Sunbathing and circadian rhythms

For those of us living in the Southern Hemisphere, December is seen as the official start of summer.  It is also the point at which all the usual fear-mongering around sun exposure and skin cancer ramps up.  Whilst I have no doubts that excessive ultraviolet radiation can be a factor in the overall risk for developing various forms of skin cancer, I don’t believe that it is the linear factor that we have perhaps been lead to believe it is.  In other words, I think there is more to the risk equation than just how much time you spend in the sun.

The human body is constantly trying to balance itself against stressors, both internal and external, that are breaking the body down, and its ability to build itself back up (which can be overdone if there isn’t a sufficient hand brake on those systems).  A recent paper suggests that, when it comes to suffering damage from ultraviolet radiation, humans might have a distinct time of the day where we can best withstand that stress and repair the damage, perhaps with implications as to when we expose ourselves to the sun and when we might choose to minimise that exposure.

Control of skin cancer by the circadian rhythm

While UV radiation has the ability to cause lesions to DNA which, if unrepaired, have the ability to become carcinogenic, humans also have the ability to repair this damage by effectively cutting out the damaged DNA segments.  In people who have hereditary mutations in the genes that code for this repair mechanism, there is a 5000-fold increase in skin cancer rates.

Researchers have recently identified a protein that is critical in the role of damage-recognition – xeroderma pigmentosum group A (XPA).  XPA is a rate-limiting factor in the repair sequence and is seemingly controlled by the body’s internal circadian clock.  As such, the activity of this repair protein has a distinct rhythmicity.  You can perhaps see where this is going.  If XPA is needed to repair DNA damage caused by UV radiation, but it is only produced in the body during certain times of the day due to circadian rhythms, perhaps there is an optimal time of the day where you can match the two factors up?

To test this, the researchers used a mouse model.  Mice, however, are nocturnal, compared to the diurnal rhythms of humans.  Their findings with the mice;

In mouse skin there is more DNA replication and less repair in the morning and less replication and more repair in the evening. Because UV-induced skin cancers arise from mutagenic replication of epidermal keratinocyte DNA, the same UV dose is more carcinogenic in early morning hours than when given in the early evening hours.

Understandably, being a nocturnal creature, the mice’s skin isn’t adapted to receiving large UV radiation doses during the day and repairing the damage that such a dose might cause…

…mice exposed to UVR in the early morning hours are more likely to develop skin cancer than those receiving the same dose in the evening hours because of a poor repair rate, and therefore, avoiding sunlight exposure in the morning hours will likely reduce cancer risk.

So how might this knowledge translate to humans?  Well at this point, it hasn’t been tested on humans or a model analogous to us, so it is all relatively theoretical at present…

Mice are nocturnal and humans are diurnal animals and, therefore, the core circadian clock and their outputs exhibit opposite phases. On the basis of this well-established fact, we predict that humans will have a higher rate of repair in the morning and would be less prone to the carcinogenic effect of UVR in the morning hours and it might be advisable for humans, to the extent possible, to restrict their occupational, therapeutic, recreational, and cosmetic UVR exposure to the morning hours.

We note that this is a deductive conclusion based on our findings in mice reported in this paper and the evidence that humans have a robust circadian clock in their skin, which is antiphase to the mouse clock and with the peak xpa transcript around 7:00AM.

On the basis of the differences in circadian rhythm patterns between mice and humans, and that the key protein for repair, XPA, hits its peak expression in humans at around 7:00am, it seems to make reasonable sense that we have evolved with the capacity to head out into the morning sunshine to undertake our chores with our defences set to full power.  I note my experience of tropical countries is that the locals get out and about in the morning and tend to take shelter from the heat and the sunlight later in the afternoon.

From the standpoint that we want to balance out exposure to the sun to optimise our vitamin D production without over-exposing ourselves to ultraviolet radiation and risk overwhelming our defences, and thinking about how ancestral populations living in tropical regions may have structured their day, it would seem reasonable to conclude that sun exposure across the morning, and perhaps through to the early phase of solar noon, may offer us the best health returns at the lowest risk (allowing for the fact that the above deduction is yet to be tested directly in humans).  This type of advice might be more user-friendly than a blanket ban on sun exposure by the cancer society fun police.

Of course, this means that the opposite may also hold true.  Spending time sunbathing/bedding in the afternoon may be a maladaptive behaviour from an evolutionary standpoint, and may increase our skin cancer risk.  If a skin cancer lesion does occur, all we get is a statistical reinforcement of the sun = skin cancer meme, and a complete lack of fidelity around the actual mechanism.

I note from the above paper, this line from the authors;

…DNA damage and repair are two of the multiple factors that impact cancer development.

Leaving aside the specifics of the multiple factors, we effectively have this sort of equation going on… skin cancer development risk = [DNA damage - DNA repair].  This leaves us a few areas to focus on to mitigate our risk.  The Cancer Society focuses heavily and almost exclusively on the DNA damage factor, with their strategy to mitigate this being the likes of don’t go in the sun, or if you do, cover up with clothing, hats, glasses, and/or sunscreen.  And of course, all of these are behavioural strategies where we place external physical barriers between our skin and the sun.  We forget that we have internal defences, surely bolstered by how we might shape our internal environment through the likes of the food we eat.  What sort of skin defence might we expect by eating a low nutrient, pro-inflammatory diet?

Accepting the fact that a degree of damage will occur, how well are we geared to undertake repairs?  Do you have a diet rich in anti-inflammatory agents, antioxidants, cholesterol, immunomodulators?  I can’t help but thinking that by living as close to being in line with one’s evolutionary biology as is possible in our modern environment, that we are automatically strengthening the repair side of the equation, leaving us less prone overall to developing skin cancers.  This might be further enhanced by paying attention to our natural rhythms – including our circadian rhythms and those that track with the seasons.

I’ve written previously that, despite my fair skin, I have tended to get less sunburn since eating a paleo diet.  I’ll now be more conscious of the time of day I get the bulk of my exposure from now on too.

16 thoughts on “Sunbathing and circadian rhythms

  1. PG,
    I saw a couple of studies done in Australia maybe 10 years ago that strongly implicated omega 6 fat intake as a major culprit in the skin cancer epidemic which makes sense. I have tried to locate these recently but have had no luck. Do you know of which studies I am talking about?

    If our skin cell membranes are composed of lots of omega 6 fats instead of more stable saturated fats they would be more susceptible to damage from UV exposure. I know that there are many factors involved such as timing of exposure like you wrote above but I feel that the stability of the cell membrane based on fatty acid intake is the biggest factor. Omega 3 to 6 ratio is important from an eicosanoid production stand point but even the omega 3’s are prone to damage. You would not want too many omega 3’s in the membrane would you?

    1. From Second Opinions Barry Groves:-

      The dietary connection

      In the 1970s, when kidney transplantation was pioneered, doctors first encountered the problem of tissue rejection. To combat it, they gave their transplant patients linoleic acid. This suppressed their immune systems very effectively, preventing their transplanted kidneys being rejected. But it also caused a large increase in cancers and this treatment was stopped.

      Since then, linoleic acid and oils that contain it, have been shown time and again to increase the risk of several types of cancer, including skin cancers.

      Linoleic acid is the major fatty acid in all polyunsaturated vegetable margarines and cooking oils:

      Polyunsaturated margarines are around 40% linoleic acid
      Sunflower, safflower, corn and soya oils are all more than 50% linoleic acid.

      Drs B S and L E Mackie, working on Australia’s Sunshine Coast have a great deal of experience in skin cancers. They say: “In view of the work of Black and Erickson in mice and our own work in humans, we believe that human subjects who are at high risk of melanomas and other solar-induced forms of skin cancer should be advised to be moderate in their intake of dietary polyunsaturated fats.” (28)

      Patricia Holborrow also points out that the increase in melanomas could be a result of dietary changes to PUFs.”Recently, I followed up four families that started in 1976 to use a diet with preferred oils as safflower and sunflower oil and low in salicylates and additives (that interfere with the metabolic pathway of these fats). There had been three cases of cancer resulting in two deaths in these families.” (29) “The issue is further complicated by dietary factors that are cofactors for the metabolic pathways for the fatty acids and which may in addition favour or have a negative effect on the anticancer or cancer enhancing properties of the various prostaglandins (eg the negative effects of vitamin E and the positive effects of vitamin C).” (30)

      The Australians are as paranoid about heart disease as the Americans. I was in Australia in 1995 and noticed that it is even their custom to remove the cream from milk and replace it with polyunsaturated vegetable oil.

      One of the recommendations for reducing the risk of skin and other cancers is to reduce intakes of fats and take vitamin supplements. But this approach doesn’t seem to work. The findings of a huge study by scientists at the Departments of Nutrition and Epidemiology, Harvard School of Public Health, Boston; the Division of Human Nutrition and Epidemiology, Wageningen Agricultural University, Wageningen, Netherlands; the Department of Community and Preventive Medicine, Mount Sinai School of Medicine, New York; and the Channing Laboratory, Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, of 43,217 male participants of the Health Professionals Follow-up Study, did not support the hypothesis that diets low in fat or high in specific vitamins lower risk of basal cell carcinoma. (31)

      It’s usually saturated animal fats that get the blame for all diseases today. They are not the culprits — ‘healthy’ vegetable oils are (see Polyunsaturated Fats in The Cholesterol Myth)

  2. In college my fellow biochemistry major friend and I used to go out in the sun and joke we were giving our thymine-thymine dimer repair mechanisms a work out… Once a nerd, always a nerd I suppose!

    I wonder what role (if any) the circulating levels of D3 plays in this (someone recently was talking to me about animal Vitamin D synthesis but I’ll admit I wasn’t paying that close attention- will have to look into it). I tend to think that, as a steroid hormone capable of changing the transcription of many genes, the timing of changes in blood levels might be important. This probably isn’t such an issue with synthesized Vitamin D, but I’ve heard a couple people talk about taking a Vitamin D supplement at night, which seems very counter intuitive to me. When I take Vit D, I always take it in the morning- just seems logical…

  3. I, too, have experienced less burning since switching my diet to Paleo/PHD and eating lots of coconut.

    Just a side note, I went backpacking in Yosemite last summer and burned badly on Half Dome, as I lost my sunscreen. Coconut oil slathered on the burn helped so much! I barely had any pain the next day (except the places I missed.)

  4. I was going to mention the D3 connection, buuut, the other guys got there first. I find it very interesting as it begins to link up… D3 in the morning, simulating morning sunlight exposure = better regulated circadian rhythms = improvement in conditions such as depression. Of course D3 is also crucial to Calcium homeostasis (amongst other things it increases transport of Calcium and Phosphorous across the brush border). Calcium is lower in a paleo diet, which suggests paleolithic peoples probably absorbed more of what they did consume (actually, I have no data on osteoporosis in paleoliths, does anyone know?). Bone health is a big problem in older individuals especially women. Perhaps our fear of all sunlight is making us sad and brittle..?

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