Journal of Photochemistry and Photobiology B: Biology
Photoprotection and vitamin D status
Introduction
Solar ultraviolet radiation (UVR) comprises UVB (280–320 nm) and UVA (320–400 nm), the vast majority being UVA (⩾95%). The ratio of UVB to UVA is dependent on the solar elevation angle (SEA i.e. height of the sun) which in turn is dependent on factors such as latitude, season and time of day [1]. The adverse clinical effects of terrestrial UVR (∼295–400 nm) are well documented and include sunburn in the short-term and skin cancer and photoageing in the long-term. These effects are the consequence of many molecular, cellular and immunological changes in the skin induced by UVR. Such changes include the formation of DNA photoproducts such as cyclobutane pyrimidine dimers (CPD) [2], the induction of cytokines such as TNFα and IL-10 [3] and the depletion of epidermal antigen presenting Langerhans cells [4].
One of the ways of assessing risk from UVR is the determination of the wavelength dependence, or action spectrum, for the effect in question. Action spectroscopy is time-consuming but relatively easy for determining the acute effects in human skin in vivo. This has been done, for example, for erythema [2], [5], epidermal CPD [2] and TNFα [6] induction. These studies have shown similar action spectra with UVB being 3–4 orders of magnitude more effective than UVA. One conclusion that has been drawn from these studies is that DNA is the probable chromophore for TNFα and erythema. The Commission Internationale de l’Éclairage (CIE) action spectrum for erythema [7] is shown in Fig. 1. This spectrum is a mathematical composite from several human studies but does not include the study of Anders et al. [5], which was done with lasers and so is likely to be more accurate. Action spectra can be used as biological weighting functions, to assess the true effect of a given UVR spectrum. Thus, for example, when the CIE erythema action spectrum is used to weight a solar UVR spectrum with say 5% UVB, it is this 5% that is responsible for about 85% of erythemally effective energy (EEE). The CIE action spectrum for erythema is also the basis of the exposure unit known as the standard erythema dose (SED) [7], [8] which is increasingly used as a measure of individual or population UVR exposure that is independent of individual sensitivity and the emission spectrum of the UVR source. This is in contrast to the minimal erythema dose (MED) which is a measure of individual sensitivity to a given UVR spectrum. It is very important to note that an action spectrum is only valid for spectral weighting if there are no spectral interactions between different wavebands.
Sunscreens are designed and tested for the prevention of erythema and their primary index of protection is their sun protection factor (SPF). This globally accepted labelled index is used by the public as a means of assessing efficacy, and is primarily a measure of UVB protection because this spectral region is the main cause of erythema as shown in Fig. 1. During the last decade or so there has been an increasing trend, especially in Europe, for the development of broad-spectrum sunscreens with protection well into the UVAI (340–400 nm) region. This has necessitated the development of UVA protection factors (UVA-PF) which are unrelated to erythema. One consequence of this trend is that, for a given SPF, a broad-spectrum sunscreen will have lower UVB protection than a primarily UVB sunscreen. This is illustrated in Fig. 2.
The SPF of a given sunscreen is assessed in a panel of volunteers in a laboratory under stringent conditions including the use of solar simulated radiation (SSR) with a UVB content that is higher than is found even at the most extreme conditions at the earth’s surface and sunscreen applied at 2 mg/cm2 of skin. One consequence of this is that the SPF of a primarily UVB sunscreen will have a higher SPF rating than it would if tested under natural sunlight. Furthermore, sunscreens frequently don’t reach the expected level of protection for a range of reasons, including application densities much lower than used for SPF testing [9]. Sunscreen use is widely advocated by public health bodies as a means of reducing the risk of skin cancer, especially in sun-sensitive skin types I and II. However, there is no evidence that sunscreens prevent malignant melanoma (MM) [10], some evidence that they may prevent basal cell carcinoma (BCC) [11], but there is good evidence that they prevent squamous cell carcinoma (SCC) [11]. Sunscreens would be expected to offer some protection against SCC because the action spectrum for this lesion, based on mouse data, is quite similar to that for human erythema. We lack mammalian action spectra for the induction of BCC and MM. Possible explanations for the lack of sunscreen protection against skin cancer include a lower level of protection than expected because of poor application compared to SPF testing and the use of sunscreens to extend intentional sun exposure [12]. Furthermore, few prospective studies have been done to assess the role of sunscreens on non-melanoma skin cancer because of the logistical difficulties of doing such work. Prospective studies on melanoma would require huge sample sizes.
Photoprotection strategies, including sunscreen use, are also advocated in patients with abnormal sensitivity to solar UVR. These include people with photodermatoses ranging from xeroderma pigmentosum (XP) to polymorphic light eruption (PLE). Such strategies are also recommended for organ transplant patients maintained on immunotherapy. This ever growing group of patients has a much greater risk than normal for all types of skin cancer on sun-exposed sites.
Section snippets
The photosynthesis of vitamin D in the skin
The detrimental effects of UVR exposure are well defined. Some limited data suggest mood altering properties that may depend on opioid induction [13]. The only established benefit of solar UVR and UVB in particular, is the cutaneous synthesis of vitamin D3, hereafter referred to as vitamin D for convenience. It has been estimated that the majority of vitamin D is derived from solar UVB exposure [14]. The risks and benefits of solar UVR are therefore of great interest to the dermatology
Pigmentation: natural photoprotection and its effect on behaviour
The risks of the acute and long-term effects of UVR are inversely related to constitutive pigmentation (i.e. skin colour on unexposed sites) as shown in Table 2. This is thought to be due to the photoprotective effects of epidermal melanin.
The degree of protection from constitutive pigmentation is not easy to determine, and different values can be found in the literature, but one study has suggested a relatively low protection factor of about three (black vs. white) against erythema [36]. It
Clothing
Clothing is advocated in photoprotection strategies and there are designated ways of determining the degree of protection against erythema which is labelled by the ultraviolet protection factor (UPF). Laboratory in vitro and in vivo studies have shown that clothing inhibits the production of previtamin D and serum vitamin D respectively [51], [52], and that the transmission of previtamin D effective radiation depends on fabric type [52], [53]. Several studies have shown that clothing worn for
Discussion, conclusions and recommendations
It is difficult to make comparisons between studies because, although the majority used 25(OH)D as the serum marker of vitamin D status, a wide range of detection techniques was used which can result in markedly different results [69]. Furthermore, studies have been done with different protocols. The recent work of Bogh et al. [49] has shown that baseline 25(OH)D is a major determinant of response to UVB and this must be taken into account in the design of future studies. Table 3 shows that
Acknowledgements
SB was supported by GenoMEL Network of Excellence supported by the European Commission under the 6th Framework Programme: Contract number: LSHC-CT-2006-018702. We thank Dr. Sophie Seite of L’Oréal Recherche for the data in Fig. 2. We thank Professors Mary Norval and Brian Diffey for their constructive comments.
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Melanin has a Small Inhibitory Effect on Cutaneous Vitamin D Synthesis: A Comparison of Extreme Phenotypes
2020, Journal of Investigative DermatologyCitation Excerpt :However, it should be noted that the absorption and metabolism of vitamin D is similar across individuals of all skin types (Gallagher et al., 2013). The few UVR intervention studies that have assessed the effect of melanin on vitamin D synthesis have produced inconsistent results; see Springbett et al. (2010) and Xiang et al. (2015) for reviews. One study that reported that melanin had no effect (Bogh et al., 2010) was criticized for using a non-solar UVB phototherapy source with a significant spectral content in the short-wave non-solar UVB range (i.e., 280–295 nm) (Björn, 2010).
Regular Sunscreen Use and Risk of Mortality: Long-Term Follow-up of a Skin Cancer Prevention Trial
2019, American Journal of Preventive MedicineDeterminants of vitamin D status of healthy office workers in Sydney, Australia
2019, Journal of Steroid Biochemistry and Molecular BiologyCitation Excerpt :BMI was not found to predict 25OHD, except in a small proportion who were obese, probably because most people in this study had relatively normal BMI (Table 2). Skin phototype was the major independent determinant of vitamin D status, as found in other studies reviewed [39]. Melanin is known to absorb UV radiation and experimentally, pigmentation has been shown in a number of studies to be an important cause of reduced 25OHD levels after exposure to known amounts of UV [40,41] or alternatively to require more UV exposure to make the same amount of vitamin D [42,43].
Vitamin D in inflammation mitigation and role as signaling molecule
2018, Gene ReportsCitation Excerpt :People are using hormone-disrupting cosmetics without knowing the full picture, which is hampering vitamin D synthesis. Sunscreens have sensitizers which cause inflammation, and they are likely to be hampering vitamin D synthesis in the skin (Springbett et al., 2010). The link between air pollution and vitamin D deficiency is reported (Hoseinzadeh et al., 2018), inflmmation being the convergence point.
The prevalence of vitamin D deficiency among dark-skinned populations according to their stage of migration and region of birth: A meta-analysis
2016, NutritionCitation Excerpt :Univariate regression suggested that length of time in migrated country and age of population studied affects prevalence. Although people with dark skin require more exposure to sunlight than white-skinned people to maintain adequate serum vitamin D levels [63], we found that prevalence of VDD was lower among migrants from SSA than for migrants from the EME. Studies in dark-skinned native traditional living African populations [64], Gambian women [65], and Nigerian children [66] have found adequate levels of circulating vitamin D. Two studies with older native Africans, found increased levels of VDD.