Conclusion
Skin pigmentation has continuously evolved alongside humans. Genetic selection for lighter skin coincides with a favorable selection for genes involved in vitamin D synthesis as humans migrated to northern latitudes, which enabled humans to produce adequate levels of exogenous vitamin D in low-UVR areas and in turn promoted survival. Early humans without access to supplementation or foods rich in vitamin D acquired vitamin D primarily through sunlight. In comparison to modern society, where vitamin D supplementation is accessible and human lifespans are prolonged, lighter skin tone is now a risk factor for malignant cancers of the skin rather than being a protective adaptation. Current sun behavior recommendations conclude that the body’s need for vitamin D is satisfied by UV exposure to the arms, legs, hands, and/or face for only 5 to 30 minutes between 10 am and 4 pm daily without sunscreen.42-44 Approximately 600 IU of vitamin D supplementation daily is recommended in a typical adult younger than 70 years to avoid deficiency. In adults 70 years and older who are not receiving adequate sunlight exposure, 800 IU of daily vitamin D supplementation is recommended.45
The hypothesis that skin lightening primarily was driven by the need for vitamin D can only be partially supported by our review. Studies have shown that there is a corresponding complex network of genes that determines skin pigmentation as well as vitamin D synthesis and conservation. However, there is sufficient evidence that skin lightening is multifactorial in nature, and vitamin D alone may not be the sole driver. The information in this review can be used by health care providers to educate patients on sun protection, given the lesser threat of severe vitamin D deficiency in developed communities today that have access to adequate nutrition and supplementation.
Skin lightening and its coinciding evolutionary drivers are a rather neglected area of research. Due to heterogeneous cohorts and conservative data analysis, GWAS studies run the risk of type II error, yielding a limitation in our data analysis.9 Furthermore, the data regarding specific time frames in evolutionary skin lightening as well as the intensity of gene polymorphisms are limited.1 Further studies are needed to determine the interconnectedness of the current skin-lightening theories to identify other important factors that may play a role in the process. Determining the key event can help us better understand skin-adaptation mechanisms and create a framework for understanding the vital process involved in adaptation, survival, and disease manifestation in different patient populations.