Vitamin D is often added to calcium supplements to increase their absorption. Despite numerous studies and guidelines published by medical organizations, it is unclear what dose of the vitamin should be recommended. No systematic studies have investigated the effect of increasing doses of vitamin D on absorption of calcium.
The aim of this randomized, double-blind, placebo-controlled trial was to determine the effect of increasing doses of vitamin D3 on calcium absorption in postmenopausal women. The study was conducted at an urban medical center in Nebraska. Participants were 163 vitamin D–deficient white women who were at least 7 year postmenopause. Vitamin D insufficiency was defined as a serum level of 25-hydroxyvitamin D (25OHD) below 20 ng/mL (50 nmol/L). Subjects were randomly assigned to 1 of 8 groups, 400, 800, 1600, 2400, 3200, 4000, or 4800 IU vitamin D3 per day or matching placebo for 1 year. Daily oral intake of calcium was increased to 1200 to 1400 mg. The primary study outcome was the change in calcium absorption on serum levels of 25OHD.
At 1 year, there was an increase in mean serum 25OHD from the baseline value of 15.6 ng/mL (39 nmol/L) to 46.5 ng/mL (112 nmol/L) among subjects receiving the highest dose of vitamin D (4800 IU). Calcium absorption was more significantly correlated with the 1-year serum 25OHD (R2 = 0.50; P = 0.0019) than dose (R2 = 0.47; P = 0.033). There was only a small increase in the calcium absorption of a 100-mg dose from 52% to 58% (6%) over a serum 25OHD range of 20 to 66 ng/mL (50–165 nmol/L).
These data show that there is no threshold for reduced calcium absorption in the serum 25OHD range of 10 to 66 ng/mL (25–165 nmol/L). The 6% increase in absorbed calcium achieved with high doses of vitamin D can be obtained by drinking a small glass of milk (100 mL) or taking a 100-mg elemental calcium tablet. The data challenge the widespread assumption that adding vitamin D to calcium supplements increases absorption of calcium except when serum 25OHD is less than 10 ng/mL (25 nmol/L).