Lead investigator, J. Christopher Gallagher, M.D., professor and director of the Bone Metabolism Unit at Creighton University Medical Center in Omaha, NE said:
"The use of calcium and vitamin D supplementation may not be as benign as previously thought. 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."
Vitamin supplements have become increasingly popular in many countries around the world. In the U.S. alone, almost 75% of women take vitamin supplements. The most frequently taken supplements are calcium and vitamin D, yet despite their popularity there is no concrete evidence about the health effects for taking these supplements over a long period of time.
Earlier studies have suggested that elevated calcium levels in the urine (hypercalciuria) may raise the risk of kidney stones, whilst higher calcium levels in the blood (hypercalcemia) are linked to numerous complications, such as bone and kidney problems.
Gallagher and Vinod Yalamanchili, M.D., a research fellow in Creighton University's Bone Metabolism Unit and leading author of the study evaluated 163 healthy, postmenopausal women aged between 57 and 85 years, who were randomly assigned to either take placebo or a vitamin D supplement of 400, 800, 1600, 2400, 3200, 4000, or 4800 international units a day. In addition, the researchers raised the participants' calcium intake from 691 milligrams per day at the start of the study to 1,200-1,400 mg/day. The levels of calcium in the blood and urine were measured at baseline and every three months for the 12-months study period.
The results revealed that at some point during the study period about 33% of the participants (n=48) developed high levels of calcium in their urine. Overall, the researchers noted 88 episodes of high urinary calcium. Earlier studies provided evidence that high calcium levels in the urine are linked to an elevated risk of kidney stones. However, the team notes that no incidents of kidney stones were observed during the one-year study period.
The findings also showed that around 10% of participants (n=16) developed high levels of calcium in the blood. Overall, there were 25 episodes of high calcium levels in the blood, although in both incidents the increases were not linked to the vitamin D dosage.
Gallagher explained: "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."
,br> He concluded stating:
"However, it is possible that long-term use of supplements causes hypercalciuria and hypercalcemia, and this can contribute 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."