The Effect of Vitamin D Supplementation During Caloric Restriction on Intestinal Calcium Absorption
Condition: Weight Loss; Vitamin D or Placebo
MeSH terms: Weight Loss
Study type: Interventional

Summary: 
      The purpose of this study is to learn how the amount of vitamin D supplementation influences
      intestinal fractional calcium absorption (a measure of the amount of calcium absorbed).
    

Detailed description: 
      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.
    

Intervention name(s): Weight Loss; Vitamin D 2500 IU; Weight Maintenance
Intervention type(s): Behavioral; Dietary Supplement; Behavioral
Intervention description(s): Diet for weight loss for 6 weeks; all weight loss participants will attend 6 weekly counseling sessions (about 50 minutes per session); Daily Vitamin D 2500 IU supplement for 6 weeks; Diet for weight maintenance for 6 weeks
Intervention MeSH terms: Vitamin D
Allocation: Randomized
Intervention model: Parallel Assignment
Masking: Quadruple (Participant, Care Provider, Investigator, Outcomes Assessor)
Primary purpose: Treatment
Keywords: bone mass; calcium absorption; bone quality; fracture risk