1.Decreased vitamin D synthesis
A. People with darker skin tend to release less D3from the skin
B. Excessive use of UV skin blockers.
C. Latitude – it has been suggested that the farther away from the equator, the lower the 25(OH)D – this has been disputed (Hagenau, et. al., Osteoporosis Int, 20; 133-140, 2009)
D. There is a seasonal change in 25(OH)D farther away from equator with the peak in the summer and nadir in the winter.
2. Decreased nutritional intake of vitamin D
3. Infants can have low vitamin D status if the mother is vitamin D deficient and the infant is not taking vitamin supplements.
4. GI malabsorption
5. Significant liver disease (e.g. cirrhosis) can decrease conversion of Vitamin D to 25(OH)D
(Source of some of this information - From PEDIATRICS Vol. 122 No. 2 August 2008, pp. 398-417)