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Written by Hanwen Feng

If there was a clear way to improve health with no medical costs, would you take it? Using vitamin D, through sunlight, the human body can do just that at no charge. Ultraviolet (UV) light from the sun assists the human body in obtaining necessary levels of vitamin D. The absorption of calcium through foods like dairy and leafy greens is mediated by vitamin D. Calcium levels have been shown to increase bone strength and maintain heart, muscles, and nerves

The vitamin D facilitated pathway for calcium absorption begins with pre-vitamin D molecules in the skin being chemically altered by UV light to cholecalciferol (vitamin D3). Cholecalciferol is transferred to the liver where it is transformed into calcifediol. From there, calcifediol is enzymatically altered in the kidneys into calcitriol. Calcitriol is the active form of vitamin D that humans use in the body for calcium absorption. Calcitriol is used to increase the genetic expression of calcium transporters whose purpose is to increase calcium concentration through absorption from the gut into the bloodstream. 

The pathway described above is the only way the body can increase calcium stores. Additionally, UV light is the easiest way to achieve calcium absorption as diets rich in calcium are hard to maintain. As a result, people who do not receive enough sunlight often need oral supplements of vitamin D to maintain healthier bones. For example, people in certain states, like Ohio, fall above the 37th parallel where sunlight is scarcer due to harsher winters. The question remains: what is the daily recommendation of vitamin D? There is variation depending on age. Individuals who are 19-70 years of age are recommended 1,000mg per day while those 71 years of age and older are recommended 1,200mg daily; there is a directly proportional relationship between age and vitamin D as elderly people require more to maintain their health. 

While it is important to discuss the biological background of vitamin D, it is also pertinent to incorporate the human adaptation perspective. The origin of our species, homo sapiens, originated in Africa approximately 200,000 years ago. Homo sapiens were not the first to leave Africa but were the first species to geographically disperse significantly out of the African continent. Adaptation occurred when homo sapiens became fitted to different environments. Through multi-generational adaptation, skin color became the controlling factor in the “vitamin D hypothesis” which states that maintaining vitamin D homeostasis propagated the skin color of homo sapiens to lighten when they reached areas of low UV exposure. 

Bipedalism had a direct effect on the way vitamin D drove skin pigmentation. The mobility of homo sapiens allowed exposure to new climates. Walking on two feet instead of quadrupedalism permitted usage of hands for tools, gathering of food, and longer stamina when walking. It is important to understand that without the additional leverage of bipedalism, humans could not have traveled far from their homes, exposing them to the stressor of climate.  

Climate exposure differences between populations within and outside of Africa evolutionarily pushed for differing quantities of melanin in the skin. Melanin decreases vitamin D absorption. At equatorial latitudes, having abundant melanin is not an issue due to abundant UV exposure. However, as our species experiences less UV exposure, the human body was not adjusted to lower vitamin D synthesis. As a result, humans with lighter skin developed to counter the UV blocking effect of melanin. The selection for skin color based on UV exposure is evident in modern populations where a wide spectrum of skin tones exist. 

Vitamin D is termed the “sunshine vitamin” as it naturally manifests health benefits and melanin level adaptations. Although skin pigmentation adaptations show the importance of Vitamin D, adaptations do not necessarily select for the best traits. Evolution depends on the current genetic pool and does not account for the spread of genetic information in populations. Then why did climate change drive a wide spectrum of skin color? It is possible that Vitamin D, being necessary for optimal health, caused a soft sweep of genetic skin diversity. However, even Vitamin D adaptations have their downside. Lighter skin assists in helping Vitamin D absorption, but it also leads to an increased chance of melanoma

Even with a seemingly perfect adaptation of melanin for Vitamin D absorption, there are still flaws. From an evolutionary perspective, it is difficult to view adaptations as a linear model of improvement. Rather, it is pertinent to encapsulate what natural selection is trading off for the benefits of certain traits.