The main cause of kidney stones is urine that becomes too concentrated with dissolved minerals, allowing crystals to form and clump together. About 80% of all kidney stones are made of calcium oxalate (sometimes mixed with calcium phosphate), making excess calcium and oxalate in the urine the single biggest driver. But “too concentrated” can happen for several reasons, from not drinking enough water to eating too much salt to having an underlying metabolic condition.
How Stones Actually Form
Your kidneys filter waste from your blood and send it out as urine. When the concentration of certain minerals in that urine rises past a tipping point, called supersaturation, those minerals can crystallize. Think of it like dissolving sugar in water: at some point, the water can’t hold any more and crystals start appearing at the bottom of the glass.
Stone formation follows a sequence. First, tiny crystals form (nucleation). Then those crystals grow and stick together (aggregation). If they attach to the inner surface of the kidney or collect in the drainage system, they keep accumulating layers of mineral and organic material over weeks or months until they become large enough to cause problems. People who form stones repeatedly tend to have urine that is consistently more supersaturated than people who don’t, which keeps the cycle going.
The Four Main Types of Stones
Calcium oxalate and calcium phosphate stones dominate, accounting for roughly 80% of cases worldwide. Struvite stones, which form in response to urinary tract infections, make up about 10%. Uric acid stones account for around 9%, and cystine stones, caused by a rare genetic condition, represent about 1%. The type of stone a person forms is closely tied to the specific chemistry of their urine, so the underlying cause varies by stone type.
Dehydration: The Most Common Trigger
Not drinking enough fluid is the most straightforward path to supersaturated urine. When urine volume drops, the same amount of calcium, oxalate, and other minerals gets packed into less liquid, pushing concentrations past the crystallization threshold. This is why kidney stones are more common in hot climates and during summer months.
The American Urological Association, the European Association of Urology, and the American College of Physicians all recommend producing at least 2 liters of urine per day to prevent stones. For most people, that means drinking roughly 2.5 to 3 liters of fluid daily, though it varies with activity level and climate. Increasing water intake to hit that 2-liter urine target has been shown to reduce stone recurrence and lengthen the time between episodes for people who have already had a stone.
Diet Plays a Bigger Role Than Most People Realize
Several dietary habits raise stone risk by changing urine chemistry in specific, measurable ways.
High Salt Intake
Sodium and calcium share a reabsorption pathway in the kidneys. When you eat a lot of salt, your kidneys excrete more sodium, and calcium gets pulled along with it. The result is higher calcium levels in urine, which directly increases supersaturation. Cutting back on processed and restaurant food, which account for most sodium intake, is one of the most effective dietary changes for calcium stone prevention.
Oxalate-Rich Foods
Oxalate is a natural compound found in spinach, beets, rhubarb, potatoes, nuts, chocolate, and brewed tea. Normally, oxalate binds to calcium in the gut and leaves the body in stool. But when fat absorption is impaired, or when calcium intake is too low, calcium binds to fat instead. That leaves oxalate free to be absorbed into the bloodstream, filtered by the kidneys, and concentrated in urine, where it can pair with calcium to form crystals. This is why very low-calcium diets can actually increase stone risk rather than reduce it.
Sugar and Fructose
High intake of table sugar and high-fructose corn syrup is linked to increased stone risk through several simultaneous effects. Fructose raises uric acid levels in the blood during metabolism, lowers urine pH (making it more acidic), and increases oxalate excretion. It also reduces urinary magnesium and citrate, both of which normally help prevent crystals from forming. In one study, fructose ingestion significantly raised serum uric acid, boosted urine oxalate by a measurable margin, and dropped urine pH. Sweetened beverages are one of the most consistent dietary risk factors identified in stone research.
Animal Protein
Diets heavy in red meat, poultry, and seafood increase the acid load your kidneys have to process. This lowers urine pH, which favors uric acid crystal formation. High animal protein intake also increases calcium excretion and reduces citrate, a natural stone inhibitor in urine.
Uric Acid Stones and Urine pH
Unlike calcium stones, uric acid stones are driven primarily by acidic urine rather than by high calcium levels. When urine pH drops below about 5.5, uric acid becomes much less soluble and crystallizes readily. People with gout, type 2 diabetes, or metabolic syndrome tend to produce more acidic urine and are at higher risk. Obesity itself shifts urine chemistry toward lower pH. Uric acid stones are unique in that they can sometimes be dissolved by making the urine more alkaline, which isn’t possible with calcium-based stones.
Infection-Related Struvite Stones
Struvite stones form when certain bacteria infect the urinary tract and produce an enzyme called urease. This enzyme breaks down urea into ammonia and carbon dioxide, which makes the urine highly alkaline (pH 7.2 to 8.0). In that environment, magnesium, ammonium, and phosphate combine to form struvite crystals. Bacteria that consistently produce urease include Proteus, Providencia, and Morganella species. Other bacteria do so less reliably: about 84% of Klebsiella strains produce it, 55% of Staphylococcus strains, and only about 1.4% of E. coli strains. Struvite stones can grow rapidly and become quite large, sometimes filling the entire collecting system of the kidney.
Metabolic and Genetic Causes
Some people form stones because of an underlying condition that changes their urine chemistry regardless of diet. Primary hyperparathyroidism, a condition where the parathyroid glands release too much hormone, increases calcium absorption from food and calcium excretion through the kidneys. People with this condition who form stones appear to have an exaggerated calcium response, absorbing and excreting more calcium than those with the same condition who don’t form stones.
Cystinuria is a genetic condition affecting about 1 in 10,000 people. Mutations in either of two genes (SLC3A1 or SLC7A9) disrupt a protein complex in the kidneys that normally reabsorbs the amino acid cystine back into the blood. Without proper reabsorption, cystine accumulates in the urine and forms crystals. These crystals sometimes combine with calcium to create large stones. Cystine stones tend to appear in childhood or early adulthood and recur frequently throughout life.
Why Some People Get Stones and Others Don’t
Stone formation usually isn’t caused by a single factor. It’s the overlap of several: genetic predisposition, dietary habits, hydration level, body weight, and sometimes an underlying medical condition. Two people can eat the same diet, but the one whose kidneys excrete more calcium or less citrate will be at higher risk. Family history is a strong predictor. If a first-degree relative has had kidney stones, your own risk roughly doubles.
Obesity increases risk across all stone types by altering urine pH, raising uric acid production, and increasing calcium and oxalate excretion. Weight loss surgery, particularly procedures that reduce fat absorption, can paradoxically raise oxalate levels in urine because unabsorbed fat in the gut binds calcium and frees oxalate for absorption.
For most people, though, the combination of drinking too little water, eating too much sodium, and consuming excess sugar or animal protein creates the conditions for that first stone. Since recurrence rates are high (roughly 50% within 5 to 10 years without intervention), understanding the specific cause behind your stone type is the most useful step toward keeping it from happening again.