vitamin D from food sources are somewhat more than those in the United States. This may be due to the Canadian food supply having mandatory fortification of margarine with vitamin D in addition to fortification of milk.

Further, differences in serum 25OHD concentrations between the United States and Canada are evident. The estimates for Canadians are consistently higher than those for the United States. Although differences in the food supply may account for some of these differences, it is noted that the analyses for the Canadian data are based on the use of the “Liaison” kit,¹⁷ whereas the U.S. data are derived from the “DiaSorin RIA” kit.¹⁸ Direct comparison of the two kits within the CHMS laboratory at Health Canada indicates a 6 to 9 percent difference, with the Liaison measuring values higher than the RIA kit.¹⁹ Other researchers have also performed comparisons with various outcomes. The differences may be laboratory-specific because Wagner et al. (2009) found no difference, although data from Carter et al. (2010) suggest a 5 percent bias, with the RIA kit giving higher values. It is notable that the serum 25OHD levels in Canada are not lower than those in the United States, as would be predicted if higher latitudes were responsible for reduced serum 25OHD levels.

Finally, Appendix I contains information about the proportion of per-