Based on these findings, doctors treating such patients should advise them not to reduce their calcium intake, Dr. Mathew D. Sorensen, an assistant professor of urology at the University of Washington in Seattle and the study’s lead author, told Reuters Health.
Many patients will reduce their calcium intake after they have a kidney stone, Dr. Sorensen explained in an interview. “It’s such a natural thing, patients have a stone, stones are made of calcium.” However, studies published in the 1990s demonstrated that people with low calcium intakes were actually at increased risk of stone formation.
While the degree of intestinal calcium absorption is thought to play a key role in stone formation, Dr. Sorensen and his team explain in the April issue of the Journal of Urology, the association has not been studied directly. To better understand the relationship, they looked at data from the Study of Osteoporotic Fractures, which includes nearly 10,000 postmenopausal women who have been followed up every other year for more than 20 years.
The current analysis includes 5,452 study participants who underwent an oral radioactive calcium assay. At visits 2, 3 and 4, a total of 490 women (6.1%) reported a history of nephrolithiasis.
Fractional calcium absorption decreased as calcium intake increased — whether from food or supplements — the researchers found. Also, within every calcium intake quintile, fractional absorption was higher in the women with a history of nephrolithiasis (p < 0.001).
Women in the highest four quintiles of calcium intake were 45% to 54% less likely to have a history of kidney stones (p=.03), the report indicates, and women who had taken calcium supplements were 21% to 44% less likely to have a history of nephrolithiasis compared to the women who never took calcium supplements (p < 0.001).
Multivariate analysis showed that fractional calcium absorption, body mass index, and calcium supplementation were each independently associated with nephrolithiasis.
While fractional calcium absorption can range from 10% to 70%, Dr. Sorensen explained, most women absorb about 30% of the calcium they consume. Women who take in less calcium are likely to have higher fractional calcium absorption, while those who consume more absorb less of it.
“We often see patients in clinic who reduce their intake of calcium … after having a kidney stone,” the researcher added. “That really can be a dangerous thing for women to do at least in terms of their stone risk, and it probably applies to men as well.”
While many patients are told to increase their fluid intake after having a kidney stone, physicians should also advise them to cut down on their salt intake, eat less animal protein, and be sure to get adequate calcium, Dr. Sorensen added. “Those are good recommendations for anybody that has a stone,” he said.
Also, he said, it’s important for patients to take calcium with meals.
“Taking a calcium supplement by itself (not with a meal) probably provides just a calcium load, without the benefit of binding other substances like oxalate,” which can also promote stone formation, Dr. Sorensen said. “In clinical practice we give some stone patients calcium supplements to intentionally try to promote this binding process, but they should always be taken with a meal.”
One unique aspect of the study, the researcher noted, is that it’s the first to include thousands of women who completed oral calcium absorption assays. Previous studies, he pointed out, have included only a handful of women. “It really gives us an insight into the mechanisms of the role that calcium plays in protecting, in this study, women, but probably all patients, from forming kidney stones,” Dr. Sorensen said.