Vitamin D is one of the most important underlying factors for human pigmentation. We humans need regular supplies of this vitamin, which is seldom found in diet (fish essentially) and therefore Nature has provided us with sort of a photosynthetic skin, which generates the essential nutrient thanks to the energy of the Sun.
That is why humans loss partly their tan as they migrated northwards, where ultra-violet radiation is scarce, at least in winter.
I have on occasion discussed at my old blog Leherensuge several newly found roles of this critical vitamin besides bone formation: brain development, immune system and cholesterol. I have also mentioned a reason for the dark skin dominant in tropical peoples besides sunburns and skin cancer (vitamin B9, folate).
Well, there is yet another reason for the critical importance of vitamin D in our metabolism and coloring, though maybe not too different from one of the above (immune system): an specific defense response has been linked to vitamin D levels:
Olof Hertting et al., Vitamin D Induction of the Human Antimicrobial Peptide Cathelicidin in the Urinary Bladder. PLoS ONE 2010. Open access.
The urinary tract is frequently being exposed to potential pathogens and rapid defence mechanisms are therefore needed. Cathelicidin, a human antimicrobial peptide is expressed and secreted by bladder epithelial cells and protects the urinary tract from infection. Here we show that vitamin D can induce cathelicidin in the urinary bladder. We analyzed bladder tissue from postmenopausal women for expression of cathelicidin, before and after a three-month period of supplementation with 25-hydroxyvitamin D3 (25D3). Cell culture experiments were performed to elucidate the mechanisms for cathelicidin induction. We observed that, vitamin D per se did not up-regulate cathelicidin in serum or in bladder tissue of the women in this study. However, when the bladder biopsies were infected with uropathogenic E. coli (UPEC), a significant increase in cathelicidin expression was observed after 25D3 supplementation. This observation was confirmed in human bladder cell lines, even though here, cathelicidin induction occurred irrespectively of infection. Vitamin D treated bladder cells exerted an increased antibacterial effect against UPEC and colocalization to cathelicidin indicated the relevance of this peptide. In the light of the rapidly growing problem of resistance to common urinary tract antibiotics, we suggest that vitamin D may be a potential complement in the prevention of UTI.