Kidney stones form when certain minerals in your urine become so concentrated that they crystallize and stick together. This usually happens through a combination of not drinking enough water, eating certain foods, and having metabolic or genetic factors that tip your urine chemistry out of balance. Most stones are made of calcium and oxalate, but the specific type you develop depends on what’s driving the imbalance.
The Basic Process Behind Stone Formation
Your kidneys filter waste products out of your blood and flush them out in urine. When you’re well-hydrated and your urine chemistry is balanced, minerals like calcium, oxalate, and uric acid stay dissolved. But when urine becomes too concentrated, or when levels of these substances get too high, they can form tiny crystals. Those crystals attract more material, gradually building into a stone that can range from a grain of sand to a golf ball.
Two things make this more likely: too much stone-forming material in your urine, or too little of the substances that normally prevent crystals from clumping together. Citrate is one of the body’s key natural inhibitors. Low urine volume is the single most common factor across all stone types, because it concentrates everything.
Not Drinking Enough Water
Dehydration is the most straightforward cause. When you don’t drink enough fluids, your kidneys produce less urine, and the minerals in that urine become more concentrated. Urologists typically recommend that people who’ve had a stone aim to produce 2.5 to 3 liters of urine per day, which translates to roughly 3 liters or more of fluid intake depending on your activity level and climate. Most people produce far less than that without conscious effort.
Hot weather, heavy exercise, and physically demanding jobs all increase your risk simply because you lose more water through sweat. This is one reason kidney stone rates are higher in warmer regions and peak during summer months.
How Your Diet Creates Stones
What you eat has a surprisingly direct effect on urine chemistry. Several dietary patterns increase your risk.
Salt and Calcium in Urine
High sodium intake forces your kidneys to excrete more calcium. When you eat a lot of salty food, the extra sodium that passes through your kidneys drags calcium along with it, raising the calcium concentration in your urine. Research in the American Journal of Clinical Nutrition confirmed that switching from a low-sodium to a high-sodium diet significantly increased the calcium-to-creatinine ratio in urine, a direct marker for stone risk. Processed foods, restaurant meals, and cured meats are the biggest sources of hidden sodium for most people.
Animal Protein and Uric Acid
Diets heavy in red meat, organ meats, shellfish, sardines, and anchovies are high in compounds called purines. Your body breaks purines down into uric acid. When uric acid builds up in urine and the urine becomes too acidic, uric acid stones can form. Beer and alcohol also raise purine levels. Even if you don’t form a pure uric acid stone, excess uric acid can serve as a seed for calcium stones to build on.
The Calcium Paradox
This one surprises most people: eating more calcium from food actually lowers your stone risk, while taking calcium supplements raises it. A large study found that women in the highest fifth of dietary calcium intake had a 65% lower risk of kidney stones compared to those who ate the least calcium. Meanwhile, participants taking calcium and vitamin D supplements had a 17% higher rate of new stones.
The reason comes down to timing and location. Calcium from food mixes with oxalate in your intestines, binding to it so neither gets absorbed into your bloodstream. That bound calcium-oxalate complex passes harmlessly through your stool. But calcium supplements, especially taken between meals, get absorbed into your blood and eventually filtered by your kidneys, where they can combine with oxalate in your urine and crystallize. The takeaway: dairy, leafy greens, and other calcium-rich foods are protective, not harmful.
Oxalate-Rich Foods
Oxalate is a natural compound found in spinach, rhubarb, beets, nuts, chocolate, and tea, among other foods. On its own, eating moderate amounts of these foods isn’t usually a problem. But if you eat high-oxalate foods without enough dietary calcium to bind them in the gut, more oxalate gets absorbed into your blood and filtered into your urine, where it can pair with calcium and crystallize.
Medical Conditions That Raise Your Risk
Obesity and Metabolic Syndrome
Kidney stone rates have risen in parallel with the obesity epidemic, and the connection is more than coincidental. A study published in the American Journal of Kidney Diseases found that people with all five traits of metabolic syndrome (obesity, high blood pressure, elevated blood sugar, unhealthy cholesterol, and high triglycerides) were about three times more likely to have kidney stones than people with none of those traits. The prevalence climbed steadily: 3% of people with zero traits had stones, compared to 7.5% with three traits and 9.8% with all five. Excess body weight tends to make urine more acidic and increases calcium and uric acid excretion, both of which promote stone formation.
Overactive Parathyroid Glands
Your parathyroid glands regulate calcium levels in your blood. Normally, they release a hormone that pulls calcium from your bones when blood calcium dips too low, then stop when levels normalize. In hyperparathyroidism, the glands ignore the “stop” signal and keep pumping out hormone, flooding the blood with calcium. Your kidneys then filter that excess calcium into your urine, creating ideal conditions for calcium stones. If you keep forming calcium stones without an obvious dietary explanation, this is one of the conditions your doctor will check for with a simple blood test.
Gut Conditions and Fat Malabsorption
People with Crohn’s disease, ulcerative colitis, or those who’ve had gastric bypass surgery face a higher stone risk through an indirect mechanism. When the small intestine can’t properly absorb fat, that unabsorbed fat binds to calcium in the gut, leaving oxalate free to be absorbed into the bloodstream instead. The kidneys then filter out the excess oxalate, and it combines with calcium in the urine. This process can dramatically increase oxalate levels and lead to frequent calcium oxalate stones.
Urinary Tract Infections
Certain bacteria that cause urinary tract infections produce an enzyme that breaks down urea (a normal waste product in urine) into ammonium and other byproducts. This makes the urine highly alkaline, with a pH above 7.2, which creates the right conditions for a type of stone called a struvite stone. These stones can grow quickly and become very large, sometimes filling an entire section of the kidney. They’re more common in women and in people who use catheters. Unlike other stone types, struvite stones are driven by infection rather than diet or metabolism, so treating the underlying infection is essential.
Genetics and Family History
If your parents or siblings have had kidney stones, your risk roughly doubles. Some people inherit a tendency to absorb more calcium from food, excrete more oxalate, or produce urine with less citrate. A condition called cystinuria, which is entirely genetic, causes the kidneys to leak an amino acid called cystine into the urine, forming a rare but recurrent type of stone that typically starts in childhood or early adulthood.
Why Some People Get Stones Repeatedly
About half of people who form one kidney stone will form another within five to ten years. This happens because the underlying conditions that created the first stone, whether dietary habits, low fluid intake, or a metabolic tendency, usually persist unless you actively change something. The risk factors compound over time. Someone who eats a high-sodium diet, doesn’t drink much water, and carries extra weight is stacking multiple causes on top of each other.
The most effective single change for prevention is increasing fluid intake enough to consistently produce pale, dilute urine. Beyond that, reducing sodium, eating calcium-rich foods with meals, moderating animal protein, and maintaining a healthy weight each chip away at the risk. For people with recurrent stones, a 24-hour urine collection can identify exactly which minerals are out of balance, allowing targeted dietary adjustments rather than guesswork.