What causes kidney stones, and how do you actually prevent them?

Kidney stones are common, painful, and prone to coming back — but for most people the single most effective preventive step is unglamorous: drink enough water that your urine stays pale and plentiful. That is the honest headline, and the evidence behind it is stronger than for almost anything else on offer.

What a stone actually is

A kidney stone is a hard deposit that forms when the substances dissolved in urine become concentrated enough to crystallise and clump together. The chemistry depends on the stone type. Most are calcium oxalate or calcium phosphate stones; others are uric acid stones or, more rarely, cystine stones, which run in families [s2]. The type matters because it steers prevention — what helps one kind of stone is not identical to what helps another, which is why anyone with recurrent stones benefits from knowing the composition of the one they passed [s1].

Why they form

The common thread across stone types is supersaturation: urine carrying more mineral than it can keep dissolved. Low urine volume is the most direct driver, because concentrated urine supersaturates more easily. That is why fluid intake sits at the centre of prevention, and why diet — how much sodium, animal protein, calcium and oxalate you take in — modifies the risk on top of it [s2]. Individual susceptibility varies, so two people with similar habits do not carry identical risk.

The best-tested defence: fluid

The cleanest evidence comes from a five-year randomised trial in people who had formed their first calcium stone. Half were asked simply to drink more water, with no other dietary change; half received no treatment. Recurrences occurred in 12 of 99 patients in the high-water group versus 27 of 100 in the untreated group, and when stones did recur, they took longer to appear — an average interval of 38.7 months versus 25.1 months [s3]. Drinking more water, in other words, roughly halved recurrences and delayed the rest.

How much is "enough" is individual, but the general public-health target is a useful anchor: unless you have kidney failure, many clinicians suggest six to eight 8-ounce glasses of water a day, adjusted with a professional for your situation [s2]. The practical test is the urine itself — aim for output that is pale and copious rather than dark and scant.

Diet, beyond water

A few dietary levers have reasonable support for calcium stones. Cutting sodium helps, because high salt intake raises the calcium excreted into urine; the general guidance is to aim for less than 2,300 mg of sodium a day, and one teaspoon of table salt already contains 2,325 milligrams [s2]. It is also worth reducing very high-oxalate foods such as spinach, rhubarb, nuts and wheat bran if you form calcium oxalate stones [s2].

The most counter-intuitive point is calcium. Cutting dietary calcium tends to backfire: calcium in food binds oxalate in the gut and stops it reaching the urine, so getting enough calcium from food can actually lower stone risk rather than raise it [s2]. That is the opposite of what many people assume, and it is a common reason well-meaning "stone diets" fail.

When diet is not enough

For people who keep forming stones despite fluids and diet, guidelines describe an evidence-based medical step-up. Both the urological and internal-medicine guidelines recommend starting with increased fluid intake, then adding targeted drug therapy — such as a thiazide diuretic, potassium citrate, or allopurinol — chosen according to the stone type and what a metabolic work-up finds in the urine [s1][s4]. These are prescription decisions that depend on testing, not something to self-select.

When to seek care

Severe flank or back pain, especially in waves, is the classic sign of a stone on the move and warrants prompt medical assessment. Fever with that pain, or an inability to pass urine, is an emergency, because an infected or fully obstructed system needs urgent treatment. Blood in the urine should always be checked by a clinician rather than watched. None of this is a substitute for individual medical advice — it is a map of where the strong evidence points.

The bottom line

Kidney stones are largely a plumbing problem: keep the urine dilute and you keep the minerals dissolved. Fluid first, then sensible limits on salt and very high-oxalate foods, and — importantly — enough dietary calcium, not less [s1][s2]. For stubborn recurrences, testing-guided medication adds further protection [s4]. The dull advice is the effective advice.

Sources

• [s1] Medical Management of Kidney Stones: AUA Guideline, Journal of Urology, 2014-08-01
• [s2] Eating, Diet, & Nutrition for Kidney Stones, NIDDK
• [s3] Urinary Volume, Water and Recurrences in Idiopathic Calcium Nephrolithiasis: a 5-year Randomized Prospective Study, Journal of Urology, 1996-03-01
• [s4] Dietary and Pharmacologic Management to Prevent Recurrent Nephrolithiasis in Adults: A Clinical Practice Guideline From the American College of Physicians, Annals of Internal Medicine, 2014-11-04