Short answer: They can raise the risk a little. In the Women's Health Initiative trial, 1,000 mg of calcium plus vitamin D a day raised kidney stone risk by 17% compared with placebo. Calcium from food goes the other way: in large studies, people who ate the most calcium had fewer stones. The NIDDK says calcium from food does not cause calcium stones and can help prevent them.
The Women's Health Initiative calcium trial (Jackson 2006) randomly assigned 36,282 postmenopausal women aged 50 to 79 to 1,000 mg of calcium (as calcium carbonate) plus 400 IU of vitamin D3 a day, or placebo, and followed them for about 7 years.
| Outcome | Result |
|---|---|
| Kidney stones | Hazard ratio 1.17 (about 17% higher risk) |
| Hip bone density | 1.06% higher |
| Hip fracture (all participants) | Hazard ratio 0.88, not statistically significant |
| Hip fracture (women who kept taking it) | Hazard ratio 0.71 |
The authors concluded the supplement gave a small gain in hip bone density, did not significantly reduce hip fractures overall, and increased the risk of kidney stones. See Women's Health Initiative for the wider study.
Two large cohort studies point the other way for dietary calcium. In 45,619 men (Curhan 1993), those with the highest calcium intake from food had a 44% lower risk of symptomatic kidney stones than those with the lowest (relative risk 0.56, or 0.66 after adjusting for other factors).
In 91,731 women followed for 12 years (Curhan 1997), the highest dietary calcium intake went with a lower risk (relative risk 0.65), while taking calcium supplements went with a higher one (1.20). In 67% of supplement users, the calcium was not taken with a meal or was taken with meals probably low in oxalate.
The authors suggested timing may explain part of the gap: calcium eaten with food binds oxalate in the gut, so less is absorbed. These were observational studies, so they show links, not proof of cause.
For people who have had calcium oxalate or calcium phosphate stones, the NIDDK says that even though calcium sounds like the cause of calcium stones, it is not. In the right amounts, calcium can block other substances in the digestive tract that may cause stones. It suggests getting enough calcium from foods and talking with a health care professional about how much you need for both stone prevention and bone health.
The NIDDK also points to sodium, animal protein and, for oxalate stones, high-oxalate foods as things to look at, and says most Americans eat too much sodium.
Bone medicine labels often expect calcium and vitamin D to be adequate, and some trials gave both to everyone; see calcium and vitamin D with bone medicines. If you have had a kidney stone, how much should come from food vs a supplement is worth settling with the prescriber.
Dose Track can log a calcium supplement with its time, which also helps with spacing rules such as those for calcium and Fosamax.
Read more on Dose Track: Women's Health Initiative · Bone mineral density · Dietary supplement · Missed a Fosamax dose