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After a bit of a hiatus from any sort of regular blogging, and in light of the fact that we here in the Southern Hemisphere are on the icy slope to winter, it is somewhat ironic that I am diving back in with a post on sunlight. Although I love being in the sun, with all its warmth and feel good factor, I’m as likely guilty as the next person for focusing far too much on vitamin D production when it comes to the actual hard physiology of it all.
And as we are all but effectively in a UVB shadow now until perhaps September, my only vitamin D is going to come from a small white capsule.
Yet there are plenty of other reasons to be getting out in the sunlight, regardless of the time of year. A recent free paper outlined many of those beneficial reasons, and once again underscored my feeling that we have taken the anti-sun message too far the other way. Here in the paleodiet land, we are forever focused on the impact of food on our immune system function, yet we rarely consider the big yellow orb in the sky as a potent immunomodulator beyond the effects of vitamin D.
Human skin is highly interactive with ultraviolet radiation (UVR), with both positive and negative implications to these interactions (context, much?). It is well recognised that over 90% of our vitamin D needs should be met by the conversion of cholesterol-derived vitamin D precursors to more active vitamin D forms via UVB radiation interacting with these precursors. But beyond this steroid hormone production, what else occurs with this UV-skin interaction?
We know that unless you are a ginger or an emo, chronic exposure to UVR increases our skin pigmentation (tanning) (my ability to tan and hold some colour has increased no end since switching to a paleo lifestyle). I knew that tanning was largely mediated by UVB radiation and it was my understanding that UVA, with its ability to penetrate more deeply into the skin and well beyond the layers where melanocytes produce the tanning pigment, melanin, played very little part in the tanning process. Perhaps not…
There are three phases of tanning: immediate pigment darkening (IPD), persistent pigment darkening (PPD) and delayed tanning (DT).
IPD occurs during the first minutes of exposure to UVA, and then fades within few hours. PPD appears within hours of higher doses of UVA exposure and persist up to several days or weeks. DT develops over 3–7 days after UVB exposure, and then remains for weeks.
So the darkening of our skin that we appreciate when we are trying to tan is very much part of the delayed tanning process, and relates to UVB exposure. The more immediate tanning reactions, IPD and PPD (which I can’t say I tend to get much of with my skin type), are mediated more by UVA exposure, and partly by the production of reactive oxygen radicals (you generally don’t want too many of these larking about in your body).
High doses of UV radiation (be it from the sun or from artificial sources, such as tanning beds), can lead to redness and burning, both localised and systemic immunosuppression (which can be both good and bad), DNA damage, ageing of the skin, and the initiation of cancer. The skin isn’t without its defences though, adapting to chronic UV exposure by increasing the production of tanning pigments, thickening of the skin (the horny layer), increasing antioxidant defences, increasing DNA repair mechanisms, and secretion of inflammatory cytokines to kickstart an immune response.
While UVA and UVB-derived tans are visually identical, it is only the UVB-pigmentation that offers much in the way of protection against further photodamage to DNA, and to sunburn. The protection of this tan is equivalent to using a SPF 2-3 sunscreen. The UVA-derived tan is not photoprotective in the sense that we would consider our UVB tan to be. Instead it is hypothesized that the immediate changes in skin pigmentation stimulated by UVA radiation is an attempt to protect many of the photosensitive compounds we have near the skin surface, including the likes of folates and B-group vitamins. When these compounds are hit by photons of UVA spectrum light, they can generate reactive oxygen and nitrogen species that can cause damage by themselves or enhance the damaging effects of UVB radiation. The UVA-induced immediate pigment darkening my be an attempt to protect these photosensitive compounds from photo-oxidation.
With chronic exposure to UV radiation, we see changes in the elasticity of skin. This may be the result of UV-induced degradation of elastin fibres in the skin and an accumulation of abnormal elastic tissue (solar elastosis). These changes in the skin’s elasticity can be considered a biomarker for cumulative sun exposure. As much as saggy sun-damaged skin might be considered unsightly for those who have it, there is a degree of protection that it appears to afford;
…solar elastosis is a protective factor for sporadic basal cell carcinoma. Furthermore, it has been positively associated with melanoma survival. Prognosis for melanoma of the back (low solar elastosis) is worse than that for the face (high solar elastosis). Melanomas with elastosis occur at later ages than melanomas without elastosis. Consequently, sun exposure is associated with increased survival from melanoma.
Melanoma is more frequent among people with indoor occupations than among people getting large accumulated UV exposures (farmers, fishermen, etc.). Chronic UV exposure reduces and/or delays the development of melanoma. Holiday sun exposure is not always associated with an increased melanoma risk, and even a protective effect of regular weekend sun exposure has been observed, particularly for limb tumors.
Outdoor activities (without sunburn, associated with increased risk of melanoma) in childhood are associated with a lower risk of melanoma. The observed effects could be mediated independently by photoadaptation (development of solar elastosis) and by higher vitamin D levels.
The above statements, particularly the ones which I have put in bold type, seem to echo the sentiments of a 2009 Medical Hypothesis paper;
Cutaneous malignant melanoma (CMM) has been increasing at a steady exponential rate in fair-skinned, indoor workers since before 1940. A paradox exists between indoor and outdoor workers because indoor workers get three to nine times less solar UV (290–400 nm) exposure than outdoor workers get, yet only indoor workers have an increasing incidence of CMM.
Of course, despite the type of statements above, year after year we have to suffer through the same fear-mongering rubbish from the cancer society zealots encouraging you not to go outside in the sun unless you are wearing a hazmat suit and welders mask (actually, it wouldn’t be so bad if that is what is being recommended… instead, we have them telling us to smear ourselves in photoreactive sunscreens all the time).
Screw the cancer society… getting out in the sun feels good. And for very good reason;
…exposure of keratinocytes to UV radiation leads to production of an opioid β-endorphin via stimulation of the POMC promoter. This β-endorphin released into the blood during UV exposure may reach the brain in sufficient concentrations to induce mood enhancement and relaxation.
The authors go on to say that only one out of four studies looking at sun exposure and β-endorphin release has been able to show any relationship between the two. However, as they also point out, many people faced with a large venous sampling needle following a lie in the sun, will likely find their endorphin levels suddenly tanking. Anecdotally, I think most people understand how much better and more relaxed we feel having had some sunlight on the face and in the eyes.
The use of the sun for therapeutic purposes is nothing new, dating back (at least in documented medical times) to ancient Egyptian, Greek, and Roman times. And there are many buildings dotted around New Zealand which were, until relatively recently, tuberculosis hospitals with large sun-facing balconies. Modern phototherapy is still used, mostly for skin conditions. With many of these skin conditions characterised by a high cellular turnover, one of the modes of action for the beneficial effects of UV radiation, involves UVB radiation reducing the rate of DNA synthesis in these cells and thus reducing their turnover rates. UVB radiation can also have immunosuppressive effects, effectively reducing inflammation.
Time in the sun can also play a role in regulating vasodilation of blood vessels, immune defences, neurotransmission, and programmed cell death, all via the effect of UVA radiation on the gaseous compound, nitric oxide. Human skin and the skin vasculature make up the largest storage organ in the body for nitric oxide derivatives such as nitrite and nitrosothiols. Nitric oxide is rapidly released following UVA exposure, with the UVA radiation able to recycle nitrites and nitrosothiols back into nitric oxide. It is thought that this ultraviolet radiation-mediated nitric oxide pathway results in higher concentrations of nitric oxide being released than those produced enzymatically by nitric oxide synthases in the plasma.
As nitric oxide plays an important role in mediating vasodilation, it is thought that sunlight exposure may offer health benefits relating to this role;
UVA-exposure of human skin releases nitric oxide into the circulation. In the bloodstream, nitric oxide can reach the nervous system. In this way, UVA can influence transmission of nerve signals indirectly.
Like all things, however, there is a point at which something positive can become something negative. Indeed, getting too much UVA radiation may lead to oxidative damage to blood vessels and skin, with a potential role in the induction and progression of skin cancer.
Obviously getting sensible, vitamin D-producing sun exposure is important to health. But it is clear that there is more to life under the sun, and its positive effect on our biology, than just the vitamin D story alone. For that reason, simply popping a vitamin D capsule and believing you are getting the same health benefits as sun exposure, are likely to see you come up short. We evolved naked under the sun. Best we continue, sensibly, that relationship and interaction with our sun god. If nothing else, the endorphin hit alone is worth it.