In Canada, Faulkner et al. (2000) surveyed milk samples and found that 20 percent of skim milk, 40 percent of 2 percent fat milk, and 20 percent of whole milk, contained the recommended level of vitamin D. Samples collected by the Canadian Food Inspection Agency from 1999 through 2009 and analyzed for vitamin D indicated that during the last 4 years of sample collection, 47 to 69 percent were within the range specified by regulation (personal communication, S. Brooks, Health Canada, April 30, 2010). In addition, over the past 5 years, the average vitamin D content of analyzed milk samples fell within this range. Over time, manufacturers in the United States have added vitamin D to other foods, and the food industry is increasingly marketing foods fortified with vitamin D (Yetley, 2008). Based on data from a U.S. Food and Drug Administration (FDA) survey that provides information on the labels of processed, packaged food products in the United States, Yetley (2008) reported that almost all fluid milks, approximately 75 percent of ready-to-eat breakfast cereals, slightly more than half of all milk substitutes, approximately one-quarter of yogurts, and approximately 8 to 14 percent of cheeses, juices, and spreads are fortified with vitamin D in the U.S. market. Many product labels included in the survey indicated that the form of added vitamin D was vitamin D3. However, some milk substitutes are fortified with vitamin D2 Cereal labels did not specify the form of added vitamin D. Levels of vitamin D ranged from 40 IU per regulatory serving for cereals and cheeses to 60 IU per regulatory serving for spreads and 100 IU per regulatory serving for fluid milk. Several food categories had within-category ranges of 40 to 100 IU of vitamin D per regulatory serving. Serum vitamin D and 25OHD have low penetrance into breast milk, together comprising 40 to 50 IU of antirachitic activity per liter, most of which is contributed by 25OHD (Leerbeck and Sondergaard, 1980; Hollis et al., 1981; Reeve et al., 1982; Specker et al., 1985). Data from the USDA report the vitamin D content of human milk to be 4.3 IU/100 kcal.2 However, the vitamin D biological activity may be higher than the analyzed values, because human milk contains small amounts of 25OHD in addition to vitamin D3 (Reeve et al., 1982); further, the biological activity of 25OHD is approximately 50 percent higher than that of vitamin D (Blunt et al., 1968).
Recently, the National Center for Health Statistics (NCHS) posted an analytical note on its NHANES web page informing users about two issues that should be addressed when analyzing and using serum 25OHD data from NHANES.10 The first involved making direct comparisons between serum 25OHD levels from NHANES III (1988 to 1994) and those from the 2000 to 2006 NHANES because of a reformulation of the DiaSorin radioimmunoassay (RIA) kit11 that resulted in shifts in assay results between the two time periods. Second, the note also cautioned that the data from the 2000 to 2006 NHANES were likely affected by drifts in the assay performance (method bias and imprecision) over time. A standard reference material (SRM 972, Vitamin D in Human Serum) and calibration solution (SRM 2972, 25-Hydroxyvitamin D2 and D3 Calibration Solutions) are currently available from the NIST. The NCHS plans to incorporate use of this SRM into future measures of 25OHD in NHANES. The web page also indicates that guidance concerning a correction factor and data interpretation regarding this shift and drift will be forthcoming from the agency. In the case of the Canadian survey of 25OHD levels in the Canadian population, the analytical methods used to determine 25OHD concentrations were the Diasorin total 25OHD (Liaison) kit12 (personal communication, S. Brooks, Health Canada, December 18, 2009). 1e1e36bf2d