Vitamin D doses up to 2,400 IU daily did not increase calcium absorption, a study found.
A dose response study of vitamin D on calcium absorption showed that there was no threshold level of serum 25-hydroxyvitamin D (25OHD) for calcium absorption, suggesting that active transport of calcium is saturated at very low serum levels of less than 5ng/mL.
Researchers examined 198 Caucasian and African American women, ages 25 to 45 years, with vitamin D insufficiency, serum 25OHD less than 20 ng/mL, in a randomized, double blind study of vitamin D3 levels of 400, 800, 1600 or 2,400 IU, or placebo. A calcium supplement was given to increase mean calcium intake at baseline from 706 mg/day to 1,031 mg/day. Calcium absorption was measured at baseline and after 12 months. Results appeared online in the Journal of Bone and Mineral Density.
Mean baseline serum 25OHD was 13.4 ng/mL (33.5nmol/L) and increased to 40 ng/mL (100nmol/L) on the highest dose of 2,400 IU. There was no increase in 12-month calcium absorption compared to baseline on any dose of vitamin D in either race of women, and there was no significant relationship between 12-month calcium absorption and final serum 25OHD. There was no evidence of a threshold decrease in calcium absorption or serum 1,25(OH)2D among the lowest groups when serum 25OHD levels were divided into groups 0-5, 6-10, 11-15, and 16-20ng/mL.
“There is no need to recommend vitamin D for increasing calcium absorption in normal subjects,” the authors noted. “Very efficient calcium absorption at very low levels of serum 25OHD explains why people do not develop osteomalacia provided that dietary intake of calcium and phosphorus is adequate.”
Studies have long questioned the role of vitamin D and calcium supplements, and earlier this year, the U.S. Preventive Services Task Force recommended against taking daily supplementation of 400 IU or less of vitamin D3 and 1,000 mg or less of calcium for the primary prevention of fractures in community-dwelling postmenopausal women.