EVIDENCE THAT VITAMIN-D-3 INCREASES SERUM 25-HYDROXYVITAMIN-D MORE EFFICIENTLY THAN DOES VITAMIN-D-2

In all species tested, except humans, biological differences between v itamins D-2 and D-3 are accepted as fact. To test the presumption of e quivalence in humans, we compared the ability of equal molar quantitie s of vitamin D-2 or D-3 to increase serum 25-hydroxyvitamin D [25(OH)D ], the measure of vitamin D nutrition. Subjects took 260 nmol (approxi mate to 4000 IU) vitamin D-2 (n = 17) or vitamin D-3 (n = 55) daily fo r 14 d. 25(OH)D was assayed with a method that detects both the vitami n D-2 and D-3 forms. With vitamin D-3, mean (+/-SD) serum 25(OH)D incr eased from 41.3 +/- 17.7 nmol/L before to 64.6 +/- 17.2 nmol/L after t reatment. With vitamin D-2, 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 2 5(OH)D with vitamin D-3 was 23.3 +/- 15.7 nmol/L, or 1.7 times the inc rease obtained with vitamin D-2 (13.7 +/- 11.4 nmol/L: P = 0.03), Ther e was an inverse relation between the increase in 25(OH)D and the init ial 25(OH)D concentration. The lowest 2 tertiles for basal 25(OH)D sho wed larger increases in 25(OH)D: 30.6 and 25.5 nmol/L, respectively, f or 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 less than or e qual to 0.03 for comparison with each lower tertile). Although the 1.7 -times greater efficacy for vitamin D-3 shown here may seem small, it is more than what others have shown for 25(OH)D increases when compari ng 2-fold differences in vitamin D-3 dose. The assumption that vitamin s D-2 and D-3 have equal nutritional value is probably wrong and shoul d be reconsidered.