Cadmium is among the most toxic metals in our environment, contributing to a wide range of diseases. It is particularly toxic to the kidneys. Of 38 men and women working in a battery factory, nearly half were shown to have kidney stones (Isr J Med Sci, 1992; 28: 578-83). Another study found that copper workers had a 18.5 per cent prevalence of upper urinary tract stone disease associated with a very significant hypercalciuria (too much calcium in the urine) (Urology, 1978; 11: 462-5).

But perhaps the greatest influence is the environment inside your body, as controlled by your diet.

Vegetarians as a group seem to have a lower risk of kidney stones (Eur Urol, 1982; 8: 334-9). Meat increases intestinal calcium absorption and thus the risk of abnormally high concentrations of calcium in the urine. This risk appears to be mainly a consequence of meat's high content of sulphur containing amino acids (J Nutr Sci Vitaminol (Tokyo), 1990; 36: 105-16). The metabolic by products of meat digestion also include oxalate and uric acid, both of which are implicated in kidney stones.

Not all studies, however, have found an association between eating meat and forming stones. In a randomised, controlled trial of a kidney stone prevention diet, low in animal protein and high in fibre, patients were divided into two groups one with dietary intervention and advice to increase fluid intake, and one which was given the latter advice only. The researchers found that a high fibre, low protein diet had no real advantages over the simple advice to increase fluid intake alone (Am J Epidemiol, 1996; 144: 25-33). Another study found that it was not whether or not individuals ate meat that made the difference, but whether they included fresh fruit and vegetables in their diet. Among meat eaters, those who eat more fruits and vegetables have a lower incidence of kidney stones (Urol Res, 1975; 3: 61-6).

The calcium debate

Dr Gary Curhan and his team from the Harvard School of Public Health in Boston have led the way in dietary research into kidney stones. Two studies by Dr Curhan have challenged the old presumption that lowering calcium intake will reduce the risk of forming stones. The first was a prospective study of 45,619 men aged 40 to 75, who had no history of kidney stones. During four years of follow up, 505 cases of new, symptomatic kidney stones arose. The researchers found several dietary links. The higher the intake of animal protein, the greater the risk of kidney stones, whereas a greater intake of potassium and fluid diminished the risk (N Eng J Med, 1993; 328: 833-8).

But the most dramatic result found was that those with the highest dietary calcium intake were at no greater risk of developing stones than the population as a whole. In fact, a higher intake of dietary calcium was strongly associated with a reduced risk of kidney stones. Individual foods high in calcium, such as skim or low fat milk, cottage or ricotta cheese and yoghurt, were found to be protective, as were non dairy sources of calcium, such as oranges and broccoli. These findings led the authors to conclude that the routine restriction of calcium rich foods for stone formers had no basis.

As an explanation for their surprising results, the researchers propose that a lot of calcium in the diet is protective due to its effect on oxalate absorption. With more calcium, oxalate absorption is reduced (meaning less is excreted in urine) whereas when calcium intake is restricted, oxalate absorption goes up leading to more oxalate in the urine.