Burning daylight: balancing vitamin D requirements with sensible sun exposure

Vitamin D3 (cholecalciferol) is a provitamin obtained through exposure of the skin to sunlight and from dietary sources. Vitamin D is essential for calcium homeostasis and bone health, and deficiency has been associated with several chronic diseases.1 Vitamin D requires metabolism in the liver to 25-hydroxyvitamin D (25[OH]D),2 the major circulating form and indicator of vitamin D status.3

There is debate about what serum level of 25(OH)D constitutes deficiency or sufficiency. While deficiency can be regarded as a 25(OH)D level below 50 nmol/L (< 20 ng/mL),4 a recent Institute of Medicine report suggested a level of 50 nmol/L represents adequacy,5 and that further randomised trials are needed before a higher level can be recommended. However, it has elsewhere been suggested that a level of 75 nmol/L (30 ng/mL) should represent sufficiency.6 To maintain a 25(OH)D level of 75 nmol/L, a daily intake or synthesis of at least 1000 IU (25 μg) of vitamin D is required.6

Exposure of skin to ultraviolet B (UVB) radiation in sunlight is the body’s principal vitamin D source.7 UVB penetrates outer layers of skin, causing temporary redness (erythema). UVA radiation penetrates deeper layers of the skin, causing photoageing8 and increased risk of cutaneous malignant melanoma.9 Although UVB is lower early and late in the day, UVA may be relatively high at these times.10 The Ultraviolet Index (UVI) measures ultraviolet radiation (UVR) intensity, and ranges from 1 to about 14 in Australia (higher towards lower latitudes in the north, and decreasing towards higher latitudes in the south).

Melanoma incidence is associated with higher UVI, lower latitude11 and history of sunburn.12 Australian skin cancer prevention campaigns advise sun protection (shade, protective clothing, broad-brimmed hats, sunglasses, sunscreen) during “peak UVI periods”, typically promoted as between 10 am and 3 pm, or when the UVI reaches 3.4,13,14 Conversely, bone health experts recommend short periods of unprotected exposure of the face, arms and hands to sunlight three to six times per week to prevent vitamin D deficiency.15,16 Adherence to protective guidelines without compensatory vitamin D intake increases the risk of deficiency, as clothing,17 shade18 and sunscreen19 reduce vitamin D synthesis. Although evidence is limited, high rates of vitamin D deficiency have been reported in Australians.20,21 Vitamin D deficiency is more prevalent in winter, and insufficiency is common in summer and autumn, suggesting that regular sunlight exposure or intake is required year-round.

A practical tool enabling individuals to maintain adequate vitamin D status that also considers the deleterious effects of UVR is warranted. Here, we aim to identify sunlight exposure duration required for fair-skinned individuals in Australian cities to generate 1000 IU of vitamin D throughout the year, and to determine the feasibility of achieving this within 30 min at times when sun protection is not required.

The cumulative or standard erythemal dose (SED) of UVR determines the magnitude of vitamin D synthesis. Minimal erythemal dose (MED) is the measure of cumulative UVR exposure that causes erythema, and is equal to about 2 SED. Whole-body exposure to 1 MED is equivalent to an oral dose of about 20 000 IU of vitamin D in fair-skinned individuals.22

Data on hourly UVI and SED between 1 June 1996 and 30 December 2005 were obtained from the Australian Radiation Protection and Nuclear Safety Agency23 for seven cities with data loggers — Darwin (12.4°S), Townsville (19.3°S), Brisbane (27.5°S), Perth (31.9°S), Sydney (34.0°S), Adelaide (34.9°S) and Melbourne (37.7°S). Data from Tasmania were unavailable for the study period. When SED was below 0.005, a value of zero was given. Missing values, indicating data monitor failure, were excluded from calculations.

The Lund–Browder burn diagram24 was used to estimate skin surface exposure percentages (Box 1). The face was not included, as sun protection of the eyes and face is always recommended.14 Estimated body surface areas for children and adolescents did not differ greatly from adults.

Current guidelines suggest exposure of 15% of the body to one-third of an MED to produce 1000 IU of vitamin D, four to five times a week.13,16 Excluding the face equates to 11% body exposure, which would reportedly produce 733 IU of vitamin D.22 We calculated that when hands, arms and neck on one side of the body (11% of the body) are exposed to the sun, a dose of 0.455 MED is required for fair-skinned individuals to achieve 1000 IU of vitamin D, whereas including one side of the lower legs (17% of the body) decreases the required dose to 0.294 MED (Box 1). We then determined the duration of sun exposure required to achieve each of these MEDs for each hour of daylight between 6 am and 5 pm, by season and city.

Average monthly SEDs for each hour of daylight during January, April, July and October were used to represent summer, autumn, winter and spring, respectively. Periods when the average UVI (UVIav) was < 3 were recorded in tabular format as green if synthesis of 1000 IU could be achieved in ≤ 30 min, and blue if it could be achieved in 31–60 min. Periods when the UVIav was ≥ 3 were coded red to reflect sun protection guidelines. Periods when it was impossible to achieve 1000 IU of vitamin D synthesis within 1 hour were recorded as grey.

Our data showed it was possible for fair-skinned individuals to obtain 1000 IU of vitamin D within 30 min of exposure of 11% (Box 2) or 17% (Box 3) of the skin to the sun, but there were few occasions when this could be achieved at times not excluded by sun protection guidelines. More opportunities to achieve synthesis of 1000 IU in ≤ 30 min occurred, and required duration was shorter, when 17% of the body was exposed to the sun, relative to 11%, as seen by the greater number of green cells in Box 3 relative to Box 2.

UVIav often reached 3 outside the publicised peak UVI period of 10 am–3 pm (Box 2, Box 3). In summer, the UVIav was ≥ 3 between 8 am and 5 pm in all cities except Townsville and Sydney (8 am–4 pm) and Darwin (9 am–5 pm). In winter in Darwin, Townsville, and Brisbane, the UVIav was ≥ 3 from 10 am until 4 pm, 3 pm and 2 pm, respectively. In contrast, in winter in Sydney, Adelaide, and Melbourne, the UVIav remained < 3 throughout the day, allowing exposure without protection during the warmer parts of the day.

UVIav and duration of sun exposure required to achieve 1000 IU of vitamin D synthesis varied according to time of day, season and city (Box 2, Box 3). Opportunities when this could be achieved within 30 min when the UVI was < 3 were limited.

With 11% body exposure, there were no opportunities in summer and autumn to achieve 1000 IU of vitamin D synthesis in ≤ 30 min except Townsville in summer, whereas during winter and spring there were some opportunities in most cities (Box 2).

When lower legs were included in exposure, 1000 IU synthesis could be achieved within 30 min on at least one occasion each day for almost all seasons (with the exceptions of Adelaide, Perth and Melbourne in summer; Townsville in spring; and Darwin and Sydney in autumn) (Box 3). As the UVI was < 3 during the middle part of the day in winter in Perth (except at midday), Sydney, Adelaide and Melbourne, sun exposure of ≤ 30 min during the middle of the day could achieve 1000 IU of vitamin D synthesis.

We found that duration of sunlight exposure required for fair-skinned individuals to achieve 1000 IU of vitamin D synthesis varies according to time of day, season and city.

Overall, when 11% of the body is exposed to the sun, there are few opportunities to synthesise 1000 IU of vitamin D within 30 min at times of the day when sun exposure without protection is not discouraged. In nearly all cities during summer and autumn, there are no opportunities to achieve this, whereas there are more opportunities during winter and spring, particularly in southern cities such as Sydney and Melbourne. For exposure durations of 31–60 min, most cities have at least one opportunity each day to synthesise 1000 IU; however, as these periods mostly occur early and late in the day, we advise caution due to the potentially damaging effects of UVA light at these times.10 It may be preferable to instead increase the amount of skin exposed to the sun to reduce the required duration.

We consider 30 min as the maximum duration that individuals could feasibly spend in the sun each day on a regular basis. Formation of pre-vitamin D plateaus in fair-skinned individuals within 15–45 min of exposure to UVR, after which further exposure causes the breakdown of pre-vitamin D into biologically inactive lumisterol and tachysterol.2

Our findings extend previous guidelines,1,16 which suggest that the majority of fair-skinned individuals in Australia can achieve vitamin D requirements at 10 am or 3 pm, and individuals with no other risk factors for skin cancer can achieve their requirements at 12 noon. However, our findings suggest that UVIav is often ≥ 3 at these times, and the risk of overexposure is therefore higher. We found that UVIav often reached 3 outside commonly promoted peak UVI periods, indicating sun protection behaviour should extend beyond these times.

Our UVI data cover a period of 10 years, and therefore are likely to be robust and based on “typical” cloud cover, although daily variation in cloud cover and pollution levels will alter the required exposure durations. As reported previously,25 we found that the UVIav remains below 3 throughout the day in Sydney, Adelaide and Melbourne during winter, suggesting that there may be a wider safety margin to allow sufficient sunlight exposure to achieve adequate vitamin D synthesis in winter in these cities.

Our recommendations take into account Australian guidelines that suggest sun protection is not required when the UVI is below 3.4 Our suggested doses of 0.455 and 0.294 MED are unlikely to cause sunburn, as 1 MED produces only a faint pinkness in fair-skinned individuals.26 We recommend individuals check real-time UVI data,27 as the actual level may vary from the long-term average.

A limitation of our study is our inability to take into account physiological factors that contribute to vitamin D deficiency, such as increased age,28 increased skin pigmentation,29 fat malabsorption,30 and obesity,31 and as such our recommendations are likely to be an underestimate for individuals affected by these factors.

It could be argued that the recommendation of avoiding sunlight exposure when the UVI is ≥ 3 could be more nuanced, as skin is damaged relative to the total exposure, which is dependent on both UVR intensity and duration of exposure. Although brief exposure when the UVI is ≥ 3 could be identical to longer exposure during periods of low UVI in terms of both vitamin D synthesis and skin damage, exposure during high UVI periods requires careful timing to avoid overexposure. Additionally, the achievements made in educating the public about the dangers of high UVI could be lost if alternative recommendations were to be made.

Australians should be aware of methods to obtain sufficient vitamin D while minimising skin damage. Given the difficulty of achieving adequate vitamin D synthesis in some individuals, increasing oral intake with daily supplementation or consuming food naturally containing or fortified with vitamin D may be required.

2 Duration (min) required to achieve synthesis of 1000 IU of vitamin D with one side of the hands, arms and neck (11% of the body) exposed to the sun (0.455 MED), by city, season and time of day*

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3 Duration (min) required to achieve synthesis of 1000 IU of vitamin D with one side of the hands, arms, neck and lower legs (17% of the body) exposed to the sun (0.294 MED), by city, season and time of day*

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