Vitamin D

After Vitamin D has been synthesized (this is the skin's response to sunlight), in the kidneys/liver it is converted to its active form. Active vitamin D regulates Ca2+ metabolism. In the gut, Vitamin D encourages Ca2+ uptake and simulatenously reducing the amount of reducing the excretion of Ca2+ in the kidneys. 

 
Molecular Biology of the Cell, 5th ed. B Alberts, A Johnson, J Lewis, M Raff, K Roberts, P Walter. Published by Gardland Science, Taylor & Franci Group, New York, 2008.