What Causes Kidney Stones?

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A kidney stone usually announces itself the loud way — and once the pain is over, one question takes its place: why did that happen, and is it coming back? Here is the short answer first: a kidney stone is a cluster of mineral crystals that grew in urine which sat too concentrated for too long — and the things that concentrate urine and load it with crystal-building minerals are knowable, and many of them are changeable. This page is the after page: what a stone actually is, the honest list of what causes them, why kidney disease and stones feed each other in both directions, and — as the second half — what genuinely lowers the odds of a second one. If a stone is hurting you right now, this is not the page to start with: start with what a kidney stone feels like, and get care first. Read this one when the episode is behind you and the why is what's keeping you up.

What a Kidney Stone Actually Is

One breath, plainly: a kidney stone is a hard little cluster of crystals — minerals that were dissolved in your urine, came out of solution, and grew. Urine is the body's liquid waste stream, and it normally carries dissolved minerals — calcium, oxalate, uric acid, and others — out of the body invisibly, the way salt stays invisible in a glass of salt water. But leave a saturated solution standing and crystals begin to form; that is kitchen chemistry as much as kidney chemistry. When urine is concentrated — too little water for the amount of mineral dissolved in it — or when one ingredient runs unusually high, tiny crystals can form, clump, and grow layer by layer inside the kidney. Most stay small and wash out unnoticed. The ones that lodge in the narrow tube between kidney and bladder, the ureter, are the ones that make the night nobody forgets. So every cause on this page works by one of two routes: it concentrates the urine, or it overloads one of the ingredients. Hold onto that, and the whole list below makes sense.

The Master Cause: Not Enough Fluid, Day After Day

If this page had to name one cause above all others, it is this one: not drinking enough fluid overall, so the urine stays concentrated hour after hour, year after year. Stones are most common in people whose urine volume runs low — hot climates, physical work, a lifetime habit of barely drinking, a bladder that makes extra trips feel like trouble so the glass stays empty on purpose. The kidneys still have to clear the same daily load of minerals; with less water to carry them, the minerals sit closer together, and closer together is where crystals start. It is rarely one dry day that does it. It is a pattern — concentrated urine as a way of life — that gives crystals the seasons they need to grow. Whatever else is true of your stones, this is the ground they grew in, which is why the prevention half of this page starts here too.

The Salt Road

The second road is the kitchen salt shaker — and the far bigger hidden salt in restaurant meals, deli meats, canned soups, and packaged food. The connection is direct chemistry: when you eat more sodium, your kidneys clear more sodium — and calcium rides out with it. More salt in means more calcium in the urine, and more calcium in the urine is more raw material for the most common kind of stone there is. This is the same sodium this Foundation writes about for hearts and blood pressure, arriving at the kidney from the other direction: the salt that strains the heart also loads the urine. A person can drink faithfully and still grow stones on a high-salt diet, because the water arrives but so does the calcium it has to carry.

The Four Kinds of Stones

"Kidney stone" is a family name, not a single thing, and the kind matters — because each kind has its own causes, and its own prevention. The four families, plainly:

• Calcium stones — by far the most common. Usually calcium paired with oxalate, a substance found in many plant foods and also made by your own body. These grow when urine runs concentrated, calcium runs high, or oxalate runs high — the master cause, the salt road, and diet all feed this family. Despite the name, they are very often not caused by eating too much calcium — a paradox this page returns to in the prevention half, because getting it wrong makes stones more likely, not less.
• Uric acid stones. Uric acid is the waste product behind gout, and in urine that runs acidic it can crystallize on its own. Rich diets heavy in red meat and shellfish feed it; so do gout, diabetes, and the metabolic cluster this Foundation's readers know well. This is the family where the kidney audience and the heart audience overlap most: the same ground that grows gout and strains the heart grows these stones.
• Struvite stones — the infection stones. These form only when certain bacteria live in the urinary tract and change the urine's chemistry, building fast-growing stones that can fill the kidney's inner spaces. They are tied to repeated or untreated urinary infections, which is one reason infections in this readership are never something to simply ride out.
• Cystine stones — rare, and inherited. One honest line: a small number of people inherit a condition that spills an amino acid called cystine into the urine, and it crystallizes. It usually shows up young; it is not caused by anything eaten or done.

The Body Conditions That Feed Stones

Some stones are less about the kitchen than about the body itself. Three conditions earn a plain sentence each:

• An overactive parathyroid. Four small glands in the neck govern the body's calcium. When one runs overactive, it pulls calcium out of the bones and into the blood — and from the blood into the urine. Repeated calcium stones are sometimes the first sign of this fixable gland problem, which is why it belongs on the workup list below.
• Gout. High uric acid in the body means high uric acid available to the urine — feeding uric acid stones directly, and making the urine chemistry friendlier to calcium stones as well.
• Bowel surgery and inflammatory bowel disease. When the bowel absorbs fat poorly — after some bowel operations, or in conditions like Crohn's disease — it also absorbs more oxalate than it should, and that extra oxalate leaves through the urine, where it feeds calcium oxalate stones.

The Medicine Shelf

A shorter, careful section, because it is the one most easily misread: a few medicines and supplements can tilt urine chemistry toward stones — and the correct response is always to ask, never to stop. Certain water pills (the loop kind, prescribed often for heart failure and swelling) increase the calcium the kidneys clear into urine. Calcium supplements and high-dose vitamin D, especially taken on their own away from meals, can raise urine calcium in some people. Very high-dose vitamin C is partly converted by the body to oxalate. None of that makes any of these medicines bad — several of them are protecting the heart of the person taking them — and stopping a prescribed drug over a stone can do far more harm than the stone did. It makes them a conversation: bring the full list, supplements included, to the clinician reviewing your stone, and let the two of you weigh it together. Ask, don't stop.

Kidney Disease and Stones Feed Each Other — Both Directions

This is the section that makes stones a Cardio Renal Foundation subject and not just a urology one, because the traffic runs both ways. Stones can injure kidneys: a stone that blocks the flow of urine backs pressure up into the kidney behind it; an infection that takes hold behind a blockage is one of the true emergencies in this whole territory; and repeated episodes, repeated procedures, and infection stones that fill the kidney's spaces can scar tissue that does not grow back. And damaged kidneys change the urine: kidneys that filter less well handle acid, calcium, and water differently, shifting the chemistry stones grow in — which is part of why stone formers and kidney patients are so often the same people at different chapters of one story. A stone, in other words, is not always a one-off accident of a dry summer. Sometimes it is an early chapter of kidney disease, and sometimes kidney disease is what set the table for it. Either direction earns the same response: a stone deserves a kidney workup, not just pain relief and a pat on the back.

Why This Research Matters

A stone is, at bottom, a chemistry burden made visible — mineral load, fluid balance, and filtration meeting in one small, unforgettable object. The Cardio Renal Foundation supports research toward a solution beyond dialysis — work aimed at the fluid and chemical burdens that failing kidneys hand to the heart and the rest of the body. That is a direction, not a promise: nothing in a laboratory changes what a person who has just passed a stone should do this month, which is to learn which kind hers was and change what can be changed. But the research and the reading point the same way: burdens that can be measured can, one day, be lifted better — and in the meantime, they can very often be prevented from forming at all.

Keeping the Next One From Forming

Here is the pond's second half — prevention, which is really the causes list read backwards. Stones recur: without changes, a first stone is commonly followed by another within the years ahead. With the right changes, aimed at the right kind, the odds fall a great deal. The honest levers:

• Steady fluids through the day. The master cause reversed: enough fluid, spread across the day rather than loaded at once, that the urine stays dilute and pale. The usual clinical target is making roughly two to two and a half liters of urine a day — but read the caution below before treating any number on this page as yours.
• The senior caution, stated plainly: some readers of this page have been given a fluid limit by their own doctor — most often for heart failure or advanced kidney disease — and for them, drinking more on general advice can be genuinely dangerous. If you have a fluid limit, your doctor's number wins over any general advice, including this page's. Prevention for you is a plan made with your care team, balancing the stone against the heart — not a jug filled on the internet's say-so.
• Less salt. The salt road reversed: less sodium means less calcium spilled into the urine. The biggest wins are usually the hidden sources — restaurant meals, processed meats, canned and packaged foods — more than the shaker at home.
• Normal calcium from food — do not cut it. The paradox, plainly: people who slash dietary calcium often form more calcium stones, not fewer. Calcium eaten with a meal binds oxalate in the gut and carries it out in the stool; remove the calcium and the oxalate is absorbed instead, and leaves through the urine — feeding the very stones the diet meant to prevent. The sensible target for most stone formers is the normal recommended amount of calcium, from food, eaten with meals. (Calcium in pill form is a separate question — the medicine-shelf conversation above.)
• Lemon, and citrate, in plain words. Citrate is a natural stone-blocker: it grabs calcium in the urine and keeps crystals from forming and clumping. Lemons and limes carry citric acid, and a daily squeeze of lemon in water is a small, real step in the right direction for many people — a helper on the margins, not a treatment, and no substitute for the workup when stones repeat.
• Know what kind yours was. The single most useful prevention step costs nothing: catch the stone. Strain the urine during an episode, save whatever grit or pebble comes out, and ask for a stone analysis. A laboratory can name the stone's kind from a fragment the size of a grain of sand — and the kind decides everything downstream: which chemistry to change, which foods matter, whether a medicine would help. Prevention aimed at the wrong kind of stone is guesswork; prevention aimed at your kind is a plan.

First Stone or Second: Get the Workup, Not a Guess

So what should actually happen after a stone? For a first stone, and certainly for a repeat one, the episode deserves more than a scan and a shrug. A proper workup is calm, ordinary medicine:

• Blood tests — calcium (with the parathyroid question in mind), uric acid, and the kidney numbers themselves, creatinine and eGFR, because of the two-way street described above.
• A stone analysis, if any fragment was caught — the kind, in writing.
• The 24-hour urine, in plain words: for repeat stone formers, a jug you collect every drop in for one full day, which the laboratory reads as a complete chemistry portrait of your urine — how much water you actually make, and how much calcium, oxalate, uric acid, sodium, and citrate it carries. It sounds like a nuisance and it is the single most informative test in stone prevention: it replaces guessing about your chemistry with your chemistry.

And one sorting job belongs in this section too. If the painful episode was never actually sorted out — was it a stone at all, or a urinary infection, or an infection up in the kidney itself? — the three are confused for each other constantly, and they do not take the same response. This Foundation keeps a plain-words comparison for exactly that fork in the road: kidney stone, UTI, or kidney infection — how to tell the difference. Fever with flank pain, in particular, moves any of these out of the wait-and-see class and into same-day care.

So — what causes kidney stones? Crystals, growing where urine sat too concentrated and too loaded: not enough fluid, day after day, at the center; salt pulling extra calcium into the urine beside it; the stone families — calcium, uric acid, struvite, cystine — each with its own feeders, from gout and rich diets to urinary infections and one rare inherited trait; body conditions like an overactive parathyroid and a struggling bowel adding their share; and a medicine shelf worth a conversation, never a sudden stop. Underneath it all, kidney disease and stones feed each other in both directions, which is why a stone earns a workup and not just sympathy. And the second half of the answer is the hopeful half: dilute urine, less salt, normal calcium from food, a little citrate, and a stone analysis that tells you which kind you are preventing. Most second stones are not fate. They are chemistry — and chemistry can be changed.

At the Cardio Renal Foundation (Cardio Renal Foundation Inc, EIN 33-4359765), we publish education for exactly this reader — the one who has passed the stone and is asking why, and whether it has to happen again. Support from readers like you helps us keep publishing it.

This article is informational only and is not meant to replace the advice of your qualified healthcare provider. Nothing here diagnoses any condition or recommends any treatment for you personally. Never start, stop, or change a prescribed medicine without speaking to the clinician who prescribed it. Research supported by this Foundation is investigational only. As with any issues related to your health, check with your doctor.