In 1997, Dr Curhan analysed an even larger cohort this time of women with no previous kidney stones and found very similar results with a surprising twist (Ann Int Med, 1997; 126: 497-504). Among the 91,731 women studied, 864 cases of kidney stones were documented over 12 years, and once again dietary calcium was found to be protective. However, researchers also observed that the intake of supplemental calcium actually elevated the risk of stone formation. All calcium, it appears, is not equal. The opposing effects of different types of calcium found in this study may have something to do with timing: if calcium supplements are taken at the same time that oxalate is consumed in the diet.

Most recently, another team of researchers in the US studied 1,309 women aged between 20 and 92 years. They were able to rule out such factors as hypertension, bone mineral density, high oxalate food consumption, fractures, ascorbic acid from food supplements and even geography as being significantly associated with stone formation. Instead, the most significant factor was dietary calcium; stone formers consumed almost 250 mg less calcium per day than women without stones. They also had a lower energy intake (Am J Epidemiol, 1998; 147: 914-20).

Encouragingly, while family history has been shown to increase the relative risk of forming kidney stones by two and a half times (particularly in men), family history alone is not enough to reduce the protective effect of dietary calcium (J Am Soc Nephrol, 1997; 8: 1568-73). Indeed, more calcium rich foods may be the best protection against hereditary stones.

Subtle damage to the kidneys can take place over a number of years without producing any painful symptoms. A first sign that something is wrong is often the formation of kidney stones. While this condition is often derided with humour (many men say it's the closest they'll ever get to labour pains), kidney stones are no joke. They can damage the kidney, obstructing the passage of urine and leading to infection. While most humans can survive with one kidney or with each one working at sub optimum levels why take the chance? Simple lifestyle modifications can make the difference between functioning adequately and functioning optimally.