There are few vitamins that have received as much attention in the scientific literature, as well as in the mainstream press, as vitamin D. Its only contender may be vitamin C, the compound that’s found in fruits and vegetables such as lemons and oranges. Vitamin D differs from other nutrients that we humans need to take in in order to function at our best in that it is produced in our skin as a result of sun exposure. It’s also found in some foods; however, sunlight is generally considered to be our most important source of this vitamin, which acts as a hormone inside our bodies.
One of the reasons why vitamin D has received so much attention is that it plays a crucial role in many of the processes that take place inside the human body. A body that is devoid of vitamin D is a frail body: its more susceptible to immune disturbances, infections, and disease than a body that is well-stocked with this powerful compound, given that all other variables are equal. Hence, we clearly have a problem if a lot of people develop a vitamin D deficiency. Unfortunately, this is exactly what has happened in modern times…
The ground zero for the evolution of our kind is Africa. That’s where it’s believed our tree-dwelling ancestors lived, and it’s also where many of the major evolutionary processes of our genus took place. Some member of now extinct hominin species did venture into other parts of the world; however, it is safe to say that Africa is the continent that has played the most important role in shaping the evolution of the human lineage.
Hence, it goes without saying that much of our evolution took place in a very sunny environment. It wasn’t until some 60.000 years ago that members of our species migrated out of Africa and started colonizing the far corners of the world. One of the problems that these early migrators faced was the lack of sunlight in places such as Europe. The skin pigmentation that worked well in Africa didn’t work so well in a cold habitat in which little sunlight appeared.
When compared to a person with light skin, someone with dark skin requires a lot more sun exposure to keep the cellular processes of his body working at a peak level. Hence, it’s not surprising that there has been selection for lighter skin in areas of the world that are far away from equator and where sunlight is scarce.
Most contemporary humans, including public health authorities, unfortunately pay little to no attention to these things and are unaware of the fact that our sunlight/vitamin D requirements were determined in the past as a result of the environmental pressures that acted upon our ancestors.
Most contemporary humans have suboptimal levels of vitamin D circulating in their bodies
In the process of transforming our environment, we’ve stepped out of the milieu in which we evolved to live. As opposed to our primal ancestors, who spent all of their time outside, most of us who live in the modern industrialized world spend the majority of our lives inside closed buildings and vehicles, with the consequence that we are rarely exposed to the sun.
Furthermore, global travel has produced many mismatches in space. Some dark-skinned people have moved to northern countries, such as Norway, where I live, a country where the sun rarely shines, particularly during the winter. The opposite is also true; many people with light skin frequently travel on vacation to sunny parts of the world, where the sun shines brightly, “forget” that their skin is not accustomed to intense sun exposure, and as a result, get burned and eventually wrinkled.
Combine all of that with the many misguided recommendations that are put out by certain public health agencies and authorities saying that no sun exposure is better than some sun exposure, and it shouldn’t come as a surprise that a lot of contemporary humans are exposed to suboptimal amounts of sunlight. In particular insufficient exposure has become a widespread issue, with recent reports indicating that nearly 1 billion people worldwide now have a vitamin D deficiency (<20 ng/mL) or insufficiency (20-30 ng/mL) (1). The figures are particularly concerning with respects to dark-skinned people, with as many as 95% of African Americans having a low level of vitamin D (2). If we add all of the people who have a lower than optimal concentration of vitamin D circulating in their bodies, but not so low that it’s strictly classified as a vitamin D deficiency or insufficiency, the 1 billion number would undoubtedly increase.
Recent research has shown that traditionally living people in East Africa, more specifically the Maasai and the Hadza, have a mean serum 25-hydroxyvitamin D concentration of 115 nmol/l (~46 ng/mL) (3); a concentration that is markedly higher than the one found in most westerners. It’s comparable to the levels measured in Caucasian lifeguards and Hawaiians who spend a lot of time in the sun (3). Whether the mean vitamin D concentration of traditional African populations represents the optimal level for all people is questionable; however, research has shown that it is aligned with the level that is desirable with respect to the prevention of many diseases and health problems, including cancer (4).
Vitamin D deficiency and the diseases of civilization
The consequences of the above problems are multitudinous. Vitamin D plays an important role in many of the processes that take place inside the human body, including several related to immunity, cellular regeneration, bone tissue turnover, and gut homeostasis. There is little doubt in my mind that the vitamin D/sunlight deficiency pandemic has contributed to driving the increased global incidence of many of the diseases of civilization. With that said, it’s important to point out that the results from the studies in this area don’t all point in the same direction. Many studies have indeed found an association between vitamin D levels and various adverse health events (5, 6), but not all (7). This is to be expected, as it can be difficult to assess single nutrient-health relationships.
One important step that we can take to correct this major public health problem is to bring principles of Darwinian medicine into the medical system and the public’s mind. By acknowledging that our vitamin D/sunlight requirements were adjusted over evolutionary time in response to changes in the environments in which we evolved, we can understand how much sun exposure different bodies need in order to function according to their evolutionary design.
In many western countries, the idea that no sun exposure is better than some sun exposure has been allowed to spread, with the result that a lot of people fear the sun. This now common belief has no evolutionary support. It’s obviously wise to avoid getting burned; however, it’s not wise to completely avoid the sun. We have evolved a need for sunlight. It’s true that we can get vitamin D from other sources, such as fatty fish and supplements; however, the sun is in many ways the best source.
Most people in the world today don’t eat much vitamin-D rich foods, and even if they did, they would have to consume fairly large quantities of them to obtain a lot of vitamin D. Moreover, it’s important to remember that vitamin D isn’t the only thing we get as a result of exposure to the sun. Regular sun exposure has been linked to numerous health benefits, including improvements in mood and cognitive functioning. 3, 5
Another often overlooked issue has to do with the dangers of sun lotions. There’s little doubt that sunscreens are useful in some instances; however, I would argue that they are overused. Sunscreens contain many chemicals that the human skin has little or no evolutionary experience with. The evolutionary health model predicts that regular exposure to these compounds could elicit a range of harmful health effects. A growing body of research indicates that this prediction holds true (8, 9, 10). If you have the choice between covering up your body with clothes in order to avoid getting burned and lashing on a concoction of chemicals on your skin that protect it from sun damage, the former option is definitely the preferred one.
Finally, it’s important to point out that chronic inflammation, as well as conditions such as diabetes and kidney disease, may inhibit the absorption of vitamin D found in foods, as well as the production of vitamin D from sunlight. This is very important to acknowledge, seeing as many contemporary humans are chronically inflamed and sick, in part because they don’t exercise, consume unhealthy foods, and/or otherwise expose their bodies to novel environmental stimuli that initiate inflammatory reactions. Hence, these people may have to cool down their bodies in order to obtain the full benefits that the sun and vitamin D rich foods have to offer.
The bottom line is that we have to start paying attention to our evolutionary heritage. Only then can we fully understand what it takes to build a robust, well-nourished human.