The following dietary advice should be given to a first time calcium oxalate stone former with normal serum calcium and a normal 24-hour urine collection except:
A. Maintain a fluid intake of between 2.5–3.0 L/dayAnswer C
All stone formers independent of their risk of developing further stones should follow general preventative measures to modify their risk. Patients should be encouraged to maintain a fluid intake of 2.5–3.0 L/day which should be increased as necessary to ensure a diuresis of 2.0–2.5 L/day. Most fluids can be consumed although some carbonated drinks, such as cola, contain phosphoric acid which may increase the risk of stone formation. Lemon juice increases urinary citrate and so reduces the risk of calcium oxalate stones. Although orange juice also increases urinary citrate, it raises oxalate levels and so is not recommended.
Patients should be advised to eat a healthy and balanced diet. However, foods rich in oxalate (for example, chocolate, nuts, rhubarb, tea) should be limited or avoided particularly in those patients with hyperoxaluria. Excessive dietary animal protein may cause hypocitraturia, hyperuricosuria, hyperoxaluria and acidic urine thereby encouraging stone formation. High salt intake increases the risk of urolithiasis by causing increased tubular calcium excretion and hypocitraturia. Therefore, not more than 3–5 g sodium should be consumed per day.
Patients often ask about dietary calcium and whether hard water (containing a high mineral content typically including calcium carbonate) in their locality caused their kidney stone. The data regarding water hardness are controversial but suggest that any increased lithogenic salt excretion may be neutralised by a greater excretion of stone inhibitors such as citrate and magnesium. On the other hand, there is good evidence that restricting dietary calcium actually increases the risk of stone formation through the reciprocal absorption of oxalate in the gut. A randomised study that compared the 5-year risk of stone recurrence in patients with a normal calcium, low salt and low protein diet to a low-calcium diet found a relative risk of 0.49 (95% CI 0.24-0.98, p-0.04). At least 1000 mg calcium should be consumed each day although calcium supplementation is generally not recommended except in some cases of enteric hyperoxaluria.
Calcium oxalate stones can be predominately calcium oxalate monohydrate or calcium oxalate dihydrate. Patients who have calcium oxalate monohydrate stones are often found to have hyperoxaluria in their metabolic workup, whereas those with calcium oxalate dehydrate stones are more likely to have hypercalciuria.