Women who take calcium or vitamin D pills daily are at greater risk of developing kidney stones, a new study has warned.
Researchers at the Creighton University Medical Center in the US found that women who took the vitamin supplements for over a year were found to have higher calcium levels in their blood and urine, a condition called hypercalciuria which could increase the risk of kidney stones.
“The use of calcium and vitamin D supplementation may not be as benign as previously thought,” said lead study author Dr Christopher Gallagher, director of the Bone Metabolism Unit at Creighton.
“Pending further information, people should not exceed the guidelines suggested, which are 800 international units of vitamin D, and 800-1,200 mg per day of calcium,” he said.
According to the researchers, taking vitamin supplements has become a widespread practice throughout many parts of the world. Despite their popularity, the precise health effects of long-term calcium and vitamin D supplementation still remain unclear, they said.
Past research has indicated that elevated calcium in the blood, or hypercalcemia, could increase the risk of kidney stones. It is associated with many complications, including bone and kidney problems.
In the new study, presented The Endocrine Society's 94th Annual Meeting in Houston, Gallagher and his team studied 163 healthy, postmenopausal women aged between 57 and 85 years.
All subjects were randomly given a vitamin D supplement of 400, 800, 1600, 2400, 3200, 4000, or 4800 international units a day, or placebo. Their calcium intake was then raised from a daily initial intake of 691 mg to 1,200-1,400mg.
The researchers measured blood and urinary calcium levels at the beginning of the study, and then every three months for one year. It was found that 33 per cent of subjects developed high urinary levels of calcium at some time in the study.
These participants had 88 episodes of high urinary calcium or hypercalciuria, which has already been linked to an increased risk of kidney stones.
It's not clear whether the extra calcium, the vitamin D or both together cause these problems. However, it's possible that long-term use of supplements causes hypercalciuria and hypercalcemia, and this can contribute to kidney stones, Gallagher said.
Thus, it's important to monitor blood and urine calcium levels in people who take these supplements on a long-term basis, he added.