Drinking green tea can help prevent the formation of large kidney stones claim Chinese scientists. 

Kidney stones affect more than 5 per cent of the world population, and the prevalence is rising says Xudong Li at Sichuan University, Chengdu, China. The key component of kidney stones is calcium oxalate crystals and Li has found green tea is effective in directing the crystallisation to form less stable crystals. 

The team grew calcium oxalate crystals under different conditions and studied them using a variety of analytical techniques. Unusually, they found that the dihydrated form of calcium oxalate was formed when large amounts of green tea were present. Previous studies have shown that calcium oxalate tends to form a monohydrate structure which is thermodynamically stable and can lead to the formation kidney stones. The dihydrated form is less stable and so less likely to grow into problematic sizes before being extracted in urine. 

The shape of calcium oxalate crystals changes when green tea extract is present

The more green tea, the more elongated the bipyramidal shape of the crystals became, says Li. He proposes that this change is due to the phenol groups in the green tea hydrogen bonding to the oxalate groups. This inhibits the nucleation and growth of the monohydrate crystal, forcing the growth of the flat tetragonal bipyramids and creating a layered effect. 

'Our results suggest that drinking green tea might be a good habit for the prevention of human stone formation,' says Li. 

He continues that his team, 'are now focusing on the evaluation of green tea extract based modifiers on calcium oxalate crystallization and very interestingly have obtained spherical calcium oxalate dehydrate crystals.' The team are now planning to investigate this phenomenon with other biomimetic crystals, such as calcium carbonate and hydroxyapatite. 

Charlotte Beard 

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