Take a look at a kidney stone under a microscope, and you’ll understand why the pain of passing a stone is unforgettable. Most stones are spiked with razor-sharp crystals. No wonder those who’ve gone through the experience say the agony is equivalent to a knife in the back. Kidney stones develop when urine concentrations of minerals and other dissolved substances get so high that the minerals can no longer remain dissolved. Stones can also form if the pH (acid-alkaline balance) of urine is too high or too low. In all cases, the minerals form insoluble crystals and precipitate, or drop out, of the urine, exactly the same way too much sugar drops to the bottom of a glass of iced tea. The crystals collect in the kidney ducts, slowly solidifying into stones.
Most doctors these days rely on both dietary measures and drugs, often diuretics (which decrease urinary calcium and increase urine flow), to keep kidney stones from coming back.
Know Your Stone
While some dietary changes seem to help prevent all kinds of kidney stones, a few work for only certain types of stones. So it’s important to know the kind of stone you have formed, doctors say. The only way to do that is with laboratory analysis of a captured stone. The most common type of kidney stone, made of calcium oxalate, is found in more than 80 percent of cases.
“It’s also important to know why you’re forming stones. The only way to do that is with urine and blood tests and measures of levels of some hormones, such as parathyroid hormone, which regulates body levels of calcium,” explains Freda Levy, M.D., clinical associate professor at Methodist Medical Center in Dallas. “People form stones for lots of different reasons, including metabolic abnormalities and infections.”
Check with your doctor to make sure you’re selecting the best dietary changes for your specific condition before you try any of these measures, she adds.
Magnesium May Counterbalance Calcium
The chemistry behind kidney stone formation is complex. Some doctors believe that the ratio of calcium to magnesium, another essential mineral, in the diet is important. They recommend that people who’ve had one or more bouts of calcium oxalate stones make sure that they get at least the Daily Value of magnesium, 400 milligrams, through diet and supplements, if necessary.
Most kidney specialists believe that there’s only a minor role, if any, for magnesium in the treatment of kidney stones. They might recommend supplemental magnesium to someone whose urine is low in magnesium and high in calcium, which is a rare condition, says Fred Coe, M.D., professor of medicine and physiology and chief of nephrology at the University of Chicago Pritzker School of Medicine.
But some researchers and some doctors with an interest in nutrition believe that magnesium’s potential for preventing stones has not been fully appreciated. They maintain that getting an optimum amount can help prevent stones in many people.
“Doctors think it doesn’t work because they don’t try it,” says Stanley Gershoff, Ph.D., professor of nutrition and dean emeritus at Tufts University School of Nutrition in Medford, Massachusetts.
In a study that Dr. Gershoff did years ago, 149 people who had had at least two stones annually for five years saw their stone formation drop dramatically when they started taking 300 milligrams of magnesium a day. (They also took 10 milligrams of vitamin B6 a day, which is discussed below.) The people were followed for 41¼2 to 6 years. Over 90 percent had no stones during that period, Dr. Gershoff says. Only 12 people continued to make stones, but with much less frequency, he adds. “I think magnesium is definitely worth a try,” he says.
Studies also show that magnesium-deficient animals are more likely than normal to develop calcium oxalate crystals in their kidneys, making stones more likely.
In Dr. Gershoff’s studies, urine from people taking supplemental magnesium was capable of holding more than twice as much calcium oxalate in solution compared with urine from people not taking magnesium. This finding held even when the pH and the amount of calcium in the urine were adjusted so that they were exactly the same for both groups.
“Magnesium helps prevent calcium oxalate from crystallizing, although exactly how it does that isn’t known,” Dr. Gershoff says. One theory, that magnesium competes with calcium to bind with oxalate and forms a soluble compound that can be excreted from the body, is intriguing but not proven, Dr. Gershoff says.
He recommends that anyone who has passed a calcium oxalate stone take 300 milligrams of supplemental magnesium a day. “That amount worked just fine in our study,” he says. Some other doctors recommend taking 400 to 500 milligrams daily.
Studies show that most men get about 329 milligrams a day and most women get about 207 milligrams a day through foods.
Stick to the lowest dose that works for you and get medical supervision, especially if your kidneys have been damaged or if you have a heart problem, Dr. Gershoff says.
Good food sources of magnesium are green vegetables, nuts, beans and whole grains.
| Food Factors Many doctors consider the following dietary adjustments proven and effective kidney stone stoppers. Here's what they recommend. Stay well-watered. The more water that you take in, the less chance that minerals in your urine will form crystals that lead to stones. "Aim for at least a half-gallon of water a day or an eight-ounce glass every other waking hour," says Fred Coe, M.D., professor of medicine and physiology and chief of nephrology at the University of Chicago Pritzker School of Medicine. If you care to measure, you should be producing about two liters of urine a day. (Large plastic soda bottles contain almost two liters, or roughly a half-gallon.) Drinking enough water helps prevent all types of stones, and it's especially important for people who live in hot, dry climates. Shake the salt habit. Too much salt raises urine calcium levels, which ups your risk of kidney stones. Some doctors aim for a maximum daily salt intake of about 2,400 milligrams, about half of the usual intake. To go that low, avoid most processed foods, especially lunchmeats, soups and frozen dinners, and toss out your saltshaker. Don't be so sweet. "Sweet treats raise urine calcium levels at the same time they decrease urine volume, causing a very high concentration of calcium in the urine," Dr. Coe explains. If you must eat dessert, make it a small one. And avoid sweet snacks. Don't have a cow (or a pig or a chicken). For people eating the typical American diet, high meat consumption is associated with calcium oxalate stones, studies show. Animal protein increases the concentration of both calcium and uric acid in the urine, Dr. Coe says. "Many men with kidney stones are big meat-eaters," he adds. "We try to get them under ten ounces a day, and the less, the better." Don't cut back on calcium. Cutting back on dairy products and other calcium-rich foods used to be standard advice for people with kidney stones. Turns out, though, that people who get more calcium in their diets are less likely to develop kidney stones than people who get less calcium. If you're taking calcium supplements, though, don't go above 1,000 milligrams a day without your doctor's okay, Dr. Coe says. Go easy on C. The Daily Value for vitamin C is a mere 60 milligrams, but many people ingest more to take advantage of vitamin C's many healing benefits. It's especially important for people who've had kidney stones to not jump on the vitamin C bandwagon with too much enthusiasm. Some doctors recommend that if you are taking vitamin C supplements, you stay below 500 milligrams. That's because one by-product of vitamin C metabolism may be oxalate, which is half of the most common kidney stone, says Dr. Coe. Scratch oxalates off your grocery list. Beans, cocoa, instant coffee, parsley, rhubarb, spinach and black tea are all loaded with stone-causing oxalates. And ask your doctor about others. "We give our patients a list of the oxalate contents of about 200 foods," Dr. Coe says. Don't worry about coffee--or beer. Although both of these beverages up calcium excretion, they also increase urine volume, so there's no increase in calcium concentration in the urine. In fact, some people rely on beer's strong diuretic effects to flush out kidney stones, Dr. Coe says. |
Vitamin B6 Provides Anti-oxalate Protection
Along with magnesium, some doctors recommend vitamin B6 to people who get kidney stones.
“A vitamin B6 deficiency throws up a roadblock in the body’s metabolism, so more oxalic acid is made, which means that high amounts get into the urine,” Dr. Coe explains. Oxalic acid then combines with calcium to form insoluble calcium oxalate, the stuff from which stones form.
In one study, conducted in India, researchers found that people with a history of kidney stones who took 40 milligrams of vitamin B6 a day were much less likely to form stones than they were prior to beginning the vitamin. (A few people required up to 160 milligrams a day before they stopped forming stones.)
Most kidney stone specialists, however, discount the idea that people in the United States could be so shortchanged when it comes to vitamin B6 that they develop kidney stones as a result. “It might be given to someone whose urine is very high in oxalic acid, but in my opinion, most stone-formers aren’t B6-deficient,” Dr. Coe says. People with stones are seldom tested for B6 deficiency or asked about their intakes of B6-rich foods such as fish, bananas and nuts. (Studies show that in the United States, both men and women get less than the Daily Value of 2 milligrams of B6. Men average 1.87 milligrams a day; women, 1.16 milligrams a day.)
If you’re supplementing vitamin B6, stick to no more than 50 milligrams a day without medical supervision, says Dr. Coe. In large doses, B6 has been associated with nerve damage. Stop taking B6 if you develop numbness in your hands or feet or unsteadiness in walking. Medical experts suggest that if you’re taking B6 supplements, make sure you’re also taking a well-balanced multivitamin/mineral formula that includes the array of B-complex vitamins. (B vitamins work in harmony with each other.) But again, be sure that the two supplements combined give you no more than 50 milligrams of B6 a day.
| Prescriptions for Healing Some doctors recommend these nutrients, in a range of amounts, as part of a program to prevent a recurrence of kidney stones. Check with your doctor first to determine whether these supplements might help you. Nutrient Daily Amount
Magnesium 300-500 milligrams Potassium 3,500-4,500 milligrams Vitamin B6 Up to 50 milligrams (including the amount in a multivitamin/mineral supplement containing the B-complex vitamins)
MEDICAL ALERT: No supplement program dissolves kidney stones that have already formed. If you have kidney or heart problems, check with your doctor before taking supplemental magnesium. People taking potassium-sparing diuretics or who have kidney disease or diabetes should not use potassium supplements without first consulting their doctors. Some doctors recommend taking no more than 50 milligrams of vitamin B6 without medical supervision. Large doses have been associated with nerve damage. Stop taking B6 if you develop numbness in your hands or feet or unsteadiness in walking. |
Protection with Potassium Power
Medical experts agree that eating grains, vegetables and fruits helps avert kidney stones, and one reason for this may be that vegetarian fare offers lots of potassium. Low levels of this mineral can increase the risk of stone formation.
“People with low potassium levels, and especially those on potassium-draining diuretics such as thiazides, are likely to be prescribed potassium supplements and to be told to get more potassium in their diets,” explains Lisa Ruml, M.D., assistant professor of medicine and a researcher in the Department of Mineral Metabolism at the University of Texas Southwestern Medical Center at Dallas. “Low potassium can lead to low urine citrate, which is the direct reason for increased stone risk.”
Doctors who recommend potassium as a preventive for kidney stones generally suggest aiming for 3,500 to 4,500 milligrams daily. You can get this amount by eating at least five servings of fruits and vegetables, including plenty of citrus fruits and juices, every day.
One form of this mineral, potassium citrate, which is available by prescription, may be helpful not only for people with low blood levels of potassium but also for many who form calcium oxalate stones, Dr. Ruml adds.
In a study done by researchers at the University of Texas Southwestern Medical Center at Dallas, people cut their chances of forming new stones to close to zero during three to four years of daily potassium citrate therapy.
“Potassium citrate changes the pH of urine, making it able to hold more calcium oxalate without forming crystals because citrate is increased,” Dr. Ruml explains. “Instead of forming stones, the calcium oxalate is excreted in the urine. We use potassium citrate now in most of our patients who get calcium stones.”
Potassium citrate supplements should be taken only under medical supervision, Dr. Ruml says. People taking potassium-sparing diuretics or who have kidney disease or diabetes should not use potassium supplements of any kind without first consulting their doctors.
