system
Given the following post, respond as if you're commenting on Reddit:
human
Is there any advantage to not creating melanin? Why don't our bodies stay tan?

I know that melanin is supposed to help suppress the sun's rays, but why does your body stop producing it when not consistently in sunlight?
gpt
Vitamin D- or more specifically, the conversion of a Vitamin D precursor to a useable form in the body.  UV light triggers this conversion; melanin blocks UV light, so more melanin means less active Vitamin D production.  The body strikes a balance between avoiding sunburn and avoiding Vitamin D deficiency.  That's why people of an ethnicity that evolved in an area with attenuated sunlight magnitude have lighter skin.  They produce less melanin, since converting Vitamin D is more of an issue than getting sunburned.  This also occurs with increased production of melanin due to tanning.  The body is receiving more than enough UV radiation to convert its Vitamin D, and needs to protect itself from burning.