Kidney stones are rocklike deposits that form in the kidneys, the two bean-shaped organs that filter waste and extra fluid from the body.
Kidney stones typically develop when too much waste enters the urine without enough fluid in the kidneys to dilute it.
Waste can include minerals and other substances that combine to form crystals. These crystals can clump together, forming kidney stones that range in size from a grain of sand to a pea to as large as a Ping-Pong ball.
While in the kidney, the stone may not cause any symptoms. But if a larger stone travels down the ureter, the tube that transports urine from each kidney to the bladder, it can cause pain, problems urinating, and blood in the urine, among other symptoms.
Main Types of Kidney Stones
There are two main types of kidney stones: calcium stones and noncalcium stones, explains Ganesh Shidham, MD, a professor in the division of nephrology at the Ohio State University Wexner Medical Center in Columbus.
Calcium Stones
Calcium stones account for about 80 percent of all kidney stones.
Calcium can bind with a chemical called oxalate to form calcium-oxalate stones. Oxalate is found in foods such as nuts, spinach, chocolate, and certain teas, says Dr. Shidham. Calcium oxalate stones can form due to high doses of
vitamin D, dietary choices, or intestinal or metabolic disorders.
Calcium can also bind with phosphate in the kidneys to form calcium phosphate stones.
Phosphate is a chemical compound found in foods such as milk, eggs, and chocolate.
Calcium phosphate stones may develop in people with renal tubular acidosis, a condition in which the kidneys do not properly filter acids from the blood, or be a result of taking certain medicines.
Noncalcium Stones
Stones that do not contain calcium include:
- Uric Acid Stones These form when the urine contains too much acid. This can happen in people who eat a lot of protein.
- Cystine Stones These can develop in people who have a condition called cystinuria, in which the urine contains too much of the amino acid cystine.
- Struvite Stones These can form due to a urinary tract infection, from the waste products of bacteria. For struvite to develop, your urine needs to be alkaline. These are also called infection stones and consist of magnesium ammonium phosphate.
“If you pass a kidney stone, don’t throw it away. Take it to your doctor for testing. Knowing the type of stone helps doctors decide how to prevent future stones,” says Shidham. He maintains that a 24-hour urine test is very important in helping your doctor determine what is causing the stones. “About 96 percent of people who form stones have a chemical imbalance in their urine. Fixing these problems can lower the chance of getting another stone by 50 to 80 percent.”
What Increases Your Risk of Kidney Stones?
Kidney stones can occur in almost anyone, says Anil Agarwal MD, clinical professor of nephrology at the University of California San Francisco School of Medicine. While common conditions such as diabetes and high blood pressure can increase the risk of kidney stones, otherwise healthy individuals can also develop them, notes Dr. Agarwal.
Kidney stones can develop in both men and women, but they’re more likely to develop in men, according to the Urology Foundation.
Stones are also more common in white people than in African American people.
A number of lifestyle factors may also contribute to kidney stones.
Not Drinking Enough Water
Often, people with kidney stones aren’t drinking enough water, says Agarwal. “Or they may be doing hard physical labor or working out, sweating, and losing fluids that way.”
Not drinking enough water causes the urine to become concentrated, meaning the levels of waste products, including excess calcium, are high. “The solution? Drink more water,” says Shidham.
Not Consuming Enough Calcium
People often believe that if they have calcium stones, they shouldn’t be eating foods high in calcium, says Shidham. But eating a low-calcium diet may increase your risk of stones by up to 50 percent, according to the National Kidney Foundation.
Low calcium levels can cause oxalate levels to rise in the kidneys, leading to stones.
“If you have calcium stones, you should not stop eating normal calcium foods,” advises Shidham. “Studies show that a normal calcium diet is better than a low-calcium diet for stone risk. However, it’s best to avoid calcium supplements unless your doctor advises them.”
A High-Salt or High-Sugar Diet
Some causes of increased calcium in the urine may include a diet high in salt or refined sugars, says Shidham.
Eating Large Amounts of Oxalate-Rich Foods
A high amount of oxalate-rich foods can potentially contribute to the oxalate levels in your urine, causing calcium-oxalate stones. Shidham says that these include:
- Nuts, including almonds
- Spinach
- Beets
- Dark chocolate
- Cocoa
Because some of these foods offer many health benefits, doctors may suggest reducing how much you eat (rather than eliminating them entirely). Or they may recommend eating oxalate-containing foods alongside those that are rich in calcium, so that the calcium and oxalate bind in the stomach or intestines before they reach the kidneys.
Drinking Colas
Drinking colas, which contain phosphate, can also increase calcium levels in the urine and make calcium-phosphate stone formation more likely, says Agarwal.
Consuming Too Much Protein
People who eat a lot of protein, especially animal protein, may produce uric acid stones, says Agarwal.
Shidham cautions that too much animal protein and protein supplements can raise urinary calcium and uric acid levels and lower protective citrate levels, increasing the risk of stones.
A Lack of Stone Formation Inhibitors
Some natural substances present in normal urine have the capability to block kidney stone formation, but some people lack those inhibitors in their urine, Agarwal explains. “So, even though a lot of us are running around and not drinking water, not everyone gets a stone because of those inhibitors of stone formation present in urine,” he says. Notably, citrate, a form of citric acid, is an important inhibitor of stone formation. “If you’re excreting very low amounts of citrate (one of the most important inhibitors) in the urine, you have little protection from stone formation,” Agarwal adds.
Low citrate content could be due to certain medications, chronic diarrhea, kidney acid-base problems, or high protein intake, according to Shidham.
Family History and Genetics
Kidney stones can run in families, too, so genetic factors may also play a role, says Agarwal.
Other Health Conditions Can Raise the Risk of Kidney Stones
In addition to diabetes, high blood pressure, and cystinuria, other health conditions linked to developing kidney stones include:
- A urinary tract blockage
- Recurrent urinary tract infections
- Hyperuricosuria, in which the urine contains too much uric acid
- Kidney cysts, in which fluid-filled sacs form on the kidneys
- Cystic fibrosis
- Obesity
- Inflammatory bowel disease
- Digestive tract surgery, such as gastric bypass, which can lead to changes in digestion that affect how your body absorbs calcium and water
- Enteric hyperoxaluria, which is when the intestine excessively absorbs oxalate, leading to high calcium levels in the urine
- Gout, a condition that causes painful swelling of the joints due to deposits of uric acid
- Hyperparathyroidism, in which the parathyroid gland releases too much parathyroid hormone, causing extra calcium in the blood
Doctors should ask you about any underlying medical conditions when diagnosing kidney stones, Shidham says.
Agarwal warns that most people with kidney stones don’t realize that they come back frequently after their removal or normal passage in the urine.
“A consultation with a kidney doctor and commonly available blood and urine tests can easily identify specific risk factors,” Agarwal explains. “Once an individual understands their risk factors, common interventions like getting plenty of hydration, supplementation with citrate, and avoidance of diets rich in animal protein, oxalate, and phosphate can reduce or eliminate the risk of recurrent kidney stones.” And he concludes that following a healthy lifestyle and diet can reduce the risk in the first place.
