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Kidney stones are common, painful, costly, and preventable. Our experience suggests that only a small proportion of stone formers are counseled by a knowledgeable professional about prevention, resulting in the ample recurrence of stones.
Perhaps not completely congruent with what physicians believe, patients consistently state that they are interested and motivated to prevent stones.
Recent links between kidney stones and diabetes, hypertension, coronary artery disease, and chronic kidney disease indicate that nephrolithiasis is far from a merely inconvenient disorder.1 While the causal basis and directions for these associations have not been confidently established, stone formers should have their diet, activity, body mass index (BMI), and comorbidities assessed, and have appropriate individualized preventive regimens prescribed. In addition to preventive efforts, patients with renal and ureteral calculi should have their risk for other associated conditions addressed.
In this update, we focus on some recent developments regarding calcium and uric acid stones, rather than offer a comprehensive survey. The recent review by the Agency for Healthcare Research & Quality (AHRQ) is an important resource, although its conclusions are limited by its attention only to data produced by high quality randomized clinical trials (RCT).2
Given the surprising paucity of such trials, clinicians must also rely on observational studies, as well as our knowledge of the factors that influence urine chemistry, recognizing the limitations of our attempts to extrapolate from non-interventional studies to clinical outcomes. Elsewhere we have reviewed developments in cystinuria.3
Dietary effects on calcium stone formation
The influence of diet on urine chemistry has been repeatedly demonstrated, and subsequently there is little mystery about how dietary modifications change urine profiles to prevent kidney stones. We recently reviewed the data regarding diet and stones.4 There is only one RCT that demonstrated a beneficial effect of diet on stone recurrence.5
In that study, men with hypercalciuria were randomized to either a diet with a restricted intake of animal protein, sodium, and oxalate, but up to 1,200 mg of calcium per day, or a diet of 400 mg of calcium and restricted oxalate intake. The higher-calcium diet was associated with roughly 50% less stone recurrence at three years.
That is currently the diet that we most frequently recommend to patients, though we concentrate on discussing the components most relevant to stone formers based on their individual 24-hour urine collections.
For the study, researchers did not measure bone mineral density (BMD), but we speculate that the participants in the higher-calcium intake group would have had better BMD at the end of the study.
Increased dietary calcium intake is associated with a lower incidence of kidney stones, and this has been known for 20 years.6 The leading hypothesis about this effect has been the ability of ingested calcium to bind oxalate in the intestinal lumen and diminish its absorption by the intestine, and thereby its excretion by the kidney. The possibility that some other property of dairy foods besides calcium content is responsible for this inhibitory effect has also been suggested. Recent data have demonstrated that non-dairy calcium is also associated with reduced stone formation, supporting the role of calcium itself in stone prevention.7
Weight gain has been consistently associated with an increased risk for recurrent stones.8 Given the associated risk of the metabolic syndrome and hypertension, it is reasonable to prescribe weight loss to appropriate patients. However, no studies to date have addressed whether weight loss actually leads to fewer stones.
It is also important to stress that both RCTs and studies of urine chemistry demonstrate that increasing fluid intake to 3L per day to achieve urine volumes of greater than 2L per day should be emphasized.9 Patients should learn to be quantitative about fluid intake and not be told simply to drink “a lot.” The substantial numbers of stones prevented and the sizable cost factor averted—by getting patients to drink more than 2L of water per day—were recently quantitated.10