March 4, 2014
News Review From Harvard Medical School -- DASH Diet May Help Prevent Kidney Stones
A standard healthy diet may help prevent kidney stones as well as a special diet, a small study suggests. People who have had kidney stones often are advised to eat a diet low in "oxalate." This chemical binds to calcium to form the most common type of stone. But the diet also prohibits many healthy foods. Researchers randomly divided 51 people into 2 groups. One group followed a low-oxalate diet. The other followed the DASH diet, which helps prevent high blood pressure. This diet is high in fruits, vegetables, low-fat dairy and whole grains. It is low in fat, salt, sugar and meat. In all, 41 people completed the 8-week study. People on the DASH diet had more oxalates in their urine than those on the low-oxalate diet. But the DASH group was less likely to have oxalates bound to calcium in the urine. These are the compounds that form stones. Researchers said that calcium and oxalates, eaten at the same meal on the DASH diet, may have bound together in the digestive system. Thus, they would be less likely to pass through the kidneys and into urine. The American Journal of Kidney Disease published the study. HealthDay News wrote about it March 1.
By Howard LeWine, M.D.
Harvard Medical School
What Is the Doctor's Reaction?
By some estimates, about 10% of Americans will have a kidney stone at some point in their lives. People tend to get their first stones in their 20s and 30s. A first stone usually means a second one. There's an 80% chance you'll have another stone within 10 years of your first.
About 80% of kidney stones contain calcium. Usually this is combined with a chemical called oxalate. Doctors used to think that high-calcium diets caused high calcium levels in the urine and this led to calcium-containing stones. But that's not the case. It's the higher amounts of oxalate and other things in the urine that lead to the most common kind of stones.
Today people with calcium-oxalate stones are told to avoid oxalate-rich foods. These foods include spinach, walnuts, beets, strawberries, chocolate and tea. But only 10% of the oxalate in the urine comes from digested food. So it's unclear how significant dietary sources really are.
This new study asks whether a generally healthy diet could do just as well as a low-oxalate diet to decrease the risk of forming calcium-oxalate kidney stones. The researchers chose a popular diet recommended to help keep blood pressure in the normal range. It's called the DASH diet.
DASH stands for Dietary Approaches to Stop Hypertension. The diet contains lots of fruits and vegetables, as well as whole-grain foods and low-fat dairy products. It's low in fats (especially saturated fats), salt, sugar, refined grains and meat.
Based on their findings, the researchers concluded that the DASH diet had at least as much potential as a low-oxalate diet for decreasing stone formation. Compared with a low-oxalate diet, the DASH diet resulted in higher levels of oxalate in the urine. But the chemical properties of urine also were different with the DASH diet. These properties decreased the ability of calcium to combine with oxalate. So this means less chance that they could actually form stones.
What Changes Can I Make Now?
More and more research suggests that Mediterranean-style diets are generally the most healthful. The DASH diet is very similar to Mediterranean-style diets, with one important exception.
The original DASH diet suggested reducing all fats. Now we know that not all fats are bad. In fact, monounsaturated fats, such as olive oil, are actually healthy when used in moderation.
Both the DASH and Mediterranean diets promote whole-grain foods. But you don't want to eat too many whole-grain foods if you have had a calcium-oxalate stone. Whole grains, especially bran, contain lots of oxalate.
Another way to help prevent kidney stones is just the opposite of what doctors used to tell people. Today, doctors advise eating more calcium-rich foods. Dietary calcium provides some protection by binding to oxalate in the intestine. This helps prevent oxalate absorption into the blood. Therefore, less oxalate goes to the kidneys and out into the urine. But don't take calcium supplements. They don't do the same thing.
The most important advice to prevent any type of kidney stone is to drink plenty of fluids and avoid even mild dehydration. Watery urine helps calcium and those substances that bind to it stay dissolved.
What Can I Expect Looking to the Future?
This study just looked only at how the DASH diet might lead to less binding of calcium oxalate in the urine, compared with a low-oxalate diet. The study had only a few participants. It also did not examine whether one diet was better than the other in actually preventing kidney stones that cause symptoms. That's the study that needs to be done in the future.