Published: Oct 30, 2013 | Updated: Oct 30, 2013
By Michael Smith, North American Correspondent, MedPage Today

Full Story:  http://www.medpagetoday.com/Endocrinology/GeneralEndocrinology/42575

Increasing vitamin D levels had no effect on calcium absorption in young women, researchers reported.

In an ancillary result from a randomized clinical trial, women who started out with vitamin D insufficiency were brought up to normal levels through supplements over a 1-year period, according to Christopher Gallagher, MD, of Creighton University Medical Center in Omaha, and colleagues.

But the supplementation had no effect on calcium absorption at any dose, Gallagher and colleagues reported online in theJournal of Bone and Mineral Research.

The bottom line is that calcium absorption is very efficient, even at very low levels of vitamin D, the researchers argued.

“There is no need to recommend vitamin D for increasing calcium absorption in normal [people],” they concluded.

The study enrolled 198 Caucasian and African-American women, ages 25 to 45, with levels of serum 25-hydroxyvitamin D (25OHD) below 20 ng/ml.

They were randomly assigned in a double-blind fashion to placebo or one of four daily doses of cholecalciferol (vitamin D3) — 400, 800, 1600, or 2,400 IU.

The participants were also given a daily calcium supplement to bring average calcium intake up from 706 mg/day at baseline to 1,031 mg/day during the 12-month study.

The primary endpoint of the study was to how much vitamin D3 was needed to increase serum 25OHD levels and normalize serum parathyroid hormone.

But a secondary outcome was to measure changes in calcium absorption because adequate calcium and phosphorous are required to avoid osteomalacia, the investigators noted.

The investigators measured calcium absorption at baseline and after 12 months using a single isotope method.

In a multivariate analysis, the researchers found that vitamin D dose did not predict the final 12-month calcium absorption.

Also, race was marginally predictive with African-American women tending to have lower absorption than Caucasians (P=0.053).

Total calcium intake (P<0.009), baseline weight (P=0.019), and baseline calcium absorption did predict absorption at 12 months (P=0.012).

But baseline serum levels of 25OHD and calcitriol (1,25(OH)2D) were not predictive.

Findings were similar for serum levels of 25OHD at 12 months, Gallagher and colleagues reported.

The study is the first randomized trial to look at the issue in younger women, the investigators concluded, and a study in older women, using similar doses, found much the same thing.

At higher vitamin D doses, however, there was a small increase in calcium absorption, they noted. Gallagher and colleagues also noted that analyses of adolescent girls and children have also found no increase in calcium absorption with higher levels of vitamin D.

They cautioned that the single isotope method of measuring calcium absorption is probably a less accurate technique than the double isotope approach, although results of the two types are highly correlated.