24335055
BACKGROUND	The maximal calcium absorption in response to vitamin D has been proposed as a biomarker for vitamin D sufficiency .
OBJECTIVE	The 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 .
METHODS	This was a placebo-controlled , dose-response , randomized , double-blind study of the effect of vitamin D on calcium absorption in healthy postmenopausal women .
METHODS	Seventy-six healthy postmenopausal women were randomly assigned to placebo or 800 IU ( 20 g ) , 2000 IU ( 50 g ) , or 4000 IU ( 100 g ) vitamin D for 8 wk .
METHODS	The technique of dual isotopes of stable calcium was used with a calcium carrier to measure calcium absorption at baseline and after 8 wk .
RESULTS	Seventy-one women with a mean SD age of 58.8 4.9 y completed the study .
RESULTS	The mean calcium intake was 1142 509 mg/d and serum 25 ( OH ) D was 63 14 nmol/L at baseline .
RESULTS	A statistically significant linear trend of an increase in calcium absorption adjusted for age and body mass index with increasing vitamin D dose or serum 25 ( OH ) D concentration was observed .
RESULTS	A 6.7 % absolute increase in calcium absorption was found in the highest vitamin D group ( 100 g ) .
RESULTS	No evidence of nonlinearity was observed in the dose-response curve .
CONCLUSIONS	No evidence of a threshold of calcium absorption was found with a serum 25 ( OH ) D range from 40 to 130 nmol/L .
CONCLUSIONS	Calcium absorption in this range is not a useful biomarker to determine nutritional recommendations for vitamin D.

