excessive intake. In addition, reliable data on the practice and impact of discretionary fortification on the part of food manufacturers is lacking.

1. Enhance dietary assessment methods and comparability for calcium and vitamin D intake, and methods for the measurement of calcium and vitamin D in foods and supplements. Methods related to dietary assessment have come far in recent years, and research in this area should continue. DRI development as it pertains to the North American population would benefit from targeted efforts to strive for comparability between the U.S. and Canadian surveys.

2. Investigate food and supplement sources of calcium and vitamin D for bioequivalence, bioavailability, and safety. The ability to assess whether different fortification delivery systems and food production methods affect the factors such as bioavailability or safety for both calcium and vitamin D is an important component of dietary intake assessment. Information on the practice of discretionary fortification by food manufacturers is needed.

3. Improve the standardization of the assay for serum 25OHD. Currently, different assays for the determination of serum 25OHD levels are in use, and they provide disparate results. In turn, reported measures are confounded by the need to understand the assay used and research reports contain results that are not readily compared. The role of standard reference materials and inter-laboratory collaboration is an important aspect of overcoming the challenges that the assay methodologies present.

Clinical practice was outside the scope of this committee convened to develop DRIs, which was tasked primarily with describing a distribution of requirements and upper levels of intake. However, as noted in Chapter 8, the cut-point levels of serum 25OHD intended to specify deficiency and sufficiency for the purposes of interpreting laboratory analyses and for use in clinical practice have been subject to a wide variation in specification without a systematic, evidence-based consensus development process. The importance of this specification to both the well-being of the North American population and to ensuring that the population is confident in their health and nutriture results in the committee calling attention to this research need. Its broad impact requires that it be addressed by a coalition of stakeholders under the auspices of a science-based organization such as the National Institutes of Health in conjunction with equivalent science-based organizations in Canada.