Medical Articles

Kidney Stones Diet

Afia’a Mohammed Al Masri, Chief Dietician at Al-Ahli Hospital/Qatar


The kidneys’ main function is to filter the blood to remove excess mineral salts and other soluble wastes. Also they produce the urine that dissolves these wastes and excretes them through the urinary tract. Kidney stones form when the urine becomes so saturated with a certain mineral that no more of it can dissolve into the urine. The undissolved portion of the mineral forms crystals that then clump together and grow into hard stones. Kidney stones usually develop in the kidneys. However they can form anywhere in the urinary tract. This condition is medically known as urolithiasis or nephrolithiasis. 

When kidney stones are quite tiny, they may pass unnoticed with the urine. Often however, they grow too large to pass easily through the urinary tract, and some stones have rough or sharp edges. When these stones are passing through the urinary tract, it can be quite painful. In some cases, kidney stones cannot pass on their own, and treatment with specialized medical equipment or surgery may be necessary. 

There are various types of kidney stones. Because treatment for each differs, it is important for the physician to determine the stone’s mineral content and to identify any medical conditions that may have contributed to stone formation. Preventive treatment may be with medications and/or changes in the diet. About 80% of all kidney stones are composed of calcium and other minerals, usually a combination of calcium and oxalate. In some cases, dietary adjustments help to prevent the recurrence of these types of stones. 

Special Considerations

1. Increase fluid intake. This is the most important preventive measure for all patients who develop kidney stones. It hinders the formation of stones by diluting the urine. Patients should drink enough fluid to produce two quarts or more of urine each day. As a guideline, drink 8-10 oz of fluid (on average =250 ml=1 cup) every hour while awake and 8-10 oz once during the night if awakened for some reason. At least 50% of the total fluid intake should be water. In warmer climates and for physically active people, an even higher fluid intake is recommended. 

2. Calcium: Calcium from food sources is absorbed during digestion in the intestines. The body uses this mineral for many important functions. Any excess that has been absorbed is excreted or passed through the kidneys. The biggest portion of calcium in the diet comes from milk and foods made from large amounts of milk, such as cheeses and yogurt. The calcium in these foods is usually easily absorbed. Other foods, such as dark green leafy vegetables, contain significant amounts of calcium. However, they also contain other substances which prevent the body from readily absorbing the calcium. So, the amount of available calcium in green leafy vegetables is less than in milk. Certain antacids and over-the-counter medications also contain calcium that may or may not be in a form the body can absorb. Consume your daily needs for calcium which is approximately 2 cups of milk or yogurt with 60 grams of cheese or 4 table spoons of Labna.

If the physician has recommended a calcium-controlled diet, the idea is to keep calcium intake within a narrow range, not too much and not too little. On this diet, men are advised to limit calcium intake to 800 mg per day. Prior to menopause, women should limit calcium to 1000 mg per day; and after menopause, these women should have 1200 mg of calcium a day. 

3. Oxalic acid or oxalate is found mostly in foods from plants. Calcium combines with oxalate in the intestines. This reduces calcium’s ability to be absorbed. Sometimes oxalate or calcium oxalate stones form because there is not enough calcium in the intestines. Then, too much oxalate goes to the kidneys to be excreted through urine, this is called hyperoxaluria. In certain cases of oxalate or calcium oxalate stones, the physician may recommend reducing oxalate intake along with a slight increase in calcium. It is recommended that these patients have no more than 50 mg of oxalate per day in the diet. To do this, foods with high or moderate amounts of oxalate should be reduced or eliminated from the diet. 

Although there are many foods that contain large amounts of oxalate, eight foods have been shown to be most at fault for raising urine oxalate levels. They are rhubarb, spinach, strawberries, chocolate, wheat bran, nuts, beets, and tea. 

4. Sugar, sodium, and animal protein: It has been found that too much of these may also aggravate the development of calcium or calcium oxalate stones. Natural sugars found in food are not a concern. However, people who get kidney stones may benefit from avoiding packaged foods with large amounts of added sugars, and from reducing sugars added in food preparation and at the table. 

Reducing sodium in the diet appears to reduce the amount of calcium excreted in the urine. Consequently, people who develop stones containing calcium may benefit from keeping sodium intake between 2300 to 3500 mg a day. A diet high in animal protein affects certain minerals in the urine that may promote the formation of kidney stones. Therefore, people who tend to develop kidney stones should avoid eating more protein than the body needs each day. The physician or registered dietitian can recommend a daily protein intake for individual patients. 

5. Insoluble fiber: There are two types of fiber: soluble (dissolves in water) and insoluble. Both provide important functions in the body, but it is insoluble fiber (found in wheat, rye, barley, and rice) that may help to reduce calcium in the urine. It combines with calcium in the intestines, so the calcium is excreted with the stool instead of through the kidneys. Insoluble fiber also speeds up movement of substances through the intestine, so there will be less time for calcium to be absorbed. 

6. Vitamin C: Your body may turn extra vitamin C into oxalate. Avoid high doses of vitamin C supplements (more than 2,000 mg of vitamin C per day). 

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