There is currently no firm evidence that vitamin D supplementation decreases or increases cancer occurrence in predominantly elderly community-dwelling women .
Vitamin D₃ supplementation  decreased cancer mortality  and vitamin D supplementation  decreased all-cause mortality , but these estimates are at risk of type I errors due to the fact that too few participants were examined , and to risks of attrition bias originating from substantial dropout of participants .
Combined vitamin D₃ and calcium supplements  increased nephrolithiasis , whereas it remains unclear from the included trials whether vitamin D₃ , calcium , or both were responsible for this effect .
BACKGROUND The evidence on whether vitamin D supplementation is effective in decreasing cancers is contradictory .
OBJECTIVES To assess the beneficial and harmful effects of vitamin D supplementation for prevention of cancer in adults .
Evidence that vitamin D3 increases serum 25-hydroxyvitamin D more efficiently than does vitamin D2.
In all species tested , except humans , biological differences between vitamins D2 and D3 are accepted as fact . To test the presumption of equivalence in humans , we compared the ability of equal molar quantities of vitamin D2 or D3 to increase serum 25-hydroxyvitamin D [ 25(OH)D ] , the measure of vitamin D nutrition .  Subjects took 260 nmol ( approximately 4000 IU ) vitamin D2  ( n=17 ) or vitamin D3  ( n=55 ) daily for 14 d. 25(OH)D was assayed with a method that detects both the vitamin D2 and D3 forms . With vitamin D3 , mean ( + /-SD ) serum 25(OH)D increased from 41.3+/-17.7 nmol/L before to 64.6+/-17.2 nmol/L after treatment . With vitamin D2 , the 25(OH)D concentration went from 43.7+/-17.7 nmol/L before to 57.4+/-13.0 nmol/L after . The increase in 25(OH)D with vitamin D3  was 23.3+/-15.7 nmol/L , or 1.7 times the increase obtained with vitamin D2 ( 13.7+/-11.4 nmol/L ; P=0.03 ) . There was an inverse relation between the increase in 25(OH)D and the initial 25(OH)D concentration . The lowest 2 tertiles for basal 25(OH)D showed larger increases in 25(OH)D : 30.6 and 25.5 nmol/L , respectively , for the first and second tertiles . In the highest tertile [ 25(OH)D > 49 nmol/L ] the mean increase in 25(OH)D was 13.3 nmol/L ( P < 0.03 for comparison with each lower tertile ) . Although the 1.7-times greater efficacy for vitamin D3 shown here may seem small , it is more than what others have shown for 25(OH)D increases when comparing 2-fold differences in vitamin D3 dose . The assumption that vitamins D2 and D3 have equal nutritional value is probably wrong and should be reconsidered
There is currently no firm evidence that vitamin D supplementation decreases or increases cancer occurrence in predominantly elderly community-dwelling women .
Vitamin D₃ supplementation  decreased cancer mortality  and vitamin D supplementation  decreased all-cause mortality , but these estimates are at risk of type I errors due to the fact that too few participants were examined , and to risks of attrition bias originating from substantial dropout of participants .
Combined vitamin D₃ and calcium supplements  increased nephrolithiasis , whereas it remains unclear from the included trials whether vitamin D₃ , calcium , or both were responsible for this effect .
BACKGROUND The evidence on whether vitamin D supplementation is effective in decreasing cancers is contradictory .
OBJECTIVES To assess the beneficial and harmful effects of vitamin D supplementation for prevention of cancer in adults .