The Effect of Vitamin D Supplementation During Caloric Restriction on Intestinal Calcium Absorption

The extent of change in the amount of calcium that is absorbed with an increase in vitamin D
supplementation is not known. This information is important for determining appropriate
vitamin D requirements for optimal calcium absorption. During caloric restriction in
postmenopausal women, we found that serum parathyroid hormone (PTH) explains 22% of the
variance for the reduced calcium absorption (equivalent to 400 IU of Vitamin D per day). It
is possible that higher dietary Vitamin D will increase serum 25-hydroxy-Vitamin D (25(OH)D)
levels, offset serum PTH and thereby prevent a reduction in calcium absorption. This study
seeks to determine if a high Vitamin D intake (a supplement of 2500 IU per day) can increase
true fractional calcium absorption (TFCA) in postmenopausal women on a standard
high-carbohydrate weight loss diet compared to weight maintenance, with the hypothesis that
a high Vitamin D intake will raise serum 25(OH)D and offset the decline in TFCA during
caloric restriction (vs. an increase in weight-stable women), and serum PTH will no longer
be a major factor explaining changes in TFCA.

Participants will be recruited for both weight loss and weight maintenance, and all will be
randomly assigned to take either 2500 IU per day vitamin D supplement or matching placebo.
All weight loss participants will attend 5-6 weekly counseling sessions (about 50 minutes
per session). All participants will be asked to take a daily vitamin/mineral supplement and,
depending on their usual food intake, they may be asked to take a calcium tablet to meet the
recommended intake throughout the study period. To measure calcium absorption before and
after the 6 weeks of weight loss, participants will go to the study site after an overnight
fast where an IV will be placed and the participant will receive an infusion of a stable
calcium isotope and consume a 4 oz beverage that also contains a stable isotope of calcium.
Blood will be drawn, and then the participant will be asked to collect all urine for the
next 24 hours. Body composition (fat, muscle mass, and bone mineral density) will be
measured by a dual-energy x-ray absorptiometry (DXA) machine and peripheral quantitative
computer tomography (pQCT).

Participants will only be recruited in the winter and spring of each year.