97.5 percent of the population) causes a closer examination of the level of intake appropriate for an RDA value.
For this life stage group (> 70 years), the reduction in fracture risk is the most important indicator of interest, not only because of the actual event, but also because of the high mortality and morbidity associated with fractures. The factors that may have an impact on fracture risk range from functional status to neurological, metabolic, and physical determinants. Such factors enhance uncertainties about vitamin D nutriture. Changes such as impaired renal function, less efficient synthesis of vitamin D in skin, lower endogenous production of active vitamin D, increased PTH as well as age-related changes in body composition affect the daily requirement of vitamin D. Moreover, a sizeable proportion of this population can be categorized as frail compared with other age groups, and the concerns for bone health are increased. Factors of increased institutionalization also come into play. Although there is insufficient evidence to point to any one of these factors as a contributor to increasing the variability at which 97.5 percent coverage of the population occurs, when taken as a group of unknowns, it would be inappropriate to ignore the concern when considering the level of vitamin D commensurate with an RDA for this group.
For this reason, the level of uncertainty should be taken into account during the specification of the RDA for vitamin D for persons more than 70 years of age. There are very few data that are relevant to adjusting for such uncertainty. There are no dose–response data that would allow comparisons for adults more than 70 years of age regarding the effects of intakes of 600 IU of vitamin D per day with that of a higher level of intake such as 800 or 1,000 IU/day. Moreover, the evidence for fracture risk in relation to vitamin D intake for this older life stage is confounded by study protocols that do not allow separation of the effect of calcium from vitamin D; as discussed previously there is reasonably compelling evidence that calcium alone in this age group can modestly reduce the risk of fracture. Therefore, it is not surprising that the inclusion of calcium with vitamin D treatment generally, albeit not consistently, reduces the risk of fractures among the oldest adults, especially when vitamin D nutriture is considered in the context of serum 25OHD concentrations (Tang et al., 2007; Avenell et al., 2009; AHRQ-Tufts, Tang et al., 2007). Even the 10 trials that examined vitamin D alone (Lips et al., 1996; Peacock et al., 2000; Meyer et al., 2002; Trivedi et al., 2003; Avenell et al., 2004; Harwood et al., 2004; Grant et al., 2005; Law et al., 2006; Lyons et al., 2007; Smith et al., 2007), when pooled by Avenell et al. (2009), showed no statistically significant effect on fracture risk. As shown in Table 5-5, which is focused on studies with subjects more than 70 years of age and vitamin D intakes as opposed to serum 25OHD concentrations, such studies are generally non-significant for fracture risk on the basis of both vitamin D alone and vitamin D with