The investigators aim to recruit 60 women who have agreed to participate in an existing
randomized, double-blind, placebo-controlled trial (Treatment of Vitamin D Insufficiency, HSC
2009-0055). In this trial, the investigators propose to evaluate the associations between
magnesium intake, magnesium stores, fractional magnesium absorption and calcium homeostasis.
The investigators will stratify 60 subjects in the sub-study, ensuring that approximately 20
subjects are randomized into each treatment arm (placebo, low-dose and high-dose vitamin D).
The investigators already assess calcium homeostasis throughout the study by measuring bone
mineral density, fractional calcium absorption, serum and urine calcium levels, among other
tests. In the sub-study of 60 women, the investigators will evaluate whether habitually
higher magnesium intake increases or decreases fractional calcium absorption. The
investigators will evaluate the interplay between magnesium stores, vitamin D levels and
serum parathyroid hormone levels. The investigators will directly assess magnesium stores
using serum magnesium and 24-hour urine magnesium levels, and will measure magnesium
absorption using the dual stable isotope approach.