Taking high-dose vitamin D and calcium supplements may increase the risk of kidney stones in postmenopausal women, according to a study in Clinical Endocrinology, which involved 132 women, ages 50 and older.
All took 1,200 milligrams of supplemental calcium a day (which is the RDA for women over 50 and men over 70); half took 600 IU of vitamin D a day (the RDA for adults ages 19 to 70) and half took 10,000 IU (the upper limit set by the Endocrine Society). Calcium levels in blood and urine were measured every three months. Vitamin D increases intestinal absorption of calcium, which is a component of most kidney stones.
Over the course of a year, most of the women taking high-dose D had hypercalciuria (excessive urinary calcium) at least once; they were 3½ times more likely to develop it than the low-D group. While hypercalciuria greatly increases the risk of kidney stones, this study was not long or large enough to detect differences in the rate of stone formation. Based on their findings, the authors suggested that the upper limit for vitamin D should be 4,000 IU a day, which is the cutoff set by the National Academy of Medicine.
Note that research has found that dietary calcium, as opposed to supplements, actually reduces the risk of kidney stones.
This article first appeared in the UC Berkeley Wellness Letter.
Also see Vitamin D Research: A Round-up.