Calcium and vitamin D supplements may increase the risk of kidney stones by raising calcium levels in the blood and urine, a new study finds.
Creighton University Medical Center researchers said the findings suggest people at risk of developing the painful condition not exceed recommended doses of vitamin D and calcium.
"The use of calcium and vitamin D supplementation may not be as benign as previously thought," said lead investigator Dr. J. Christopher Gallagher, director of the Bone Metabolism Unit at Creighton University in Omaha. "Pending further information, people should not exceed the guidelines suggested by the Institute of Medicine, which are 800 international units of vitamin D, and 800-1,200 milligrams per day of calcium."
Researchers noted nearly two-thirds of American women take calcium and vitamin D supplements, but the long-term health effects are unclear. Past studies have shown high levels of calcium in the urine and blood may increase the risk of bone and kidney problems, including the development of stones.
For the new study, researchers tracked 163 healthy, postmenopausal women between the ages of 57 and 85 years. The women were randomly assigned to receive a vitamin D supplement of 400, 800, 1,600, 2,400, 3,200, 4,000, or 4,800 international units a day, or placebo. Their calcium intake was also increased from 691 to 1,200-1,400 milligrams per day.
After one year, investigators found 48 of the women developed high urinary levels of calcium at some time in the study – putting them at an increased risk of kidney stones. Sixteen of the women also developed high blood levels of calcium.
"Because of the unpredictable response, it is not clear whether it is the extra calcium, the vitamin D or both together that cause these problems," Gallagher said. "However, it is possible that long-term use of supplements [ contributes] to kidney stones. For these reasons, it is important to monitor blood and urine calcium levels in people who take these supplements on a long-term basis. This is rarely done in clinical practice."
The study, which was funded by The National Institute on Aging, was presented at The Endocrine Society's 94th Annual Meeting in Houston.