The link with your environment Kidney stones are thought to be caused by family heredity or too much calcium. But climate, many unlikely foods, certain supplements and even too little calcium can play their part.
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.
For a long time, the best approach offered to patients was surgery to remove large, painful stones. Smaller stones were, and still are, treated with a number of different types of drugs, such as the thiazide diuretic chlorthalidone, which reduces urine calcium concentrations. These drugs have a host of side effects, including hypotension (hence their use in the treatment of high blood pressure), impotence, skin rashes and photosensitivity. They also alter the body's balance of important minerals, such as potassium and magnesium, and may therefore, ironically, end up increasing the risk of stone formation (Acta Urologica Belgica, 1994; 62: 25-9). Another popular medication is calcium citrate, a highly soluble calcium salt whose formation decreases urinary concentrations of free calcium.
Within the last two decades, a form of ultrasound treatment known as extracorporeal shock wave lithotripsy (ESWL) has been used to shatter stones, making their fragments easier to "pass" through the urine. The treatment is performed on a day patient basis, with NSAIDs used for pain relief. Anti biotics are often given prophylactically since bacteria, which are found in more than 30 per cent of stones, can be released into the kidney when the stones are shattered, increasing the risk of post procedure infection (Acta Urologica Jap, 1992; 38: 999-1003).
Although hailed as a revolution in the treatment of kidney stones, the risks of ESWL are probably underestimated. Most patients experience internal bleeding, which can range from a tiny haemorrhage to major bleeding requiring transfusion. Immediately after the procedure, blood flow to the kidneys can be drastically altered; usually it declines, but in about 8 per cent of cases blood pressure in the kidney rises, causing kidney hypertension (RH Hepinstall, Pathology of the Kidney, Boston, Massachusetts: Little, Brown & Co, 1992: 1592).
There is evidence that ESWL can affect the ability of the kidneys to filter out impurities efficiently (J Endourol, 1994; 8: 15-19). Other studies have shown irreversible kidney damage from the procedure (Nephron, 1993; 63: 242-3) and there has been one case of fatal kidney failure afterwards (Lancet, 1994; 344: 757-8). Damage to the kidney is reportedly in the range of 63 to 85 per cent (Am J Radiol, 1985; 145: 305-13; Radiology, 1987; 163: 531-4).
But perhaps most dispiriting of all is that ESWL doesn't cure the problem. In a French study, computed tomography scans performed two years after a group of patients had undergone lithotripsy showed that 40 per cent had a recurrence of stones and 25 per cent had scarring (Nephrologie, 1993; 14: 305-7).
This recurrence rate is not much different from what you could expect if you never had the procedure. In an observational study of the natural history of stone formation among 2,322 men, 42 per cent of patients experienced a recurrence of kidney stones (Acta Med Scand, 1975; 197: 439-45). Perhaps even more interesting was that 94.5 per cent of all stones passed spontaneously, suggesting that the need for aggressive medical action may be overstated. These researchers concluded that environment played a greater part in stone formation than family history an important point when considering treatment since there are so many beneficial changes in diet and lifestyle which those at risk can make.
A number of environmental influences contribute to the risk of stone formation. People living in, or even just visiting, a warm or hot climate have an elevated chance of developing kidney stones (Med Trop, 1997; 57: 431-5; Am J Epidemiol, 1996; 143: 487-95). A sunny climate means greater risk of dehydration and lower urine output. This leads to an imbalance in urine volume and the concentration of those compounds which form stones.
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.
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.