Vitamin D Status Is Positively Associated with Calcium Absorption among Postmenopausal Thai Women with Low Calcium Intakes

Background: Few data exist on the ability of postmenopausal women to absorb calcium from diets habitually low in calcium. Objective: The objective of this study was to evaluate fractional calcium absorption from a green leafy vegetable vs. milk in relation to vitamin D status. Methods: We measured fractional calcium absorption from both a dairy- and plant-based source in 19 postmenopausal Thai women (aged 52-63 y) with low calcium consumption (350 +/- 207 mg/d) in relation to serum parathyroid hormone (PTH) and serum 25-hydroxyvitamin D [25(OH)D] Fractional calcium absorption was measured using a triple stable calcium isotope method based on isotope recovery in a 28-h urine collection. Two extrinsically labeled test meals were ingested in random order: a green leafy vegetable (cassia) ingested along with Ca-43 or a glass of milk containing Ca-44. Women received intravenous Ca-42 with the first test meal. Results: In 19 postmenopausal women studied (mean age, 56.9 +/- 3.4 y), similar to 95% were 25(OH)D sufficient (20 Serum 25(OH)D status was positively correlated with fractional absorption from both cassia (P= 0.05, R-2 = 0.21) and milk (P = 0.03, R-2 = 0.26). Fractional calcium absorption from cassia was significantly lower than that measured from milk (42.6% +/- 12.3% vs. 47.8% +/- 12.8%, P = 0.03), but true calcium absorption did not significantly differ (120 35 mg/d vs. 135 36 mg/d). Serum PTH was significantly inversely associated with serum 25(OH)D (P = 0.006, R-2 = 0.37) even though PTH was not elevated (>65 pg/mL). Conclusions: These findings suggest that vitamin D status is an important determinant of calcium absorption among Thai women with low calcium intakes, and cassia may be a readily available source of calcium in this population. Furthermore, these data indicate that serum 25(OH)D concentrations may affect PTH elevation in postmenopausal women with low calcium intakes. This trial was registered at www.clinicaltrials.gov as NCT02291315.