Physical Determinants of Vitamin D Photosynthesis: A Review

Abstract

Vitamin D synthesis by exposure of skin to solar ultraviolet radiation (UVR) provides the majority of this hormone that is essential for bone development and maintenance but may be important for many other health outcomes. This process, which is the only wellestablished benefit of solar UVR exposure, depends on many factors including genetics, age, health, and behavior. However, the most important factor is the quantity and quality of UVR reaching the skin. Vitamin D synthesis specifically requires ultraviolet B (UVB) radiation that is the minority component (<5%) of solar UVR. This waveband is also the most important for the adverse effects of solar exposure. The most obvious of which is sunburn (erythema), but UVB is also the main cause of DNA damage to the skin that is a prerequisite for most skin cancers. UVB at the Earth's surface depends on many physical and temporal factors such as latitude, altitude, season, and weather. Personal, cultural, and behavioral factors are also important. These include skin melanin, clothing, body surface area exposed, holiday habits, and sunscreen use. There is considerable disagreement in the literature about the role of some of these factors, possibly because some studies have been done by researchers with little understanding of photobiology. It can be argued that vitamin D supplementation obviates the need for solar exposure, but many studies have shown little benefit from this approach for a wide range of health outcomes. There is also increasing evidence that such exposure offers health benefits independently of vitamin D: the most important of which is blood-pressure reduction. In any case, public health advice must optimize risk versus benefit for solar exposure. It is fortunate that the individual UVB doses necessary for maintaining optimal vitamin D status are lower than those for sunburn, irrespective of skin melanin.

100 YEARS OF VITAMIN D: Dose–response for change in 25-hydroxyvitamin D after UV exposure: outcome of a systematic review

“…They find a linear dose–response relation at these doses, as indeed could current authors if only low accumulated dose studies are included. However, the (assumed mean) change in 25OHD in ( 67 ) is much greater than observed in the studies identified for this review, at ~50 nmol/L after 10 SED (5 × 2 SED) of full-body exposure, and does not easily fit our dose–response curve. The closest comparative study in our review was study ( 44 ), also providing five doses from a UV6 lamp over about 2 weeks but with each dose of 0.85 SDD rather than 3.1 SDD (2 SED): study ( 44 ) is identified as the lowest cumulative dose in Fig.…”

“…4 . It is worth noting that the lower personal Δ25OHD in ( 67 ) (individual points identified) would fit neatly into our dose–response, and several authors have identified the large interpersonal differences in response to UV radiation ( 24 ) so it is possible that the small number of participants (10) represented in ( 67 ) has resulted in this different dose–response relationship.…”

“…The authors of study ( 67 ) also attempt a dose–response relationship, from otherwise unpublished data using a UV6 lamp and a series of doses accumulating to 10 SED, so at the lower range of the studies included here. They find a linear dose–response relation at these doses, as indeed could current authors if only low accumulated dose studies are included.…”

“…The impact of reducing the skin area exposed from whole body to the more practical everyday range of 10–35% (representing hands and face only or attire equivalent to modest shorts and T-shirt) is difficult to assess. Simple scaling by skin area exposed may not be appropriate; indeed, there is evidence that such scaling is not appropriate ( 41 , 43 , 67 ), though note that all these studies used small groups of different people exposing different skin areas, which could distort the results. A longitudinal study would be more informative.…”

Vitamin D status in Polish women with endocrine and osteoporotic disorders in relation to diet, supplement use and exposure to ultraviolet radiation

“…Cutaneous synthesis provides 90% of the body supply of vitamin D. 33,34 Due to a low exposure to sunlight in autumn and winter, supplementation is necessary to maintain optimal serum 25(OH)D concentrations. Thus, we also evaluated participants' exposure to ultraviolet radiation.…”