Kidney stones: prevention is fluid and diet

Kidney stones are the most painful condition in this handbook and the prevention is almost entirely dietary. The treatments with evidence:

Fluid. Increasing fluid intake to achieve a urine output of around 2-2.5 litres a day reduces recurrence, and this is the single best-established intervention. The fluid should mostly be water.

Dietary calcium, at a normal intake, reduces stone risk. This is counter-intuitive and frequently got wrong: the calcium binds oxalate in the gut and prevents its absorption, so a low-calcium diet increases oxalate stones. Do not restrict dietary calcium to prevent stones. Calcium supplements, taken separately from meals, are different and may increase risk.

Sodium restriction. High sodium increases urinary calcium excretion. Reducing sodium lowers stone risk.

Oxalate. Reducing very high-oxalate foods helps in people with recurrent oxalate stones: spinach, rhubarb, beet greens, nuts, chocolate, and tea in quantity. Boiling reduces oxalate in greens — and importantly, discards the oxalate into the water, which is one of the few places where not drinking the cooking water is right.

Potassium citrate is used medically to alkalinise the urine and reduce recurrence, particularly in uric acid stones and in people with low urinary citrate. It is a prescribed treatment in many cases; a household equivalent is citrus juice, which is alkalinising.

Uric acid stones relate to gout, high purine intake, and low urine pH, and to the metabolic syndrome. Allopurinol and urine alkalinisation are the treatments.

What does not have evidence. The long list of supplement products sold for stones. Cranberry is unhelpful and possibly increases oxalate stones. Vitamin C at high doses increases oxalate excretion.

The acute attack. Renal colic — severe flank pain radiating to the groin, with nausea and blood in the urine — is an emergency. Treatment is analgesia (NSAIDs are the standard and are effective), fluids, and sometimes a stone-removal procedure. A stone with fever or with no urine output is a urological emergency.