Sun Exposure

sun-exposurePublic health guidelines and conventional wisdom have led many people to cover up and restrict their sun exposure in order to avoid skin cancers, premature ageing, and all the other problems associated with excessive sun exposure. Some even seem to believe that the less sunlight they expose their skin to, the better off they are.

There’s no doubt that excessive ultraviolet radiation can be harmful, but the fact is that too little sun exposure can be equally bad. In other words, sun exposure can have both beneficial and deleterious effects, depending on the amount and type of exposure we subject our body to.

How hairlessness and differences in skin color came to be

As humans, one of our natural desires is to “look good”, and when we take a peak in the mirror, we tend to assess factors such as body fat levels, muscle tone, hair style, and “beauty” in general. Less attention is often given to how our bodies evolved, why we are like we are, or how we differ from other animals on Earth. In many respects, Homo sapiens sapiens is a very unusual primate.

It’s believed that early primates might have acquired their daily vitamin D – the vitamin produced during exposure to UVB – requirements from regular grooming and ingestion of vitamin D precursors that were secreted in oils by their skin onto their fur (1).

When our ancient ancestors ventured down from the trees in Africa and slowly began their existence on the ground, the stage was set for many subsequent changes to the human body, some of which include the development of our massive human brain, our ability to walk on two legs, and perhaps most importantly to this article, the loss of fur. The gradual disappearance of body hair that coincided with the evolution of hominins might have occurred partly to allow for better heat dissipation through sweating.

In equatorial Africa where some of the very first hominins evolved, the direct angle of sunlight delivers a potent amount of ultraviolet radiation year-round. The data suggest that the very first hominins protected themselves against this harsh sunlight by seeking shade during the brightest parts of the day, but after a while, climate changes drove our ancestors into arid, open landscaped, and the evolutionary pressure to develop deeply pigmented skin increased (2). The darkening of the skin that occurred is in many respects synonymous with a very gradual natural application of sunscreen, as it helped our primal ancestors avoid photodegredation of micronutrients and damage to sweat glands.

The evolution of abundant sweat glands, hairless bodies, and skin rich in melanin (the primary determinant of skin color) gave hominins an advantage over other animals on the African savanna, as they had a superior ability to walk and run for hours under the midday sun.

When our species began to leave Africa about 50.000-100.000 years ago, we gradually travelled into new areas of the world characterized by less intense ultraviolet radiation. Dark skin requires between five to six times more sun exposure than fair skin to produce the same amount of vitamin D, and it’s therefore widely believed that the skin lightning that occurred in higher latitudes was an adaptation that allowed for adequate vitamin D synthesis in low-UVR climes (1, 3).

Populations inhabiting areas of the world with short winter days and little solar radiation in the UVB wavelengths developed clothes and shelter to protect themselves from the cold climate, further decreasing their exposure to the sun. However, since fatty fish and other foods high in vitamin D were a part of the diet of many of these societies, some have suggested that it wasn’t before the advent of novel, less nutritious diets with the Agricultural Revolution that the evolutionary pressure to evolve lighter skin really set in (2).

Clothes and indoor living

Clothing gradually became the norm in higher latitudes, and by the 1600s, peoples in some of these cold regions of the world covered their whole body even in the summertime (1). In combination with more indoor living, rickets – a disease caused by vitamin D deficiency – started to become a growing health problem. By the late 18th century, approximately 90% of all children living in industrialized Europe and North America had some manifestation of this disease (1).

If we move further up in time, to the modern day, the prevalence of rickets has gone down, but health conditions associated with sun exposure are still widespread. Modern lifestyles (e.g., plenty of time spent indoors) and mobility (e.g., people move or travel to new areas of the world) have created a mismatch between skin pigmentation and environment for many people.

Clothing has become a social attribute all over the world, and for most people in industrialized societies, the majority of wakeful hours are spent inside a car, office building, training center, and/or residential building. If you don’t make a conscious effort to get outside, sun exposure is often limited to the rays that hit your face on the short walk out to the car in the morning or during the small coffee break at lunch out on the city street. In combination with the fact that we often hear that we should be very careful about sun exposure if we want to remain wrinkle-free and avoid skin cancer, this has led to a society where many people get very little direct sunlight into their lives.

Clearly, there are also many who get too much sun exposure, such as the tanning-obsessed crowd, those with fair skin who move to warmer places, and those people who get little to no sunlight into their daily lives and then travel to a sunny location to get their holiday “fix”.

Health concerns associated with sun exposure

Through millions of years of evolution natural selection acted to adapt the human body to various and diverse environmental conditions. Sun exposure has always been an essential part of the environment component, with varying degrees of “importance” at different latitudes. It therefore doesn’t come as a surprise that both too little and too much sun exposure can adversely affect gene expression and human health. In the last couple of decades, artificial tanning devices have also provided a novel way of exposing our bodies to UVR.

Too much

Public health recommendations have for a long time advocated that people limit their time in the sun, and most of us know that exposure to excessive UVR can have a plethora of adverse effects, including skin cancers, wrinkles, sunburn, immune suppression, and cataracts (1).

Too little

Most of the benefits of sun exposure has been attributed to the synthesis of vitamin D through a photosynthetic reaction in the skin. The active form of vitamin D plays an essential role in regulating calcium metabolism, phosphorus levels, and neuromuscular and immune system functioning.

In children, vitamin D deficiency causes poor mineralization of the collagen matrix in the bones, potentially leading to rickets (4). In adults, vitamin D deficiency has been linked to several chronic health disorders, such as osteoporosis, osteomalacia, rheumatoid arthritis, and hypertension (4, 5, 6, 7)

Vitamin D deficiency is now recognized as a pandemic (8).

Although most of the benefits of sun exposure have been attributed to vitamin D, it’s becoming increasingly clear that the production of this vitamin is far from the only positive effect we derive from the sun. Other vitamin D independent pathways, such as regulation of the immune system, control of the circadian rhythm, and degradation of folic acid also play a role (9).

Different lifestyle factors tend to work in synchrony to promote health, and sun exposure is no exception. One of the primary reasons spending time outdoors and getting adequate sun exposure is so important is because it helps us sleep better at night (1). The production of melatonin – one of the key hormones involved in “setting” our circadian rhythms – occurs at night and ceases upon exposure to daylight in the morning. UVR exposure helps boost the production of serotonin, which results in more positive moods, and since serotonin is a precursor for melatonin, adequate sun exposure can improve sleep by impacting melatonin rhythms (1). Early exposure to bright light in the morning could lead to better sleep by triggering nocturnal melatonin production earlier in the day (1).

While the adverse health effects of not getting enough sun exposure are certainly many, it’s often the more superficial concerns that take precedent in most people’s minds. A “healthy tan” is considered attractive in many cultures, a glow that certainly won’t be attained by shunning the sun.

Too new

The tanning industry promotes tanning beds as a safe alternative to regular sun exposure for both tanning as well as vitamin D production, and for many people, tanning beds have become the preferred tanning option. However, as with so many other things that are novel parts of our lifestyle, indoor tanning devices come with certain negatives; some of which are:

  • “While many assume that the lamps in tanning beds contain less or similar amounts of light to that emitted by the sun, the UVA radiation emitted by these devices can be as much as 10 to 15 times more powerful than midday sunlight. Tanning lights also emit UVB radiation, although depending on the type of tanning device, the UVB emitted may be similar to or less powerful than the UVB emitted by the sun” (10).
    Many providers do not adhere to British or European standards for UV radiation (11).
    In other words, many tanning beds provide a dose of UVA that exceeds anything we experienced throughout our evolutionary history. Also, they don’t provide any superior benefits compared to regular sun exposure in terms of UVB and vitamin D synthesis.
  • Tanning beds are a potential source of pathogenic organisms (12).
  • Regular sun exposure could provide health benefits that you don’t get from artificial sources. For example, being outdoors in the sun during the day helps “set” the body’s biological clock. Tanning indoors at “unnatural” times of the day may interfere with the sleep-wake cycle.
  • Tanning beds may have independent effects from solar exposure that increase risk for skin cancers (especially melanoma) (11, 13, 14, 15).
  • Indoor artificial tanning could be more addictive than conventional sun tanning (11). This is in part due to the fact that tanning devices are accessible at all hours of the day.

All in all, “natural” sun exposure is the preferred option when possible.

Practical applications

General recommendations:

  • Tan gradually, and avoid getting burned!
  • Pay attention to latitude, time of day, and season.
  • Avoid using sunscreens if you can. Sunscreens – even many of the “natural” brands  – contain many potentially problematic ingredients.
  • Eat a healthy diet rich in antioxidants such as beta-carotene in order to increase your protection against sun damage.
  • If you have very light skin, 10 minutes of sun exposure a couple of times per week could be sufficient to give you a good dose of vitamin D. If you have dark skin, you need a lot more. Brief, repeated exposures may be more efficient than one concentrated session in terms of vitamin D production.
  • Choose sunlight over vitamin D supplements. Vitamin D supplements don’t provide the same spectrum of benefits that you get from sunlight.
  • Cover up (e.g., clothes, hat) to avoid excessive exposure.
  • Sun tanning is superior to the use of indoor tanning devices. As a general rule, it’s best to avoid indoor tanning devices.

Picture: Creative Commons picture by Harold Navarro. Some rights reserved.


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