Summer is the top time for consumption of iced tea beverages, but Dr John Milner, of Loyola University Medical Center, USA, warned: "For people who have a tendency to form the most common type of kidney stones, iced tea is one of the worst things to drink."
Milner explained that iced tea contains high concentrations of oxalate, which is one of the key chemicals that lead to the formation of kidney stones. He noted that the most common cause of kidney stones is dehydration from not drinking enough fluids.
During the warmer summer month’s people can become dehydrated from sweating, he said, adding that this, combined with increased iced tea consumption, raises the risk of kidney stones, especially in people already at risk.
"People are told that in the summertime they should drink more fluids," he said. "A lot of people choose to drink more iced tea, because it is low in calories and tastes better than water. However, in terms of kidney stones, they might be doing themselves a disservice."
Oxalates are naturally-occurring organic acids found in plants, and animals. Oxalate is also formed during the normal metabolism processes, and is excreted by the kidneys through urine.
However, because oxalate and calcium are continuously expelled through the urine, they can combine to form calcium-oxalate crystals if the concentration of oxalate is too high to be diluted.
Some research suggests that when this occurs, kidney stones may develop. However the formation of kidney stones containing oxalate is an area of controversy. About 80% of kidney stones formed by adults are calcium-oxalate stones, yet whether restriction of dietary oxalate helps prevent the formation of stones in individuals who are at high risk is an area of research that remains divisive.
Milner advised that people at risk for kidney stones should cut back on foods that contain high concentrations of oxalates – including spinach, chocolate, rhubarb and nuts.
“They should ease up on salt, eat meat sparingly, drink several glasses of water a day and eat foods that provide adequate amounts of calcium, which reduces the amount of oxalate the body absorbs,” he said.
He noted that while hot tea also contains oxalate, it's hard to drink enough to cause kidney stones.