The cornerstone of management is to increase urine volume. The effect appears to be linear, with a point of diminishing return reached at urine volumes of more than 2.5 L per day. We prescribe an intake of 2.5–3 L of fluids per day.
There are data to suggest that the type of fluid ingested matters. Epidemiologic studies show that drinking coffee and beer decreases the risk of stones. Drinking grapefruit juice consistently increases stone risk for unclear reasons, whereas lemon juice, higher in citrate content, might have beneficial effects on urinary chemistry but its ingestion has not been shown to prevent stones.
Studies of dietary calcium's effect on stone recurrence rates have led to major changes in nonpharmacologic manipulation. Epidemiologic evidence shows an inverse relation between dietary calcium intake and recurrence rates. This is probably best explained by calcium's inhibition of intestinal oxalate absorption. A randomized controlled trial (RCT) assigned men with hypercalciuria to follow either a diet low in calcium (400 mg) and oxalate or a diet higher in calcium (1200 mg) with restricted intake of oxalate, protein and salt.1
At 5 years, the latter group had a 51% lower rate of stone recurrence than those following a low-calcium diet. Although controversy persists and replication of these findings under other clinical circumstances would be desirable, low-calcium diets are not recommended and can exacerbate the well-documented association of hypercalciuria with low bone mineral density and increased fracture rates.2
Whether calcium supplements could have a similar effect in lowering stone recurrence rates has not been tested. Taking calcium carbonate supplements with meals reduces oxaluria, whereas taking them at bedtime increases calciuria and has no effect on oxaluria.3
The preferred calcium supplement for people at risk of stone formation is calcium citrate because it helps to increase urinary citrate excretion. We recommend a dose of 200–400 mg if dietary calcium cannot be increased.
The intake of foods high in oxalate should be limited. Although there are many such foods, some more frequently than others have been shown to be most at fault for raising urine oxalate levels (). Their impact might be mitigated by accompanying them with additional fluids and dietary sources of calcium to diminish oxalate absorption. No pharmacologic intervention has definitively been shown to be effective. Of note, since vitamin C can convert to oxalate, the use of vitamin C supplements may increase oxaluria and be associated with an increased risk of stone formation; therefore, the dose of these supplements should be limited to less than 1000 mg/d.
Calcium excretion is directly linked to sodium excretion. Reductions in dietary sodium reduce calciuria, but no RCT of sodium restriction alone has been performed. The successful diet mentioned earlier that was higher in calcium and restricted intake of oxalate, protein and salt1
achieved a reduction in calcium excretion despite the higher calcium intake. This effect was attributed to the reduction in dietary sodium intake to less than 2 g/d.
The ingestion of animal protein has adverse affects on urine chemistries: it lowers citrate excretion and increases calcium and uric acid excretion. Epidemiologic data show that it correlates well with the prevalence of stone formation. The recent popularity of diets low in carbohydrates and high in animal protein have refocused attention on protein intake as a risk factor for stones.4
Patients with recurrent stones should minimize their protein intake to less than 80 g/d.
Role of diabetes and obesity in stone formation
Recent data have suggested an increased prevalence of stones among people with diabetes and among obese people. Insulin resistance may reduce urinary citrate excretion and increase calcium excretion, and a greater body mass index is associated with increases in urinary oxalate excretion. Low urine pH is associated with increased risk of uric acid stones as well. Weight loss (through a mechanism other than a diet high in animal protein) and improved diabetes control may help to prevent stone formation in appropriate patients, although this approach has yet to be tested in an RCT.