New Under the Sun:
Volume 6, Issue 2
This essay continues from last month. If you missed last month’s issue, you can check it out in the archives. See below.
WHY SKIN COLOR VARIES
Here we have the dilemma that most dermatologists ignore: too much UVB radiation can cause skin cancer, but too little can result in inadequate levels of vitamin D and all its associated health problems. How can we achieve an appropriate balance? We need to closely examine human adaptation to varying environments. We have good archaeological evidence that humans originated in the equatorial zone of East Africa. Since the equatorial zone receives the most UVB radiation and since very dark skin color protects from over-exposure to UVB radiation, we can make the logical assumption that the earliest humans had very dark skin. Vitamin D levels measured in the blood of individuals with very dark skin color living in the tropics who spend most of their days outdoors show the average level to be in excess of 50 ng/ml (for 25 hydroxyvitamin D, the form which gives the best information on individual vitamin D status), much higher than what is currently considered the normal average of 30 ng/ml. Wild-living primates have much higher levels of vitamin D than do humans, further indicating that the so-called normal average level is much too low.
From the fact that individuals living in the tropics have high levels of vitamin D, we can deduce that although very dark skin color prevents most UVB radiation from penetrating to the lower layers of the skin, some UVB radiation does penetrate if enough time is spent outdoors in an equatorial zone, allowing the body to maintain adequate levels of vitamin D while at the same time preventing skin cancer. Dark-skinned individuals have about one-tenth the number of skin cancers found in light-skinned individuals.
This adaptation worked very well for a very long time. Humans were able to spend plenty of time in the strong equatorial sun doing their daily tasks without needing to worry about skin cancer while still having enough vitamin D to maintain good health. However, as human populations grew, they began to move into new territories. Over many thousands of years, humans gradually moved into more temperate climates. New types of adaptation making this move possible included better tools, controlled use of fire, and clothing. But human bodies also needed to adapt to the changing environment.
The tropical zone begins at the Tropic of Cancer, crosses the Equator, and ends at the Tropic of Capricorn. The temperate zone is between the tropical zone and the polar zone, encompassing the region from about 24 degrees latitude to 66 degrees latitude. The subtropical zone is the lower portion of the temperate zone from about 24 degrees latitude to 40 degrees latitude. The temperate zone covers the United States, Europe, and Canada, about half of Asia, the southernmost parts of Africa and South America, the southern half of Australia, and all of New Zealand. The polar zones are within the Arctic and Antarctic Circles.
Since very dark skin was the best adaptation to living in an equatorial environment thousands of years ago, those who were very dark skinned were most likely to survive to have children and to pass on their genes to future generations. Skin color is a complex trait involving the interaction of multiple genes, each gene having multiple variations. This means that an extensive range of skin color is possible. Therefore, even though most individuals living in this ancient population would have been very dark skinned, on occasion someone would be born who had lighter skin color. Generally, this person would not survive to pass on her genes because she would develop skin cancer, in particular melanoma, and would die prior to being able to ensure that any children she had survived to adulthood. High rates of melanoma occur in the 20 – 50 age bracket, the prime reproductive years.
However, as groups gradually moved into higher latitudes, those who were somewhat lighter skinned actually had a better chance of passing on their genes than did their darker-skinned relatives. This is because the balance of maintaining just the right amount of UVB radiation exposure to ensure adequate levels of vitamin D while also preventing skin cancer was changing.
Light colored skin has very few melanosomes and those that it does have are tiny and tend to clump together instead of being evenly spaced. This allows most of the available UVB radiation to penetrate to the lower levels of the skin. Another way to understand the effect of melanosomes is to imagine skin color as different types of fabric. Very dark skin with its large, tightly packed melanosomes can be compared to the heavy, tightly-woven canvas used in tents and sun umbrellas. Very little sunlight is able to penetrate the canvas fabric. On the other hand, very light skin can be compared to fish net, more holes than fabric. Virtually all available sunlight is able to penetrate the loosely woven or knotted fish net. While this permits vitamin D levels to be easily maintained if there is adequate UVB radiation exposure, it also increases the risk of developing skin cancer. But in higher latitudes, UVB radiation is dramatically reduced when compared to that available in the lower latitudes. This means that as our ancestors gradually moved into higher latitudes, those who had very dark skin would have had difficulty maintaining adequate levels of vitamin D and would have developed a variety of health problems.
Infants who do not get enough vitamin D develop rickets. Their skeletal growth is delayed and the bones do not develop properly; the long bones are bent out of shape. Another problem that occurs is that the bones of the pelvis, the hip bones, also become improperly shaped with the result that instead of having a rounded pelvis, the pelvis is flattened. This may affect the ability to walk in males and females, but there is a worse effect in females. Pregnant women who have a flattened pelvis cannot give birth normally. In the past, this would have meant that the infant, and probably the mother, would have died during childbirth. Women with very dark skin color living at higher latitudes would have been more likely to have had rickets as an infant and would have had more difficulty giving birth as an adult than would women with lighter skin color. The balance would have shifted.
As we can see, skin color is adapted to the differing UVB radiation levels of our ancestors’ environments, darkest in the equatorial zones, and gradually lightening as we move toward the polar regions. The transition in degree of pigmentation is smooth and gradual. Of course, this has changed in the past several hundred years with individuals from all different regions living in locations far from their ancestral homeland. In addition, with the rise of urbanization and industrialization over the past 300-400 years, humans live in an increasingly unnatural environment. These changes have had a major impact on human health.
Humans are primates and primates are tropical species. While we have adapted, both physically (with changes in degree of pigmentation) and culturally (with changes in technology) to non-tropical environments, these changes are not without their costs. Humans living in a region of reduced UVB radiation, even if they have a very light skin color, have a very hard time maintaining adequate levels of vitamin D. In temperate zones, UVB radiation is at its peak during the mid-summer months. It declines to virtually nothing during the winter months and only gradually builds to the summer peak during the spring. If you live in the temperate zone you need to get your entire “budget” of vitamin D during the summer months since vitamin D can be stored in the body for several weeks. However, even if you had adequate levels of vitamin D during the summer months, your levels would be suboptimal by late winter/early spring. Even the very lightest skin color in the most northern latitudes is an inadequate adaptation. Research done in coastal Finland in the 1920s found that in those villages where fish formed a major part of the diet, there was very little rickets, but in those villages where little fish was eaten, rickets occurred at very high rates. Cold-water fish, in particular salmon, mackerel, sardines, and herring, are among the very few food items that can be eaten in normal quantities to provide supplemental vitamin D.
If you have light skin and live in a temperate zone you need to expose your unprotected skin (no sunscreen on it) to UVB radiation between 10 am and 3 pm during the summer months for about 30 minutes per day to obtain adequate levels of vitamin D (at least 50 ng/ml for 25 hydroxyvitamin D). If you have very light skin and live in the desert Southwest or in south Florida, 15 minutes per day of UVB radiation may be enough.
If you have very dark skin and live in a temperate zone, it will be almost impossible for you to get enough exposure during the summer months to maintain adequate levels of vitamin D since you may need to expose your unprotected skin for up to six hours each day. You will have quite inadequate levels of vitamin D by late winter. Modern life does not take into account this need for adequate vitamin D levels since most employed individuals are at work between the hours of 10 am and 3 pm and few take their lunch break outside. Even children are spending more of this time period indoors during the summer months.
During the 1910s and 1920s, the same period that vitamin D was discovered, the importance of sunlight/UVB radiation to the prevention of rickets was also discovered. Doctors urged parents to take their babies outside each day to get some sun. In fact, in some European countries the government in effect required parents to sunbathe their infants and children. And where previously a pale skin had been the sign of wealth, now a tan indicated that one had the leisure to be outside during the hours most others were working. Sun exposure and dosing with cod liver oil were beginning to reduce the incidence of rickets, at least among those with light skin. But because the rate was still high, in the late 1930s, the US government decided to fortify a common food product with vitamin D. After some discussion, milk was chosen because it was assumed that all children drank milk. Several European countries chose to fortify margarine. These efforts were successful. By the latter half of the 20th century, rickets was so rare among light-skinned individuals that medical students received only minimal information about it. When, as physicians, they were later confronted with signs of rickets in their practices, they didn’t know what the problem was.
As with all things, balance and moderation are required. This rule was ignored when it came to sun exposure. If some was good, then more ought to be even better. The result of over-exposure to the sun has been a sky-rocketing rate of skin cancers, especially the more serious and frequently deadly melanomas, among individuals with light skin color. Since most adults work during the day and during the week, their chance for sun exposure is limited to the weekends. On the weekends, the family treks off to the beach or the park and spends the entire day outside, often ending the day with severe sunburns. This intermittent, but intense and burning exposure to UVB radiation appears to be the primary factor in the development of melanomas. While individuals in temperate zones whose jobs require that they work outdoors, and who do so without protecting their skin, frequently develop the less serious forms of skin cancer, they rarely develop melanomas. Vitamin D (1,25 OHD) is a tumor-suppressant and has been shown to induce cellular death in some melanomas. It may be that individuals with light skin color who work outdoors have high levels of vitamin D in their blood which protect them from melanoma in temperate zones. The protection would probably be inadequate in equatorial zones where UVB radiation is several hundred times more intense.
Because of the huge increase in the numbers of skin cancers among those with light skin color, sun avoidance is now urged. In addition, parents are told that infants under six months of age should not be exposed to any sun; and once past that age, all infants and children should wear a SPF 40 sunscreen when outside. Parents are also told that they should make sure their children wear hats and cover up their skin whenever possible.
Tests have shown that a SPF 8 sunscreen blocks the skin’s ability to produce vitamin D. Therefore, if one follows the dermatologists’ recommendations, infants and children will be unable to naturally produce vitamin D. This is a very serious concern since it is extremely difficult to obtain any vitamin D from the diet, let alone enough to maintain appropriate blood levels. Later chapters will discuss diet in more detail.
Although the number of rickets cases dropped dramatically in the United States during the middle decades of the 20th century, rickets never went away and has remained a problem for many populations throughout the world. And now we are seeing increasing numbers of rickets cases, even among light-skinned infants and children. This is largely the result of parents strictly following the new dermatological guidelines. If you as a parent wish to follow these guidelines, then you must make sure that your infant or child takes vitamin D drops. The current recommended level is 200 IU/day. However, the level that was used in the past to prevent rickets was 400 IU/day. This may have been adequate when infants and children also received sun exposure, but if protected from such exposure, then even 400 IU/day may be inadequate.
Current research on vitamin D levels indicates that the minimum daily dose of vitamin D for all individuals should be 1000 IU. In the more northern latitudes of temperate zones such as Canada and northern Europe, 2000 to 4000 IU/day may be required to maintain adequate blood levels of vitamin D. Research in Hungary on children who had rickets found that even a daily dose of 1200 IU was not enough to prevent rickets.
Although I’ve talked a lot about rickets, there are many other health problems associated with inadequate vitamin D levels that have also been increasing in recent years and that will likely worsen if sun avoidance becomes the norm and no change in diet and/or supplementation of vitamin D occurs. African-American males have the highest rate of prostate cancer in the world. It strikes at a younger age and in a more severe form than is true for those in other groups. African-American females are also affected at a younger age and more severely by breast cancer, although they do not have the highest overall rate for this disease. In addition, African Americans have higher rates of hypertension and Type 2 diabetes than do most other populations. Individuals in this group have a range of skin colors from moderate to very dark. If you are in this group and follow the dermatological guidelines for yourself and your children, it is probable that your overall health and that of your children will worsen over time. In order to prevent a small number of skin cancers (skin cancer is rare in those with dark skin color) and some premature skin aging, more severe health problems, already occurring at high rates, will worsen.
SEASONAL AFFECTIVE DISORDER
Another problem associated with not getting enough sun exposure is Seasonal Affective Disorder (SAD). This condition is common in the more northern latitudes where sunlight is rare throughout a good portion of the year. Individuals suffering from SAD experience varying degrees of depression and lethargy: they tend to sleep longer, are less active, and are more irritable. These symptoms lessen or disappear when individuals are exposed to sunlight or full spectrum light boxes. Suicide rates are higher in the more northern latitudes where limited sunlight is most common and SAD is most frequent. For instance, those living in Greenland suffer from higher suicide rates than do those living in Denmark; Canada has higher adolescent suicide rates than does the United States; those living in the Northwest Territories of Canada have higher suicide rates than does the rest of Canada; and suicide and alcohol consumption rates are higher in northern than southern Europe.
Seasonal Affective Disorder appears to be associated with the season in which you are born. Research in Japan found that the majority of those with SAD were born in the winter-to-early-spring season, and that this was especially true for women. The highest rates of SAD were among women born in March. In other research, individuals with SAD were divided into two groups. One group received standard phototherapy (exposure to a full spectrum light box), while the other group received 100,000 IU of vitamin D. Those receiving phototherapy showed no significant improvement in measures of depression, but all those receiving vitamin D improved on all measures of depression.
If you are a woman living in a temperate zone like the United States and your baby is born in winter to early spring, you are giving birth at a time when your stored vitamin D levels are plummeting and when your baby will not get any UVB radiation exposure even if you take him out in the sun. It is likely that your baby will be born with low levels of vitamin D. If you don’t give your baby vitamin D drops, the amount of vitamin D in his blood will continue to decline and he could develop rickets or other health problems. Once summer comes, if you take him outside a lot, the amount of vitamin D in blood will go up. However, if you carefully follow the advice of dermatologists to avoid sun exposure and also do not give your baby vitamin D drops, the level of vitamin D in his blood will not improve and will only get worse. One of the health problems that could occur is that your baby will suffer from SAD as he grows up. If you or anyone in your family has seasonal depression (SAD), you may want to find out if they were born in the winter-to-early spring months and whether or not they get enough UVB exposure during the summer months. It appears that the best prevention for SAD is adequate exposure to UVB radiation.
Humans are adapted to sunlight and UVB radiation exposure. If we do not receive enough exposure, our health and well-being will suffer. If we receive too much exposure, we will also have health problems. A balance must be maintained and careful consideration given to the differing health issues caused by over-exposure versus under-exposure. Total avoidance of sun exposure is no better for our health, and may, in fact, be much worse, at least in temperate zones, than is total exposure. Skin color, the latitude at which you live, your daily activities, and your diet must all be considered when you determine what exposure is best for you. Balance is critical. Achieving the proper balance for you and any family members is going to require careful thought and time and effort on your part. Good health requires hard work.
© 2001-2009 Kathleen E. Fuller, PhD. All rights reserved.