Kidney stones form when minerals in your urine become so concentrated that they crystallize and stick together into solid masses. This happens through a specific chain of events inside your kidneys, and the triggers range from what you eat and drink to underlying medical conditions and genetics. Globally, over 100 million new cases are recorded each year, with men accounting for more than two-thirds of them.
What Happens Inside Your Kidneys
Your urine naturally contains dissolved minerals like calcium, oxalate, phosphate, and uric acid. Normally, these stay dissolved because your urine also contains inhibitors (citrate is the most important one) that prevent crystals from forming. A kidney stone starts when the balance tips: too many stone-forming minerals, not enough inhibitors, or both.
When your urine becomes supersaturated with a particular mineral, free-floating ions begin combining into tiny solid particles in a process called nucleation. For example, dissolved calcium links with oxalate to form calcium oxalate crystals. Once that initial seed crystal (called a nidus) exists, more minerals layer onto it. The crystal grows, clumps together with other crystals, and eventually gets stuck in the narrow tubules of your kidney. That trapped, growing mass is your stone.
The Four Main Types of Stones
Calcium Oxalate and Calcium Phosphate
These account for roughly 80% of all kidney stones. They form when calcium and oxalate (or phosphate) levels in urine are high relative to the inhibitors keeping them dissolved. Despite what you might assume, eating too little calcium can actually increase your risk. Calcium in your digestive tract binds oxalate from food before it ever reaches your kidneys. Without enough dietary calcium, more oxalate gets absorbed into your bloodstream and filtered through your kidneys, raising the concentration in your urine.
Uric Acid
Uric acid is a byproduct of breaking down purines, compounds found in high concentrations in red meat, organ meats, and shellfish. When your body produces too much uric acid or your urine is consistently acidic (low pH), uric acid crystallizes instead of staying dissolved. People with insulin resistance and type 2 diabetes tend to produce more acidic urine, which is why uric acid stones are more common in that population.
Struvite
Struvite stones are caused by urinary tract infections. Certain bacteria, including Proteus, Klebsiella, and Pseudomonas, produce an enzyme called urease that breaks down urea into ammonia. This makes the urine alkaline, which raises the concentration of ammonium and phosphate ions until they crystallize. Struvite stones can grow quickly and become very large, sometimes filling the entire interior of the kidney.
Cystine
Cystine stones are the rarest type and result from a genetic condition called cystinuria. People with this inherited disorder have a defective amino acid transporter in their kidneys, causing excessive amounts of the amino acid cystine to spill into the urine. Cystine doesn’t dissolve well, so it forms stones repeatedly, often starting in childhood or early adulthood. The condition is autosomal recessive, meaning you need to inherit a copy of the gene from both parents.
Dietary and Lifestyle Causes
Not drinking enough water is the single most straightforward cause. When urine volume drops, the concentration of stone-forming minerals rises. Urologists recommend producing at least 2.5 liters of urine per day to keep minerals diluted enough to stay in solution. For most people, that means drinking about 3 liters of fluid daily, more in hot climates or if you exercise heavily.
High sodium intake is a surprisingly powerful driver of calcium stones. The way your kidneys filter and reabsorb salt forces them to lose calcium in proportion. The more salt you eat, the more calcium ends up in your urine, pushing it closer to supersaturation. Processed foods, restaurant meals, and canned soups are the biggest sources for most people.
Certain foods are especially high in oxalate: spinach, rhubarb, wheat bran, nuts, and peanuts. If you’ve already had a calcium oxalate stone, reducing these foods lowers the oxalate load your kidneys have to handle. But cutting calcium from your diet is counterproductive. Eating calcium-rich foods at meals lets calcium bind oxalate in your gut before it reaches your kidneys.
High animal protein intake raises your risk through multiple pathways. It increases uric acid production, makes urine more acidic, and reduces citrate (your urine’s main natural inhibitor). A diet heavy in red meat, poultry, and fish pushes all three of those levers in the wrong direction simultaneously.
Vitamin C Supplements and Stone Risk
Your body converts excess vitamin C into oxalate, which your kidneys then excrete. A systematic review presented at a 2024 urology conference found that vitamin C supplements significantly raise urinary oxalate, by an average of nearly 10 mg per day compared to baseline. That increase was consistent in both people with a history of stones and those without. If you take high-dose vitamin C supplements and you’re prone to calcium oxalate stones, this is a meaningful and modifiable risk factor.
Medical Conditions That Cause Stones
Primary hyperparathyroidism is one of the most common metabolic causes. The parathyroid glands overproduce a hormone that pulls calcium from your bones into your blood. Your kidneys then filter that extra calcium, and urine calcium levels can climb to 350 to 450 mg per day (normal is under 200). Both calcium oxalate and calcium phosphate supersaturation rise sharply, and stones form repeatedly until the underlying condition is treated, typically by surgically removing the overactive gland.
Inflammatory bowel diseases like Crohn’s disease increase stone risk through a different mechanism. When your intestines are inflamed or you’ve had bowel surgery, fat isn’t absorbed properly. Unabsorbed fat binds to calcium in your gut, leaving oxalate free to be absorbed in much larger quantities. That extra oxalate floods your kidneys.
Chronic urinary tract infections set the stage for struvite stones, as described above. People with structural abnormalities in their urinary tract or who use catheters long-term are especially vulnerable because bacteria can colonize more easily.
Who Is Most at Risk
Men develop kidney stones at roughly twice the rate women do, though the gap has been narrowing. Having one stone significantly raises your odds of forming another: about half of first-time stone formers will have a recurrence within 5 to 10 years without preventive changes. Family history matters too. Even outside of rare genetic conditions like cystinuria, having a parent or sibling with stones roughly doubles your risk, likely through shared tendencies in how your kidneys handle calcium and other minerals.
Obesity and weight gain independently increase risk. Excess body weight is associated with more acidic urine, higher calcium excretion, and greater uric acid production. Gastric bypass surgery, while effective for weight loss, can dramatically increase oxalate absorption and is a well-recognized trigger for calcium oxalate stones in the years following the procedure.
Hot climates and physically demanding jobs that cause heavy sweating also contribute. When you lose fluid through sweat without replacing it, urine volume drops and concentration rises. Seasonal patterns in kidney stone cases reflect this: emergency room visits for stones peak in summer months.