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Location: Children's Nutrition Research Center

Title: Vitamin D supplementation increases calcium absorption without a threshold effect

item Aloia, John
item Dhaliwal, Ruban
item Shieh, Albert
item Mikhail, Mageda
item Fazzari, Melissa
item Ragolia, Louis
item Abrams, Steven

Submitted to: American Journal of Clinical Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/18/2013
Publication Date: 3/3/2014
Citation: Aloia, J.F., Dhaliwal, R., Shieh, A., Mikhail, M., Fazzari, M., Ragolia, L., Abrams, S.A. 2014. Vitamin D supplementation increases calcium absorption without a threshold effect. American Journal of Clinical Nutrition. 99(3):624-631.

Interpretive Summary: Determining whether more calcium is absorbed when more vitamin D is given could be an important way to assess whether someone has enough vitamin D. In this study, we evaluated how much calcium was given when healthy postmenopausal women were randomly assigned to get different amounts of vitamin D for 8 weeks. We did this using non-radioactive isotopes that were measured at our center at the CNRC. We found a small increase in calcium absorption with more vitamin D. However we didn't find any maximum amount of absorption over a large range of blood levels of vitamin D. Therefore we concluded that calcium absorption in people who are not very low in their vitamin D level is not a useful way to determine nutritional recommendations for vitamin D. This finding about the importance of carefully choosing subjects for testing of calcium absorption is likely important for both children and adults.

Technical Abstract: The maximal calcium absorption in response to vitamin D has been proposed as a biomarker for vitamin D sufficiency. Our objective was to determine whether there is a threshold beyond which increasing doses of vitamin D, or concentrations of serum 25-hydroxyvitamin D [25(OH)D], no longer increase calcium absorption. This was a placebo-controlled, dose-response, randomized, double-blind study of the effect of vitamin D on calcium absorption in healthy postmenopausal women. Seventy-six healthy postmenopausal women were randomly assigned to placebo, 800 IU (20 ug), 2000 IU (50 ug), or 4000 IU (100 ug) vitamin D3 for 8 wk. The technique of dual isotopes of stable calcium was used with a calcium carrier to measure calcium absorption at baseline and after 8 wk. Seventy-one women with a mean +/- SD age of 58.8 +/- 4.9 y completed the study. The mean calcium intake was 1142 +/- 509 mg/d and serum 25(OH)D was 63 +/- 14 nmol/L at baseline. A statistically significant linear trend of an increase in calcium absorption adjusted for age and body mass index with increasing vitamin D3 dose or serum 25(OH)D concentration was observed. A 6.7% absolute increase in calcium absorption was found in the highest vitamin D3 group (100 ug). No evidence of nonlinearity was observed in the dose-response curve. No evidence of a threshold of calcium absorption was found with a serum 25(OH)D range from 40 to 130 nmol/L. Calcium absorption in this range is not a useful biomarker to determine nutritional recommendations for vitamin D.