The current recommendations are to avoid strong sunlight and to wear sun cream if exposure to the sun is going to be prolonged. This advice is based on the assumption that exposure to ultraviolet light from the sun is a cause of skin cancer. The current ‘consensus’ surrounding the dangers of the sun have lead to paranoia regarding sunlight exposure, which has lead many to unnecessarily curtain time spent in the sun. Questions are now starting to be raised in the literature as to whether current advice is actually detrimental to health in the long term. The unintended consequences of the current policy stems from the realisation in recent decades that circulating levels the vitamin D that were once believed to be adequate, are now considered inadequate for proper metabolic function. Of course dietary sources of vitamin D are available, but these foods tend to be uncommon even in high quality diets. Therefore sunlight is an important source of vitamin D and limiting sun exposure may be detrimentally affecting health.
Sun exposure is not a problem for those who live in equatorial regions where the angle of incidence is high and the production of vitamin D from the skin possible year round. However, in higher latitudes, when the angle of incidence drops during the winter, vitamin D synthesis has been shown to fall dramatically. Therefore for most of the winter months, many people in higher latitude areas are at risk of vitamin D insufficiency, defined as having levels of the active vitamin D metabolite 25-hydroxyvitamin D of between 25 and 40 nmols/L. Certain subgroups of the population are at increased risk of poor vitamin D status because of genetic or environmental factors. As well as the elderly who tend to spend more time indoors, and the obese who can sequester vitamin D in their adipose tissue, those with dark skin are known to require longer periods in the sun in order to maintain adequate vitamin D status. In fact, studies show that those of Asians and African descent have an increased requirement sunlight exposure.
The problem of inadequate vitamin D synthesis for those with darker skin living in higher latitudes has been investigated by researchers. In this regard, studies have attempted to identify the requirement for ultraviolet light exposure in those with darker skin in order to provide adequate serum 25-hydroxyvitamin D concentrations. In one study, researchers exposed subjects of South Asian origin to six different ultraviolet light treatments in order to assess the changes in serum concentrations of 25-hydroxyvitamin D1. The amount of ultraviolet light exposure ranged from 0.65 to 3.9 standard erythema doses (SEDs), equivalent to 15 to 90 minutes of unshaded daytime summer sunlight at a latitude equivalent to Manchester, England. This routine was followed 3 times a week for 6 weeks while wearing clothes that allowed around 35 % of the skin surface to be exposed. As well as measuring the serum 25-hydroxyvitamin D levels, the researchers also estimated the dietary intake of vitamin D from food diaries.
The results of this study mirrored other studies in that the subjects had poor vitamin D status at baseline. In fact, all subjects were borderline vitamin D insufficient and had serum concentrations of vitamin D below 50 nmol/L with 90 % of subjects having 25-hydroxyvitamin D concentrations of below 25 nmol/L. More worryingly 35 % of the subjects had serum vitamin D concentrations of less than 12.5 nmol/L which would make them vitamin D deficient and at increased risk of osteomalacia and rickets. Exposure to the ultraviolet light was able to improve the serum 25-hydroxyvitamin D concentrations in all subjects which improved to greater than 12.5 nmols/L. Exposure equivalent to 45 minutes of unshaded sunlight was able to raise serum 25-hydroxyvitamin D to almost 40 nmol/L suggesting that this would be the minimum requirement to achieve sufficient vitamin D status. However 45 minutes of sunlight exposure at this strength of sun is not possible year round in Manchester.
Previous work by the same authors have shown that less than 45 minutes sunlight exposure was able to raise serum levels of 25-hydroxyvitamin D above 50 nmol/L in 90 % of white adult subjects. Although this improvement is greater than seen in the subjects of South Asian origin, it is still borderline to insufficiency and therefore not optimal. The fact that 45 minutes sunlight exposure 3 times a week for 6 weeks is not able to improve vitamin D status to optimal (a level considered perhaps as high as 100 nmol/L in most individuals), tends to suggest that dietary sources of vitamin D are required to make up the difference. As dietary sources of vitamin D are limited, two reasonable alternatives remain. Food sources of preformed 25-hydroxyvitamin D are one alternative (here). The other alternative is to take supplements to improve vitamin D status. Currently around 2000 IU of vitamin D3 is recommended over the winter to maintain adequate vitamin D status in those in high latitude regions.
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