How often do you think about your kidneys? Probably never. For most of us, the kidneys are yet another set of organs which quietly do their job day after day without incidence. Yet if the kidneys malfunction or fail for any reason, our health is profoundly affected. Kidneys are involved with several vital bodily functions. They help to remove waste and excess fluid from the body, they filter the blood and they control the body's pH balance. They also take part in maintaining the balance of essential nutrients and in regulating blood pressure.Stone formation in the urinary tract the kidneys, ureters, bladder and urethra has been recognised for thousands of years. Although we think of kidney stones as purely a result of modern diets and lifestyle, they have been found among prehistoric remains in Egypt and western Europe. From Hippocratic manuscripts it is evident that kidney stones were a common phenomenon in ancient Greece. But during the last few decades, the pattern and rate of the disease have changed markedly. In the past, stone formation was almost exclusively in the bladder, whereas today most stones form in the kidney and upper urinary tract. As many as 10 per cent of men and 3 per cent of women get kidney stones, most of whom are over 30 (New Eng J Med, 1993; 328: 884-5). The incidence of kidney stones has been steadily increasing, paralleling the rise in other diseases associated with the poor Western diet, such as heart disease, gallstones, high blood pressure, diabetes and cancer.

In a healthy individual, urine is usually saturated to the limit with calcium oxalate, uric acid and phosphates. Normally, with the secretion of protective factors and a balanced pH in the body, these compounds remain in solution and are excreted in the urine. But, if these protective factors are overwhelmed or an imbalance in pH occurs, small particles or "crystals" may separate from solution and build up in the kidney increasing the risk of stone formation. About 80 per cent of all stones are composed of calcium oxalate, either on its own or with a nucleus of calcium phosphate (known as apatite). Uric acid accounts for about 5 per cent of stones and struvite for about 15 per cent.

Environment vs heredity

As one study elegantly illustrated, it's likely that the environment plays as great a part as heredity in any individual's risk of developing kidney stones. In this general health survey of 2,500 middle aged men, a family history was significantly more common among stone formers than among controls, most prominent through the male line. However, an increased tendency was also noted among the wives of those with a family history of stones even if they had no family history of stone forming themselves (Br J Urol, 1979; 51: 249-52).

The case for environment over heredity is also underscored by ongoing research in space. Astronauts are generally at the peak of physical health. One would not expect them to fall prey to metabolic disorders. But space flight induces many physiological changes, among which are changes in the mineral balance of the body. An astronaut's diet is likely to contain less fluid, energy, protein, potassium, phosphorus and magnesium all known risk factors in stone formation (J Urol, 1997; 158: 2305-10). For astronauts, osteoporosis is the major risk from these changes they lose approximately one per cent of their bone mass per month during space flights. But they are also at greater risk of forming kidney stones (J Urol, 1993; 150: 803-7), adding important evidence to the association between environment, lifestyle and kidney stones.