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there was a marked difference in the prevalence of rickets: 85/1,000 and 55/1,000 in two communities, but none in the third. No clear explanation for the differing prevalence was obtained by the investigator (Smith, 1999).

Taken as a whole, the limited data surrounding indigenous Canadian populations suggest a basis for concern regarding vitamin D nutriture, most notably in the likelihood that typical diets are changing from traditional foods to more westernized foods. Although the assumption of minimal sun exposure underpinning the DRI values may not entirely align with this group of people who may experience considerable sun exposure in the summer, ensuring that the diet meets the DRI values should provide assurances that risk of vitamin D deficiency has been greatly reduced.

Use of Calcium Supplements

The forms and nature of calcium supplements have been discussed in Chapter 2, and their possible role in kidney stone formation as well as the emerging data regarding possible adverse cardiovascular effects have been outlined in Chapter 6. The mechanisms for differential effects of food sources and supplement forms of calcium on kidney stone formation are complex and may relate to the timing of calcium administration. Approximately 80 percent of kidney stones contain calcium combined with oxalate or, less often, phosphate (Park and Pearle, 2007). Calcium in food or in supplements taken with food is believed to bind to dietary oxalate in the digestive tract, reducing the absorption and subsequent urinary excretion of oxalate and thus risk for kidney stones (urinary oxalate may be more critical than urinary calcium with respect to calcium oxalate crystallization) (Curhan et al., 1997). When calcium supplements are not taken with food, dietary oxalate is absorbed unopposed and thus is more available for stone formation. Although dairy foods, which are the major source of calcium in much of North America, have been suggested to contain an unidentified protective compound not found in supplements (Curhan et al., 1997), this possibility has not been well studied. Obtaining sufficient calcium via dietary sources is the preferred strategy—and it remains uncertain as to whether taking calcium supplements with food may reduce the likelihood of stone formation associated with supplement use. Head-to-head comparisons of different calcium supplement formulations with respect to risk for kidney stone formation are also lacking. In any case, given the desirability of not surpassing the UL for calcium intake and given that even those not meeting their requirement for calcium are nonetheless consuming some calcium from dietary sources that range from breads to dairy products, care must be taken in selecting a calcium supplement that when combined with dietary intake does not result in a total intake above the UL. The UL for a sizable proportion of the population, including groups that commonly



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