Kidney stones form when minerals and salts in urine crystallize and stick together. Most stones are made of calcium oxalate, a compound found in many foods and naturally produced by the liver. The main driver is urine that becomes too concentrated, allowing these crystals to form and grow over time.
How Do Kidney Stones Actually Form?
Your kidneys filter waste from your blood and send it out as urine. Normally, substances like calcium, oxalate, and phosphorus stay dissolved and pass through without issue. But when your urine volume is low or certain chemicals are out of balance, these substances can clump together.
Think of it like salt in water. If you keep adding salt to a glass of water, eventually the water can’t hold any more and crystals settle at the bottom. Your urine works the same way. When it becomes supersaturated with stone-forming compounds, crystals form. Those crystals can grow into stones over weeks or months.
The size matters. Tiny crystals pass without notice. But once a stone reaches about 3 millimeters — roughly the size of a grain of rice — it can cause pain as it moves through the urinary tract. Stones larger than 5 millimeters often get stuck.
What Are the Most Common Types of Kidney Stones?
Knowing the type matters because the cause and prevention differ for each. About 80% of all kidney stones are calcium stones, mostly calcium oxalate. The remaining 20% include uric acid stones, struvite stones, and cystine stones.
Calcium oxalate stones form when oxalate binds with calcium in the urine. High oxalate foods like spinach, rhubarb, and almonds can contribute, but the bigger issue is often not drinking enough water or having too much calcium in the urine for other reasons.
Uric acid stones are more common in people who eat a lot of animal protein — red meat, poultry, eggs, and shellfish. This type also tends to form in people with gout or those who lose too much fluid from chronic diarrhea. Struvite stones are linked to urinary tract infections and can grow quickly. Cystine stones are rare and caused by a genetic disorder that makes the kidneys leak too much cystine into the urine.
What Causes Kidney Stone Formation in the First Place?
The single biggest cause is not drinking enough water. When you are dehydrated, your urine becomes more concentrated. The CDC reports that about half of all people who have had a kidney stone could prevent a recurrence simply by drinking more fluids. Aim for urine that is pale yellow, not dark.
Diet plays a major role. High sodium intake forces your kidneys to excrete more calcium into the urine, raising the risk of calcium stones. The American Heart Association recommends keeping sodium under 2,300 milligrams per day, but most Americans eat closer to 3,400 milligrams.
Obesity is another well-established risk factor. Research published in the Journal of Urology found that people with a body mass index over 30 have a 50% higher risk of forming stones compared to those in a healthy weight range. Insulin resistance changes how the kidneys handle calcium and uric acid.
Certain medical conditions also increase risk. These include hyperparathyroidism, where the parathyroid glands release too much calcium into the blood, and renal tubular acidosis, a condition that makes urine too acidic. Recurrent urinary tract infections can also set the stage for struvite stones.
What Foods Increase Your Risk?
It is not just about eating too much oxalate. The real problem is eating high-oxalate foods without enough calcium at the same meal. Calcium binds to oxalate in the gut, which keeps both from reaching the kidneys. Without calcium present, oxalate gets absorbed into the bloodstream and ends up in urine.
Here are dietary factors that raise risk:
- High sodium foods — processed meats, canned soups, fast food, salty snacks. Sodium increases calcium in urine.
- Animal protein in excess — red meat, poultry, eggs, and shellfish increase uric acid and lower citrate, a natural stone inhibitor.
- High oxalate foods eaten alone — spinach, beets, rhubarb, almonds, Swiss chard, and potatoes. Pair them with a calcium source like cheese or yogurt.
- Sugary drinks — soda, especially cola, contains phosphoric acid which can promote stones. Fructose from sweetened beverages also increases uric acid excretion.
- Vitamin C supplements — high doses of vitamin C (over 1,000 mg daily) can convert to oxalate in the body. The National Institutes of Health notes this is a real risk for stone formers.
One common myth is that cutting out all calcium will prevent stones. The opposite is true. Low calcium diets actually increase stone risk because oxalate gets absorbed instead of binding to calcium in the gut. The NIH recommends getting calcium from food, not supplements, for most people.
What Does the Research Show About Prevention?
The strongest evidence supports three strategies: drink enough water, reduce sodium, and eat a balanced diet with adequate calcium from food. A study published in the Annals of Internal Medicine followed 220,000 adults and found that those who drank the most water had a 30% lower risk of kidney stones compared to those who drank the least.
Citrate is a natural stone inhibitor. It binds to calcium and prevents crystals from forming. Foods high in citrate include lemons, limes, oranges, and melons. Some research suggests drinking lemon water daily can raise urinary citrate levels, though the effect is modest compared to prescription potassium citrate.
For people with uric acid stones, reducing animal protein and increasing plant-based foods is effective. A study in the Journal of the American Society of Nephrology found that a DASH-style diet — rich in fruits, vegetables, nuts, legumes, and low-fat dairy — reduced stone risk by up to 40% compared to a standard Western diet.
Weight loss also helps. Even a 5% reduction in body weight can lower urinary oxalate and uric acid levels. Exercise alone, without weight loss, does not appear to have the same effect based on current evidence.
How Do Kidney Stone Symptoms Differ by Stone Location?
Where the stone is stuck determines the symptoms. A stone still inside the kidney often causes no symptoms at all. Many people discover them accidentally during an X-ray or CT scan for another reason.
Once a stone moves into the ureter — the tube connecting the kidney to the bladder — pain starts. This is called renal colic. The pain comes in waves as the ureter tries to push the stone down. It typically starts in the side or back, below the ribs, and radiates to the lower abdomen and groin.
Other symptoms include blood in the urine, which happens because the stone scrapes the lining of the urinary tract. Nausea and vomiting are common because the pain stimulates the vagus nerve. Fever and chills indicate an infection and require immediate medical attention.
What Treatments Actually Work for Existing Stones?
Small stones — under 5 millimeters — pass on their own about 80% of the time. Doctors typically recommend drinking extra water and taking over-the-counter pain relievers like ibuprofen. Tamsulosin, a medication that relaxes the ureter, may help stones pass faster.
For larger stones, procedures are needed. Shock wave lithotripsy uses sound waves to break stones into small pieces that pass in urine. Ureteroscopy involves threading a small scope through the bladder and ureter to remove or break up the stone with a laser. Percutaneous nephrolithotomy is a surgical option for very large stones, where a small incision is made in the back to remove the stone directly.
The table below compares common treatments:
| Treatment | Stone Size Range | Recovery Time | Success Rate |
|---|---|---|---|
| Watchful waiting | Under 5 mm | Days to weeks | 80% pass rate |
| Shock wave lithotripsy | 5–20 mm | 1–2 days | 70–90% |
| Ureteroscopy | 5–20 mm | 1–2 days | 85–95% |
| Percutaneous nephrolithotomy | Over 20 mm | 3–5 days | 90–95% |
When Should You See a Doctor?
Any kidney stone that causes fever or chills is a medical emergency. This means the stone has caused an infection that can spread to the bloodstream. Sepsis from a kidney stone can be life-threatening within hours.
You should also see a doctor if the pain is severe enough that you cannot sit still, if you see blood in your urine, or if you have persistent nausea and vomiting that prevents you from drinking fluids. An inability to pass urine for more than 8 hours is another red flag.
For people who have had one stone, the risk of a second stone within 5 years is about 50%. A 24-hour urine collection test can identify the specific chemical imbalances driving stone formation. This test measures volume, pH, calcium, oxalate, uric acid, citrate, and sodium levels. Knowing these numbers allows targeted prevention.
Frequently Asked Questions
Can drinking more water prevent kidney stones?
Yes, drinking enough water to produce at least 2.5 liters of urine per day is the most effective prevention strategy. It dilutes stone-forming substances and reduces the chance of crystallization.
Are kidney stones painful?
Yes, kidney stones cause severe pain that comes in waves as the stone moves through the ureter. The pain is often described as sharp and stabbing, radiating from the back to the groin.
Can certain foods cause kidney stones?
Yes, high sodium foods, excess animal protein, and high-oxalate foods eaten without calcium increase risk. Sugary drinks and high-dose vitamin C supplements also contribute.
Do kidney stones go away on their own?
Small stones under 5 millimeters pass on their own about 80% of the time. Larger stones often require medical treatment such as shock wave therapy or surgery.
