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).

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 1200 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 1200 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.

• Weight Loss
• Bone Density

• Behavioral: Weight Loss
• Dietary Supplement: Vitamin D 1200 IU
• Behavioral: Weight Maintenance

• Shapses SA, Riedt CS. Bone, body weight, and weight reduction: what are the concerns? J Nutr. 2006 Jun;136(6):1453-6. Review.
• Riedt CS, Cifuentes M, Stahl T, Chowdhury HA, Schlussel Y, Shapses SA. Overweight postmenopausal women lose bone with moderate weight reduction and 1 g/day calcium intake. J Bone Miner Res. 2005 Mar;20(3):455-63. Epub 2004 Nov 29.
• Cifuentes M, Riedt CS, Brolin RE, Field MP, Sherrell RM, Shapses SA. Weight loss and calcium intake influence calcium absorption in overweight postmenopausal women. Am J Clin Nutr. 2004 Jul;80(1):123-30. Erratum in: Am J Clin Nutr. 2005 Jan;81(1):199.
• Goode LR, Brolin RE, Chowdhury HA, Shapses SA. Bone and gastric bypass surgery: effects of dietary calcium and vitamin D. Obes Res. 2004 Jan;12(1):40-7.

Inclusion Criteria:

• Postmenopausal women who are more than 2 years since last menses
• Obese or overweight
• Must live in the geographic vicinity of Rutgers University

Exclusion Criteria:

• Currently on any medication known to influence calcium or bone metabolism, including HRT, or with evidence of diseases known to influence calcium metabolism (i.e. metabolic bone disease, hyperparathyroidism, untreated thyroid disease, significant immune, hepatic, or renal disease, significant cardiac disease [i.e., heart attack or stroke in the past 6 months., abnormal EKG], active malignancy or cancer therapy within the past year)
• History of kidney stones
• Weight gain or weight loss (5% of body wt) within 3 months prior to recruitment
• Participation in other investigational studies during the 12-month study period
• Travel for longer than 2 consecutive weeks during the study period
• Usually have a very high or low intake of calcium (more than 1500 or less than 500 mg per day)