very light skin, or fair or red hair and freckles (types I and II), need little time in the sun and burn after short exposure. People with genetically darker skin (types V and VI) burn less easily but run a higher risk of deficiency in the UK, and may need more time to make the same amount. Staying out longer helps nobody: NICE says burning or tanning is unlikely to add benefit, and a tan gives little protection against later damage. Short and regular, never pink.

Blood tests measure 25-hydroxyvitamin D. EFSA's target is 50 nmol/L (20 ng/mL) under conditions of minimal skin synthesis, with an adequate intake of 15 micrograms a day to reach it without sun. In the VITAL trial, the 15,787 of 25,871 US adults with a usable baseline sample averaged 30.8 ng/mL, and 12.7% sat below 20 ng/mL.

The UV index and when to protect the skin

The UV index, developed by the WHO with the UN Environment Programme, the World Meteorological Organization and ICNIRP, is in most weather forecasts. Sun protection is recommended from 3 upwards. From 3 to 7 the WHO advises shade at midday plus a shirt, sunscreen and a hat; at 8 and above, stay out of the midday sun. The daily maximum falls in the 4 hours around solar noon, which depending on location and summer time lands between 12 and 2pm.

The WHO lists 7 measures: limit midday sun, seek shade, cover up, a broad-brimmed hat, wraparound sunglasses with 99 to 100% UVA and UVB protection, broad-spectrum sunscreen where clothes cannot cover, and no sunbeds. Shade and clothing beat sunscreen, which should never be used to stretch time in the sun. NICE advises at least SPF 30 with 4-star UVA protection, and warns that an average adult body needs about 35 ml, 6 to 8 teaspoons, for the protection on the label, that most people apply far less, and that it must go on again after water, towelling or sweat. Keep children under 6 months out of direct strong sunlight.

The WHO figures explain the care: over 1.5 million skin cancers were diagnosed worldwide in 2020 and over 120,000 deaths recorded, with about 1.2 million non-melanoma cancers and 325,000 melanomas attributed to excessive UV. NICE puts the order plainly: unless your skin type is very dark, protect your skin when out in strong sunlight for more than a short period.

Who should take a supplement in winter

The NHS advice, from the Department of Health and Social Care, is that everyone should consider a daily supplement of 10 micrograms (400 IU) from October to early March, because the sun is too weak and food alone rarely covers the need. Oily fish, red meat, egg yolks, liver and fortified spreads and cereals are the main food sources, and UK cows' milk is not fortified. The same 10 micrograms applies to pregnant and breastfeeding women and to children from the age of 1; babies up to 1 year need 8.5 to 10 micrograms.

Some people should take 10 micrograms all year, because the summer sun does not reach them: people who are not often outdoors, the frail or housebound, residents of care homes, people who usually cover most of their skin outdoors, and people with dark skin from African, African-Caribbean or south Asian backgrounds, whose skin needs longer to make the same amount. All children aged 1 to 4 should take a supplement throughout the year, and so should babies, unless they drink more than 500 ml of infant formula a day, which is already fortified.

EFSA's figures for the EU point the same way: an adequate intake of 15 micrograms a day for adults, and in 2023 a tolerable upper level of 100 micrograms a day from 11 years, pregnancy and lactation included, and 50 micrograms for children aged 1 to 10, from a review of 34 studies in which persistent calcium in the urine, the earliest warning sign, appeared at 250 micrograms a day, divided by a safety factor of 2.5. The NHS ceilings match, down to 25 micrograms for infants under 12 months. The winter dose is a tenth of the adult limit. National figures differ, so follow your own country's advice.

What a supplement can do, and when to talk to a doctor

The winter tablet is for bone and muscle health, and the evidence for larger claims is thin. The VITAL trial, in the New England Journal of Medicine in 2019, randomised 25,871 US adults, men from 50 and women from 55, to 2,000 IU (50 micrograms) of vitamin D3 a day or placebo for a median of 5.3 years. There were 1,617 invasive cancers, 793 in the vitamin D group and 824 on placebo, a hazard ratio of 0.96 (95% confidence interval 0.88 to 1.06), and 805 major cardiovascular events, 396 against 409, hazard ratio 0.97 (0.85 to 1.12). Supplementation reduced neither.

Too much over a long period does harm: the NHS warns that excess vitamin D lets calcium build up in the body, which can weaken bones and damage the kidneys and the heart. Ask a doctor before taking more than 10 micrograms if you have kidney disease, a history of kidney stones or raised calcium, or take medicines that affect calcium, such as thiazide diuretics. In pregnancy 10 micrograms is the NHS figure, and liver is the food source to avoid. Under 18s follow the age limits in the table, and a baby's supplement is a matter for the health visitor or GP.

Watch the skin too. NICE asks all adults to check for a new mole, growth or lump, or any mole, freckle or patch that changes in size, shape or colour, and to tell a doctor about unusual or persistent changes. If you burn easily, have many moles, are immunosuppressed or have a family history of skin cancer, the short-spell rule applies with less room for error. Bone pain, muscle weakness or months without daylight are reasons for a blood test. Our morning light guide covers the other reason to be outside early, while the UV index is low.

A sun routine that keeps both sides of the balance

Frequently asked

Can I make vitamin D through a window?

No. NICE states that the vitamin D-producing UVB does not penetrate glass, while UVA does and over long periods damages the skin. Time by a sunny window is pleasant, but for vitamin D you need to be outside between about April and September.

Does sunscreen stop vitamin D production?

Sunscreen blocks UVB, which is its purpose, but the guidance does not ask you to choose. NICE advises a short spell with forearms, hands or lower legs uncovered, then protection when you stay in strong sun longer. Applying less than the 35 ml an adult body needs is not a way to top up vitamin D: thin application only cuts the protection you get.

How many minutes of sun do I need for vitamin D?

No guideline gives a number, because skin type, latitude, month, hour, cloud and altitude all change the answer. NICE describes short periods with commonly exposed skin uncovered, such as forearms, hands or lower legs, longer for darker skin, and never long enough to burn or tan, which it says is unlikely to provide additional benefit.

Who should take vitamin D in winter?

In the UK the NHS advises everyone to consider 10 micrograms (400 IU) a day from October to early March. People who are rarely outdoors, live in care homes, cover most of their skin or have dark skin should take it all year, as should children aged 1 to 4 and babies not drinking more than 500 ml of formula a day; EFSA's adequate intake for adults is 15 micrograms.

Will a vitamin D supplement prevent cancer or heart disease?

The largest trial says no. In VITAL, 25,871 US adults took 2,000 IU a day or placebo for a median of 5.3 years, and the hazard ratios were 0.96 for invasive cancer and 0.97 for major cardiovascular events, neither different from placebo in a meaningful way. The reason to take 10 micrograms in winter is bone and muscle health.

Is a tan a sign that I have enough vitamin D?

No. NICE states that a tan offers little protection against further skin damage, that there is no safe or healthy way to get a tan from sunlight, and that exposure long enough to tan or burn is unlikely to add vitamin D. The WHO attributes about 1.2 million non-melanoma skin cancers and 325,000 melanomas in 2020 to excessive UV.

Sources

• NHS: Vitamin D (Vitamins and minerals) · 10 micrograms a day; sunlight sufficient from late March or early April to the end of September, not from October to early March; groups advised to supplement all year; upper limits 100, 50 and 25 micrograms by age; hypercalcaemia warning; food sources
• NICE guideline NG34 (2016): Sunlight exposure, risks and benefits, supporting information for practitioners · Published 9 February 2016; UVB from March to October, strongest 11am to 3pm, peak late June; short periods with forearms, hands or lower legs uncovered, longer for darker skin; skin types I and II burn fastest, types V and VI risk deficiency; SPF 30 and 4-star UVA; 35 ml or 6 to 8 teaspoons; under 6 months out of strong sun; UVB does not pass glass; a tan gives little protection; skin checks
• WHO fact sheet: Ultraviolet radiation · Over 1.5 million skin cancers and over 120,000 deaths in 2020; about 1.2 million non-melanoma and 325,000 melanoma cases attributed to excessive UV; protection recommended from UV index 3; 7 protective measures; 15 million blind from cataracts, some 10% from UV. The bands 0 to 2, 3 to 7 and 8 and above, the 4-hour maximum around solar noon and solar noon falling between 12 and 2pm are from the WHO UV index questions and answers page linked from the fact sheet
• EFSA 2023, EFSA Journal (full text at PubMed Central): Scientific opinion on the tolerable upper intake level for vitamin D · UL of 100 micrograms a day from 11 years including pregnancy and lactation, 50 micrograms for ages 1 to 10; infant ULs retained from the 2018 opinion, 25 micrograms up to 6 months and 35 micrograms for 6 to 12 months; 34 studies reviewed for hypercalcaemia and hypercalciuria; persistent hypercalciuria as critical endpoint, lowest observed adverse effect level 250 micrograms a day with an uncertainty factor of 2.5; restates the adequate intake of 15 micrograms and the 50 nmol/L target under minimal skin synthesis
• Manson et al. 2019, NEJM (full text at PubMed Central): Vitamin D supplements and prevention of cancer and cardiovascular disease (VITAL) · 25,871 US adults randomised to 2,000 IU vitamin D3 a day or placebo, men 50 and over, women 55 and over; median 5.3 years; invasive cancer HR 0.96 (0.88 to 1.06), 793 against 824; major cardiovascular events HR 0.97 (0.85 to 1.12), 396 against 409; baseline mean 25(OH)D 30.8 ng/mL with 12.7% below 20 ng/mL among the 15,787 participants with analysable baseline samples

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This guide is for general information. It does not replace medical advice or an examination.