Meeting Vitamin D Requirements in White Caucasians at UK Latitudes: Providing a Choice

Abstract

The body gains vitamin D through both oral intake (diet/supplementation) and synthesis in skin upon exposure to ultraviolet radiation (UVR). Sun exposure is the major source for most people even though sun exposure is complex and limited by climate and culture. We aimed to quantify the sun exposure required to meet vitamin D targets year-round and determine whether this can be safely achieved in a simply defined manner in the UK as an alternative to increasing vitamin D oral intake. Data from observation (sun exposure, diet, and vitamin D status) and UVR intervention studies performed with white Caucasian adults were combined with modeled all-weather UVR climatology. Daily vitamin D effective UVR doses (all-weather) were calculated across the UK based on ten-year climatology for pre-defined lunchtime exposure regimes. Calculations then determined the time necessary to spend outdoors for the body to gain sufficient vitamin D levels for year-round needs without being sunburnt under differing exposure scenarios. Results show that, in specified conditions, white Caucasians across the UK need nine minutes of daily sunlight at lunchtime from March to September for 25(OH)D levels to remain ≥25 nmol/L throughout the winter. This assumes forearms and lower legs are exposed June-August, while in the remaining, cooler months only hands and face need be exposed. Exposing only the hands and face throughout the summer does not meet requirements.

Keywords: climatology; dietary intake; ultraviolet radiation; vitamin D; white Caucasian.

Figure 1

The sequence of analyses that led to the final sunlight exposure calculations for white Caucasians (skin types I–IV). Calculations do not consider any explicit dietary intake of vitamin D, but are based on results from volunteers who did acquire some oral vitamin D. The median was 3.26 (10–90 percentile, 0.91–7.81) µg/day (130.4 IU/day) [11]. Therefore, these small intakes are implicit in the end results.

Figure 2

Calculated UVR doses in SEDs achievable from March-September on a randomly oriented vertical surface based on a 10-year UVR climatology [18] and daily exposures of <1 SED around noon for scenarios S1–S4 (Table 2). S1–S3 show the dose achieved after daily 9 min exposures at noon with 35% of skin area exposed March–September (S1). 10% of skin area in March–May and September, 35% June–August (S2). 10% of skin area throughout March–September (S3). S4 indicates the dose acquired after an exposure for a time equivalent to approximately 1 SED on 35% of skin area between June–August (no exposure in other months). For S4, exposure bands are labelled in minute intervals with the time to achieve 1 SED. While incident solar radiation is independent of skin area exposed, it is only effective for vitamin D synthesis when it falls on exposed skin. Therefore, the incident UVR has been scaled by the skin area for S2 and S3 to give “effective UVR” relative to the baseline skin area (35%) exposed in S1, which has a scaling factor of 1. See Method Section 2.3 for further details. The vertical color gradient key shows the number of indicative SED such that pale blue reflects achievement of the target dose (≥38 SED; S2 and S4), dark blue shows failure to achieve target (S3), and yellow-red shows the target is more than achieved (S1). The data illustrated are equivalent to E in Figure 1 for scenarios S1–S4 while the target dose (38 SED) is C in Table 3.