The scientists gave a group of people who had a history of kidney stones some caffeine, the same amount of caffeine one would find in two cups of coffee. They then tested their urine and found elevated levels of calcium. If you are prone to kidney stones and your urine has elevated levels of calcium your risk of developing more kidney stones goes up significantly.

In an interview with Reuters, Dr. Linda Massey, Washington State University in Spokane, study leader, said those prone to kidney stones should have no more than two cups of coffee a day (cups, not great big mugs).

Massey said that the more calcium you have in your urine the more likely a kidney stone will develop.

Even those not prone to developing kidney stones have elevated levels of calcium in their urine after they consume caffeine.

The researchers asked 39 people (with kidney stones) and 9 people (without kidney stones) to drink water with caffeine added to it. The people had not eaten for 14 hours. Their urine was tested two hours before and two hours after they had taken the caffeine.

The urine tests showed that both groups had elevated levels of calcium in their urine after taking caffeine – as well as elevated levels of sodium, magnesium and citrate.

The more calcium and sodium you have in your urine the higher the risk of developing kidney stones. Massey told Reuters that the opposite happens with elevated levels of magnesium and citrate (they protect you). The protection does not offset the increased risk arising from increased calcium and sodium levels, said Massey.