Vitamin supplements have become a common part of the daily routine throughout the world. In the United States about two-thirds of women ingest vitamin supplements, with vitamin D being the most frequently used one of them all, yet the health effects they may cause remain unclear.

J. Christopher Gallagher, MD, professor and director of the Bone Metabolism Unit at Creighton University Medical Center and study lead Vinod Yalamanchili, MD, research fellow at Creighton University Medical Center, studied 163 healthy and postmenopausal women between the ages 57 and 85. Each participant received a vitamin D supplement of 400, 800, 1600, 2400, 3200, 4000, or 4800 units a day or a placebo. Following the vitamin D supplement, each participant's calcium intake was increased from 691 to 1,200-1,400 milligrams per day. The blood and urinary calcium levels were measured at the beginning of the year and then every three months for one year.

Previous research has shown that high levels of calcium in the urine, hypercalciuria, may increase the risk of kidney stones and high levels of calcium in the blood, or hypercalcemia, is linked to many complications, including bone and kidney problems.

The results showed 48 participants developed high urinary levels of calcium, at an appointed time during the study. These participants also had 88 occurrences of urinary calcium. Hypercalciuria has been linked to an increased risk of kidney stones identified in previous studies. No incidents of kidney stones were reported during this one-year study.

Furthermore, about 10 percent of the participants established high blood levels of calcium during the research, which converts to 25 occurrences among 16 participants according to researchers. 

According to Dr. Gallagher because the results were so fickle, he is unsure whether it was the calcium, vitamin D or both supplements combined. Dr. Gallagher does not rule out that the possible long-term use of these supplements may cause hypercalciuria or hypercalcemia which can contribute to kidney stones. He stresses the importance to monitor blood and urine calcium levels in those who take these supplements on a long-term basis.

The results will be presented Tuesday at The Endocrine Society's 94th Annual Meeting in Houston and should be considered preliminary until published in a peer-reviewed journal.