Deficiency of Vitamin D worsens respiratory tract infections. This is thought to be one of the reasons why there are fewer Flu cases in the summer.
A metastudy in 2017 with over 10k participants found that daily vitamin D doses reduced the chance of contracting respiratory infections by 19%.
The skin of arctic tribes people are dark because they get all their vitamin D from fish instead of sunlight. Their skin does not need to lighten to try and get more sun for vitamin D production.
Vitamin D deficiency is linked with many health disorders, including depression. In a survey of almost 8,000 US residents, it was found that those with lower vitamin D levels “are at a significantly higher risk of showing depression”.