Kidney Stones: Overview and Risk Factors
Kidney stones, or nephrolithiasis, are quite common in the United States and other Western societies; nearly 10 percent of Americans will develop a kidney stone during their lifetime.
Stones form when minerals, such as calcium, or other molecules, such as uric acid, are present in the urine at abnormally high levels. These substances form crystals that gradually increase in size and can become stuck within the kidney or urinary tract. Calcium-containing stones are the most common, making up more than 75 percent of cases.
Symptoms include severe flank pain, which may also be felt in the lower abdomen, groin, or testicles; painful urination; bloody urine; and nausea or vomiting.
- Male gender: Males are three times more likely to develop stones than females.
- History of kidney stones: Individuals who have developed a kidney stone have an 80 percent chance of recurrence within 10 years. Individuals with a family history of kidney stones have a two to three times higher risk.
- Geography: Individuals in areas of elevated temperatures and high humidity appear to have an increased risk.
- Nationality: Individuals in developed countries have a much higher risk compared with individuals in developing countries. This is presumed to be due to dietary factors, specifically the absence of a Western-style, meat-based diet.
- Obesity: Compared with normal-weight persons, obese men have a 33 percent greater risk for stone formation, while obese women have a 200 percent greater risk.
- Diet: Diet plays an important role, as described below in Nutritional Considerations.
- Chronic urinary tract infections
- Dehydration (e.g., diarrhea)
- A medical history and a physical examination can suggest the disease in most cases.
- CT scan of the abdomen and pelvis is the preferred test to detect kidney stones. In some cases, an X-ray of the abdomen will identify stones, but a CT scan is far more accurate. Ultrasound may also be used to visualize stones, particularly in individuals who should avoid radiation, including pregnant women.
- Blood and urine tests may be necessary.
- If a stone is passed, it should be sent to the laboratory for analysis.
- Small stones will often pass on their own and do not require medical treatment. Increased fluid intake may promote stone passage.
- In some cases, medications may be recommended to promote stone passage. These include tamsulosin (Flomax) or an alpha blocker (e.g., terazosin).
- About 10 to 20 percent of stones require surgical removal.
- Recurrence is common. People who tend to form stones should be instructed in methods of stone prevention, which are noted below.
Kidney Stones: Nutritional Considerations
Kidney stones are increasingly common. Their incidence increases with animal protein intake, obesity, and poor fluid intake. A vegetarian diet may offer significant protection against stone formation.
In observational studies, the following factors are associated with reduced risk:
- Reduced intake of animal protein and sodium: High dietary intake of animal protein increases the risk of kidney stones. Compared with individuals eating 50 grams or less of animal protein per day, those eating the most (77 grams or more) have a 33 percent higher risk for stones. Compared with a standard (low-calcium) diet used for prevention of calcium stone formation, a diet restricted in animal protein and sodium reduces the risk for stone recurrence by half.
- Limiting oxalates: Oxalate-rich foods may contribute to the formation of kidney stones. Patients may benefit from avoiding foods high in oxalate, including rhubarb, spinach, strawberries, chocolate (especially dark), wheat bran, nuts, beets, and tea. Alternatively, certain food preparation methods may be used to reduce oxalate content. Boiling, for example, reduces oxalate content by as much as 80 percent.
- Calcium at mealtime: Interestingly, calcium intake from foods lowers the risk for calcium stones, presumably because calcium decreases the absorption of oxalates. Individuals consuming the greatest amount of calcium from foods have about a 30 percent lower risk for stone formation, compared with persons consuming the lowest amounts. In contrast to calcium consumed at mealtime, calcium from supplements taken between meals may have a very different effect and may be associated with an increased risk of stones.
- Limiting colas, coffee, and tea: Although further research is required, evidence indicates that cola consumption significantly increases the risk for stones. Individuals who avoid colas have been found to have a 15 percent lower rate of stone recurrence than those who continue to consume these beverages. Coffee and tea may also be associated with an increased risk.
- Overall fluid intake appears to be most important. Individuals who consume the highest amount of fluids each day (about 2.6 liters) have a 30 to 40 percent lower risk for stone formation than those consuming the least (about 1.4 liters).
- Vegetarian diets: Nationwide surveys have determined that the risk for stone formation is 40 to 60 percent lower in individuals following vegetarian diets. Vegetarian diets may lower the risk for kidney stones in a number of ways. These include the absence of animal protein and higher intakes of magnesium and potassium, both of which are associated with lower risk for stone formation. Vegetarian diets also provide ample amounts of whole grains high in phytic acid, a chemical in plants that is associated with a 40 percent lower risk for stone formation in persons eating the most (about 900 milligrams/day), compared with those eating the least amount (about 600 milligrams/day).