A co-worker of mine had to leave work the other day to go to a local emergency department because of severe pain, nausea and vomiting.
As it turned out, she had a kidney stone, or renal lithiasis. I asked her if she drank a lot of milk and she said no. She was told to stay away from milk because calcium is what forms kidney stones. I explained to her that yes, calcium stones are a type of kidney stone, but drinking milk isn’t the problem. She looked at me with a quizzical eye.
Let’s clear up some of the misconceptions about kidney stones and diet. There are four major types of kidney stones that can form in the body. Calcium stones are the most common.Tthey can be calcium oxalate or calcium phosphate stones. Oxalate is a substance that occurs naturally in food such as chocolate, instant coffee, cocoa, nuts, wheat bran, spinach, sweet potato, rhubarb and many other fruits and vegetables. The liver also produces oxalate. Certain dietary factors, such as high doses of vitamin D, intestinal bypass surgery and metabolic disorders can increase the concentration of calcium or oxalate in the urine.
Uric acid stones form when the diet is consistently acidic, often caused by a diet rich in purines. Purines are found in foods such as bacon, beef, pork and game meats; fatty fish such as anchovies, sardines, herring and mackerel; gravy; liver, kidney and other organ meats; as well as beer and other alcohol.
Struvite stones are the result of kidney infections. Lastly, cystine stones result from a genetic disorder that causes the amino acid cystine to leak through the kidneys into the urine, forming crystals that accumulate and eventually turn into stones.
Kidney stones form when substances in the urine such as calcium, oxalate and phosphorus become too highly concentrated. After the body uses what it needs from food, the waste products left over are carried in the bloodstream to the kidneys for excretion in the urine. Certain foods have a greater propensity than others to promote stone formation in people who are susceptible. Other factors that can affect kidney stone formation are fluid intake, body weight, environment and genetics.
The dietary recommendations for reducing your risk of kidney stones include:
- Drinking plenty of fluids
- Eating fewer oxalate-rich foods
- Consuming a diet lower in salt and animal products
- Eating calcium rich-foods
Drinking plenty of fluid is promoted as the most important thing you can do to prevent kidney stone formation. Water is best but other fluids help. Some studies suggest that citrus drinks such as orange juice or lemonade help to protect against kidney stone formation because they contain citrate, which inhibits crystals from growing into stones. Two to three quarts a day is a healthy amount of fluid to consume.
The body does make some oxalate, but eating certain foods with high levels of oxalate can increase the amount in the urine. Spinach, rhubarb, nuts and wheat bran are four oxalate-rich foods that have been shown to increase the amount of oxalate in urine and should be avoided if you are at risk for kidney stone formation.
Sodium causes the kidneys to excrete more calcium into the urine, and these high concentrations of calcium can combine with oxalate and phosphorus to form stones. It is much more beneficial to reduce sodium intake rather that reduce calcium intake.
Animal protein may increase the risk of calcium stones by increasing the excretion of calcium and reducing the excretion of citrate, which prevents kidney stones, into the urine. The acid in animal protein reduces citrate in the urine.
Adequate calcium intake from foods has been shown to lower the likelihood of kidney stones in people most at risk. Research led by Dr. Eric Taylor, a kidney specialist at Maine Medical Center in Portland, found that men and women who consumed the most dietary calcium from foods had about a 20 percent lower risk of developing kidney stones than peers who consumed the least calcium. The researchers found that people who consumed the least calcium, about 150 milligrams per day (about 4 ounces of milk) had a 30 percent higher chance of developing a kidney stone than those who consumed between 800 and 900 milligrams per day, or about the amount of calcium in three eight-ounces glasses of milk.
Given that the most common kidney stones are made of calcium oxalate, it might seem counterintuitive to recommend consuming more calcium to end up with less in your kidneys. The real culprit isn’t calcium but oxalate. Oxalate is found in many foods and it binds fervently to calcium. So the more calcium in the digestive tract, the more oxalate it can bind to and remove from the body, before the oxalate is absorbed into the bloodstream and ends up in the kidneys.
People don’t understand this process and cut back on calcium, which increases their risk, according to Dr. Matthew Sorensen, a urologist at the University of Washington in Seattle. He said he sees patients every week who have cut back on their calcium intake – it’s one of the most important myths about kidney stones that needs to be corrected. Sorensen recommends a moderate calcium level for patients, around 1,000 to 1,200 milligrams per day.
Special note for teachers, health-care professionals and others who can’t get to the bathroom when they should: Good voiding habits, including frequent urination, are important in preventing kidney stones.
Georgia Clark-Albert is a registered dietitian and adjunct nutrition instructor at Eastern Maine Community College who lives in Athens. Read more of her columns and post questions at bangordailynews.com or email her at GeorgiaMaineMSRDCDE@gmail.com.