About 1 in 15 of us will have kidney stones in our lifetime. Men are about three times more likely to be stone-formers than women, and white men between 20 and 60 are most susceptible. A kidney stone may not cause trouble. It can even pass painlessly out of the kidney through the urinary tract. But passing a stone, small or large, may cause severe pain, as well as nausea, vomiting and fever. If it blocks the urinary tract, that’s an urgent medical problem. One stone often means another is forming—and that other stones will eventually occur.
Genes play a role in kidney stones. Hot climates and dehydration (from not drinking enough and/or excess perspiration) are also predisposing factors for susceptible people. A spike in kidney stone rates has been reported among American troops in Iraq, and some scientists predict that global warming will produce a big increase in kidney stones.
Other stone-promoting factors include certain disorders such as gout and Crohn’s disease (an intestinal disorder). A study published in the American Journal of Kidney Diseases suggests that metabolic syndrome (a cluster of problems including obesity, hypertension and high blood sugar) is also associated with kidney stones. Obesity is classed as a risk factor on its own and appears to increase the risk in women more than in men. Diabetes also increases the risk on its own.
What’s in a stone
All kinds of waste products, as well as many minerals, are filtered from the blood by the kidneys and excreted in urine. If these minerals accumulate in highly concentrated urine, they may solidify. Stones are mixtures of minerals. The most common are made of calcium and oxalate (a non-nutrient found in plant foods but also made in the body) or calcium and phosphorus compounds. Phytates, which are phosphorus compounds, and oxalates are apt to combine with or bind minerals like calcium. Still other stones (less common) are made primarily of uric acid. A struvite stone, which may accompany a chronic kidney infection, is predominantly magnesium. Cystine, a building block of muscles and other tissue, is also found in some stones.
Eventually the stone finds its way to the ureter, the conduit between the kidney and the bladder. If you pass a kidney stone, it’s important to have it chemically analyzed, since different kinds of stones require different treatments and preventive measures. In any event, you will need medical advice and possibly medication, whatever type it turns out to be.
Diet and stones—scarce evidence
Diet plays a poorly understood role in kidney stones, and diets aimed at preventing stone recurrence vary widely. Advice that once was standard (such as “cut back on calcium”) is now known to be incorrect. Good evidence about kidney stone prevention is scarce, according to most researchers. Studies have usually been observational. There have been few clinical trials—which actually test the effects of different diets on stone formation—and they tend to be brief and/or small. A review by the Cochrane Collaboration could not find enough evidence to make recommendations about kidney stones and diet. It is a good idea to get a full metabolic evaluation after you are diagnosed with a stone, talk with your doctor about your kind of stone, and follow advice tailored for you.
Sound advice on kidney stones
- Drink plenty of fluids, but choose the right ones. The Cochrane review did find preliminary evidence to support this recommendation, but did not officially endorse it. However, according to Dr. Gary Curhan of Harvard, a researcher in this field, increasing fluid intake is beneficial. Some doctors measure urinary output for 24 hours and determine exactly how much more fluid the patient needs to drink throughout the day. In a review of numerous studies, published recentlyin the Annals of Internal Medicine, researchers found that increasing fluid intake halved the risk of recurrences in those who had already had a kidney stone. However, a recent study in the Clinical Journal of the American Society of Nephrology found a link between sugar-sweetened soft drinks and increased risk of kidney stones. While the evidence about this has been inconsistent, consuming lots of sugary beverages is not a good idea anyway, if only because they take the place of better beverages.
- Limit your sodium intake—less than 2,400 milligrams a day. There’s some controversy about this idea as a way to prevent stones, but it can’t hurt and many experts think it helps.
- A high calcium intake from dairy products seems to be protective. It used to be thought that calcium increased the risk, but research has shown that dietary sources may help prevent stones.Aim for about 800 milligrams of calcium from dairy. There’s no evidence that calcium from other food sources will help. A 2012 analysis from the Women's Health Initiative confirmed that consuming more calcium-rich foods, notably dairy products, helps reduce the risk of kidney stones. However, calcium supplements (especially when taken without food) may increase the risk slightly, according to previous WHI data and other research. Doctors sometimes recommend taking supplements with meals, even though it’s not known whether this will reduce the slightly increased risk seen with calcium supplements. Women who tend to form stones and take calcium should talk to their doctors about stopping the pills.
- Very high doses of vitamin C may promote stone formation in men. A recent large observational study of middle-age and older men, published in JAMA Internal Medicine, found that the use of vitamin C supplements (not vitamin C from food) doubled the risk of kidney stones. (The researchers note that the results may not be the same for women, who are at lower risk of kidney stones.)There’s no reason to take C supplements, in any case.
- Drinking a lot of lemonade is often encouraged, and some research supports the idea that lemon and orange juice might increase levels of citric acid in urine and thus help prevent calcium kidney stones. Again, follow your physician’s advice. Grapefruit juice, in preliminary studies, appears to increase the risk.
- Think twice about estrogen therapy. A 2011 study in the Archives of Internal Medicine of more than 10,000 postmenopausal women found that those given hormone therapy for six or seven years, on average, were about 20 percent more likely to develop a kidney stone than those given a placebo. It's unclear why; previously, it had been thought that the hormone might actually be protective, since the risk of stones rises in women after menopause.
- Adopt the DASH diet. A 2011 Harvard study of 3,400 health-care professionals found that the Dietary Approaches to Stop Hypertension (DASH) eating plan, which emphasizes fruits, vegetables and low-fat diary, put out more urine than those not on the diet—thanks to the higher water content of the foods they consume. In addition, the urine of those in the DASH group contained more citrate, and this compound helps inhibit calcium stones. Both higher fluid intake and citrate are associated with a decreased recurrence of kidney stones.
- Coffee, tea, beer, wine, and other alcoholic beverages used to be on the forbidden list, but newer opinion is that they are not a problem. It’s a myth that the caffeine in coffee and tea makes them dehydrating.
- Eating less animal protein was once standard advice, but now that’s not so certain. If you are forming uric acid stones (or have gout), you may need to reduce protein intake.
- Cutting out foods high in oxalate (such as spinach and other greens, peanuts, beets, wheat germ, scallops, mussels, berries, asparagus, chocolate, black tea, soy, and sweet potatoes) used to be standard advice. But there has never been good evidence to support this idea. Still, if you form calcium oxalate stones, your doctor may advise cutting out spinach and peanuts, which are very high in oxalates. Phytates (phosphorus compounds found in whole grains) may actually be protective.
Old advice now questionable
Many methods are used to treat stones that don’t pass out of the body. Some employ shock wave therapy (lithotripsy) to break up the stone into small pieces; this is usually done on an outpatient basis. There are also surgical procedures. Certain drugs can be prescribed to help prevent the recurrence of stones.
Word to the wise: Don’t fall for the many “natural” remedies advertised on the Internet. Don’t dose yourself with dietary supplements that are supposed to prevent stones. None are known to help, and some can be harmful.