interest. Need et al. (2008) studied fractional calcium absorption in 319 men (66 ± 10 years) with serum 25OHD levels less than 40 nmol/L. Fractional calcium absorption was 0.36 in men with the lowest quartile serum 25OHD levels (< 10 nmol/L) and rose significantly to 0.56 in the second quartile (11 to 20 nmol/L). No further change in fractional calcium absorption occurred with 25OHD levels of 21 to 30 or 31 to 40 nmol/L (Need et al., 2008). Kinyamu et al. (1998) performed a cross–sectional study of 376 healthy women (71 ± 4 years) and compared calcium absorption in those who took vitamin D supplements with that in non-supplemented women. Serum 25OHD level was significantly higher in women who took vitamin D (87.9 ± 28.2 nmol/L vs. 73.6 ± 23.0 nmol/L), whereas fractional calcium absorption did not differ between the two groups (Kinyamu et al., 1998). In addition, Devine et al. (2002) used a single isotope of calcium in a study of 120 older women and plotted a linear relationship between intestinal calcium absorption and serum 25OHD level. Intestinal calcium absorption rose from 35 percent at a mean serum 25OHD level of 15 nmol/L to 50 percent at 150 nmol/L. However, there were few data points at any serum 25OHD level, and an alternative fit to the data suggested an increase to 50 nmol/L and a plateau thereafter.

Hansen et al. (2008) studied 18 postmenopausal women before and after 15 days of supplementation with 50,000 IU of vitamin D₂ daily. Serum 25OHD level rose markedly from 55 ± 10 nmol/L to 160 ± 53 nmol/L (p < 0.001), while fractional calcium absorption changed only modestly from 24 ± 7 percent at baseline to 27 ± 6 percent after vitamin D repletion (p < 0.04), indicating that the large rise in serum 25OHD levels was statistically significant, as was the small rise in intestinal calcium absorption. The 3 percent absolute increase in fractional calcium absorption was considered by the authors to be a minor increment given the large increase in serum 25OHD level (Hansen et al., 2008).

In a randomized controlled study, postmenopausal women with a mean baseline serum 25OHD level of 44 nmol/L receiving 1,000 mg of calcium citrate daily were randomized to daily placebo or 1,000 IU of vitamin D₂ (Zhu et al., 2008a). Using a single-isotope method, this research group found a 36 nmol/L rise in serum 25OHD level and no increase in calcium absorption. This outcome is consistent with findings from their longer-term study (over 5 years), also demonstrating no rise in absorption (Zhu et al., 2008a). In both placebo and vitamin D groups, the calcium absorption decreased compared with baseline, most likely as a result of greater calcium intake (1 g calcium supplementation in both treatment arms). Also, Francis et al. (1996) found that 500 to 1,000 IU of vitamin D₂ did not increase calcium absorption in elderly women. In a randomized double-blind controlled pilot trial in women (mean age = 57 years; dual-isotope method) with a baseline serum 25OHD level of 52 nmol/L, it was found that 400 IU