The data – published at The Endocrine Society's 94th Annual Meeting – reveals long term intake of calcium and vitamin D supplements is associated with high calcium levels in the blood and urine – 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 principle researcher Professor J. Christopher Gallagher of Creighton University Medical Center, USA.
The researchers said that whilst taking vitamin supplements has become a widespread practice throughout many parts of the world, the precise health effects of long-term calcium and vitamin D supplementation remain unclear.
They noted that previous research has indicated high levels of calcium in the urine and blood may increase the risk of kidney stones and other conditions, including bone and possible increased risks of heart attacks.
"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," warned Gallagher.
The research team examined 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. Calcium intake was then 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 (33%) developed high urinary levels of calcium (hypercalciuria) at some time in the study. The team noted that hypercalciuria has been linked to an increased risk of kidney stones identified in previous studies – although no incidents of kidney stones were reported during their one-year study.
Additionally, 10% of study subjects developed high blood levels of calcium (hypercalcemia), they revealed.
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," said Gallagher.
"However, it is possible that long-term use of supplements causes hypercalciuria and hypercalcemia, and this can contribute to kidney stones.”