Calcium and vitamin D supplements are associated with high calcium levels in the blood and urine, which could increase the risk of kidney stones, a new study finds. The results ere presented Tuesday at The Endocrine Society's 94th Annual Meeting in Houston.
The researchers say that 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.
Previous research has indicated that high levels of calcium in the urine, or hypercalciuria, may increase the risk of kidney stones. Elevated calcium in the blood, or hypercalcemia, is associated with many complications, including bone and kidney problems.
The researchers, from Creighton University, studied 163 healthy, postmenopausal women between the ages of 57 and 85 years. All participants were randomly assigned to receive a vitamin D supplement of 400, 800, 1600, 2400, 3200, 4000, or 4800 international units a day, or placebo. Then, their calcium intake was increased from an initial intake of 691 to 1,200-1,400 milligrams per day.
Investigators measured blood and urinary calcium levels at the beginning of the study, and then every three months for one year.
They found that approximately 48 participants, or 33 percent, developed high urinary levels of calcium at some time in the study. These participants had 88 episodes of high 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.
Additionally, about 10 percent of study subjects developed high blood levels of calcium. This translates into 25 episodes among 16 participants. In both cases, the increases were unrelated to the dosage of vitamin D.
"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," the researchers said. However, it is possible that long-term use of supplements causes hypercalciuria and hypercalcemia, and this can contribute to kidney stones.
For these reasons, the researchers feel it is important to monitor blood and urine calcium levels in people who take these supplements on a long-term basis, something rarely done in clinical practice.
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