of vitamin D per day raised serum 25OHD level (by 14 nmol/L) and did not significantly increase calcium absorption compared with placebo.⁴

In a recent study, Aloia et al. (2010) performed a single-isotope assay of intestinal calcium absorption in 492 white and black women ages 20 to 80 years. They tested whether serum 25OHD or calcitriol level predicted the rate of intestinal calcium absorption in a multivariate model that included age, menopausal status, calcium intake, and other factors. The serum 25OHD levels ranged from 30 to 150 nmol/L, and were 51.62 ± 33.67 nmol/L overall or 32.87 ± 21.20 nmol/L in blacks and 67.73 ± 34.11 nmol/L in whites. Whereas calcitriol level was an important predictor of intestinal calcium absorption in the final model, 25OHD level had no effect. The authors concluded that serum 25OHD level is not an indicator of intestinal calcium absorption efficiency by itself, but 25OHD does interact at low levels with calcitriol to predict calcium absorption.

Overall, the data are mixed, but most studies show no increase in intestinal calcium absorption across a broad range of serum 25OHD levels. The single-isotope study by Need et al. (2008) indicates no increase in fractional calcium absorption above 20 nmol/L. The single-isotope studies by Heaney et al. (2003), Kinyamu et al. (1998), and Aloia et al. (2010) indicate no change in fractional calcium absorption across higher ranges of 25OHD levels—specifically, from 60 to 154 nmol/L in Heaney et al. (2003), from 50 to 116 nmol/L in Kinyamu et al. (1998), and from 30 to 150 nmol/L in Aloia et al. (2010). Others (Francis et al., 1996; Patel et al., 2001; Zhu et al., 2008a, b) demonstrate no effect on absorption of increasing the serum 25OHD concentrations by 14 to 36 nmol/L, whereas the Hansen et al. (2008) study indicates a 3 percent increase in absorption after raising the serum 25OHD level from 55 to 160 nmol/L in the short-term (15 days).

The data currently suggest that fractional calcium absorption reaches a maximum between 30 and 50 nmol/L in both children and adults. A value of 50 nmol/L allows for some uncertainty in the data and a buffer against seasonal and dietary variations in calciferol intake that, in turn, cause fluctuations in serum 25OHD levels.

Bone accretion Bone accretion resulting in bone growth and skeletal development occurs during the younger life stages. Measures of the amount of calcium needed to achieve normal bone accretion as well as the levels of vitamin D that support accretion are, therefore, relevant considerations. The topics of pregnancy and lactation among adolescent girls, who are still accruing bone tissue, are discussed in other sections below jointly with pregnancy and lactation among women.