How Kidney Stones Form—and Why They Keep Coming Back

A Painful and Growing Problem

Few medical experiences rival the agony of passing a kidney stone. These small, hard mineral deposits that form inside the kidneys affect roughly one in ten adults in the United States, and their prevalence has nearly tripled since 1980—rising from 3.2% to over 10% of the population. Globally, the trend is the same: kidney stones are becoming more common across all ages, sexes, and ethnic groups, making them one of the most widespread urological conditions on the planet.

How Stones Form Inside the Body

Kidneys filter blood and produce urine, which carries dissolved waste products—including calcium, oxalate, uric acid, and phosphate—out of the body. Normally these substances stay dissolved. But when urine becomes too concentrated, either because fluid intake is low or waste levels are high, the dissolved minerals can crystallize and clump together into solid masses.

The process is called supersaturation. When the concentration of stone-forming ions exceeds a threshold, crystals nucleate and grow. Tiny crystals may flush out unnoticed, but larger ones can lodge in the kidney, ureter, or bladder, blocking urine flow and causing intense, wave-like pain known as renal colic.

The Four Main Types

Not all kidney stones are the same. Their composition determines how they form, how they are treated, and how likely they are to return.

• Calcium oxalate stones account for 70–80% of all cases. They form when urine pH drops below 7.2 and calcium and oxalate levels are high.
• Calcium phosphate stones form in more alkaline urine and are often linked to metabolic conditions like renal tubular acidosis.
• Uric acid stones develop when urine is persistently acidic, commonly associated with gout or high-protein diets rich in red meat and seafood.
• Struvite stones are caused by chronic urinary tract infections and can grow rapidly into large, branching formations called staghorn calculi.

A rare fifth type, cystine stones, affects only 1–2% of patients and results from a genetic disorder called cystinuria.

Why Are Stones Becoming More Common?

Researchers point to several converging factors behind the global surge in kidney stones.

Diet and Obesity

Modern diets high in sodium, animal protein, and sugar promote stone formation by raising calcium and uric acid excretion while lowering protective citrate levels. The concurrent rise in obesity and diabetes further increases risk, as metabolic syndrome alters urine chemistry in ways that favor crystallization, according to research published in Reviews in Urology.

Climate and Dehydration

Higher ambient temperatures cause greater fluid loss through sweat, concentrating the urine. A study in Scientific Reports projected that warming temperatures could increase heat-related kidney stone cases by up to 3.9% by the end of the century. By 2095, an estimated 70% of Americans may live in high-risk kidney stone zones, up from 40% in 2000.

How Doctors Treat Kidney Stones

Small stones under 5 mm often pass on their own with hydration and pain management. Larger or stubbornly lodged stones require intervention:

• Extracorporeal shock wave lithotripsy (ESWL) uses focused sound waves to shatter stones into passable fragments—a noninvasive outpatient procedure that works best on stones smaller than 2 cm.
• Ureteroscopy threads a thin scope through the urinary tract to locate and laser-fragment stones, particularly effective for harder calcium oxalate monohydrate stones that resist shock waves.
• Percutaneous nephrolithotomy is reserved for very large or complex stones, requiring a small incision in the back to access the kidney directly.

Prevention: Harder Than It Sounds

The standard advice—drink more water—is simple but surprisingly difficult to sustain. Dietary changes can make a meaningful difference: studies show that patients who significantly modify their diet cut recurrence roughly in half. Citrate salts are among the most effective medical preventions, reducing two-year recurrence from 44% to just 11%, according to the Institute for Quality and Efficiency in Health Care.

Counterintuitively, low-calcium diets can actually increase stone risk by about 50%, because dietary calcium binds oxalate in the gut and prevents its absorption. The National Kidney Foundation recommends maintaining normal calcium intake while reducing sodium and animal protein.

With prevalence climbing and climate projections pointing upward, understanding how kidney stones form—and how to prevent them—has never been more relevant.