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Calcium absorption increased 18 pct with high level of vitamin D – June 2012

Dose–Response Relationship between Vitamin D Supplementation and Calcium Absorption in Postmenopausal Women

Endocr Rev, Vol. 33 (03_MeetingAbstracts): MON-379
Copyright © 2012 by The Endocrine Society ENDO 2012: June 23-26, 2012, Houston, Texas
Vinod Yalamanchili, MBBS1, John C Gallagher, MD1 and Lynette M Smith, MS2
1 Bone Metabolism Unit, Creighton University Medical Center Omaha, NE
2 Department of Public Health, UNMC College of Public Health Omaha, NE

Background: 1,25 dihydroxyvitamin D (1,25 OH2D3) is the major hormone that controls intestinal calcium absorption. Serum 1,25 OH2D3 decreases when serum 25 hydroxyvitamin D (25OHD) falls below <10ng/ml but it is not known at what serum 25OHD level malabsorption of calcium can occur. Based on cross sectional data, one study suggested that a threshold for normal calcium absorption occurred at a serum 25OHD level of 30 ng/ml but in another study it was 5-10ng/ml. We performed a longitudinal study of different doses of vitamin D on absorption to measure the effect on calcium absorption and look for a threshold effect.Methods: 163 Caucasian women, ages 57-85 years, were randomized to different doses of vitamin D3 - 400, 800, 1600, 2400, 3200, 4000, 4800 IU/day or placebo. Calcium intake was increased to 1200-1400mg/day from an average of 691 mg/day. Main inclusion criteria was that subjects had serum 25OHD level < 20ng/ml. Exclusion criteria were medical illness or medications known to affect calcium absorption. Calcium absorption was measured at baseline and after 12 months using a single isotope method with 100 mg calcium and 5 microcuries Ca45, serum Ca45 was corrected for weight or BMI. Serum 25OHD and 1,25 OH2D3 were measured by immunoassay (Diasorin). We used multivariate regression that included age, dietary calcium intake, weight, baseline and final calcium absorption and serum 25OHD.

Results: Mean baseline serum 25OHD was 15.6 ng/ml (39nmol/L) and increased as a quadratic function plateauing at 45ng/ml on the 4800IU dose. In the longitudinal study, calcium absorption increased on vitamin D but was more highly correlated with final serum 25OHD (p=0.009) than with vitamin D dose (p=0.033).

The increase in calcium absorption was approximately 18 percent between serum 25 OHD levels of 15 to 60 ng/ml and there was no evidence of a plateau.

In the baseline analysis serum 25OHD was divided into 5ng/ml groups between 3-20ng/ml and as serum 25OHD decreased below 5ng/ml there was a decrease in serum 1,25 OH2D3 but not in calcium absorption.

Conclusions: The results show that vitamin D increases calcium absorption. The association with vitamin D dose was inconsistent and absorption was more clearly related to the final serum 25OHD level. No threshold level of serum 25OHD for calcium absorption was found at baseline or longitudinally suggesting that the threshold for decreased calcium absorption occurs at a very low serum 25OHD < 5ng/ml.

Nothing to Disclose: VY, JCG, LMS
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See also VitaminDWiki

released Nov 30, 2010
Supplements which combine vitamin D and Calcium (as of Jan-2010) = green diamonds
RDA of Vitamin D and Calcium is safe = green square

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