A kidney stone is a solid mass developed from crystals that separate from the urine within the urinary tract.
Kidney stones can contain various components. The most common type of stone contains calcium in combination with either oxalate or phosphate. A less common type of stone is caused by an infection in the urinary tract. This type of stone is called a struvite or infection stone. Another type of stone, uric acid stones, are less common, and cystine stones are rare.
It is not completely understood as to why kidney stones form. The single biggest factor in the formation of kidney stones is diet. Certain foods may promote stone formation in people who are susceptible, and a person with a family history of kidney stones may be more likely to develop them. Urinary tract infections, kidney disorders such as cystic kidney diseases, and certain metabolic disorders are also linked to stone formation.
Over the past 30 years, the number of people in the United States diagnosed with kidney stones has been increasing. In the late 1970s, less than 4% of the population experienced kidney stones. By the early 1990s, the percentage of the population with stones had increased to more than 5%. The number of Americans suffering from kidney stones between 2007 and 2010 nearly doubled from 1994, according to a new study by researchers at UCLA and the RAND Corporation. Researchers reviewed responses from 12,110 individuals and found that between 2007 and 2010, 8.8% of the U.S. population had a kidney stone, which is one out of every 11 people. In 1994, the rate was one out of 20. Caucasians are more prone to develop kidney stones than African Americans, and stones are more prevalent in men. The incidence of kidney stones rises dramatically as men enter their 40s and continues to rise into their 70s. For women, the incidence of kidney stones usually peaks in their 50s. Once a person develops stones, they are more likely to get them again. Researchers suggest that the substantial increase in kidney stone cases is most likely due to increases in obesity, diabetes, and gout, which are risk factors for the condition.
Obesity plays a very big role in kidney stone formation. Obesity is associated with insulin resistance and compensatory hyperinsulinemia, metabolic imbalances that may lead to the formation of calcium-containing kidney stones. Hyperinsulinemia may contribute to the development of calcium stones by increasing the urinary excretion of calcium. Larger body size may also result in increased urinary excretion of uric acid and oxalate, risk factors for calcium oxalate kidney stones. However, the biggest problem is that people who are obese tend to have more salt and animal protein in their diet, and this predisposes them to stone formation. A new study from Johns Hopkins University shows that obesity in general nearly doubles the risk of developing kidney stones, but the degree of obesity does not appear to increase or decrease the risk one way or the other. Using a definition of obesity as having a BMI greater than 30 kg/m2, the researchers calculated the incidence of kidney stones in people who were non-obese and in those who were obese. Among the non-obese individuals, 2.6% were diagnosed during the study period with kidney stones, compared to 4.9% of the obese individuals. When the researchers arranged those in the obese group by their BMIs, ranging from above 30 kg/m2 to more than 50 kg/m2, they found that the increased risk remained constant, regardless of how heavy the individuals were.
Individuals with type 2 diabetes are at an increased risk for developing kidney stones in general, and have a particular risk for uric acid stones. Researchers at the Mayo Clinic followed 3,500 patients over a 20-year span and concluded that those with diabetes developed 40% more uric acid kidney stones than those without diabetes. People with type 2 diabetes have highly acidic urine, and this metabolic feature helps to explain their greater risk for developing uric acid stones. It was found that obesity and a diet rich in animal protein are related to abnormally acidic urine.
Prevention of kidney stones is very important. The DASH diet (Dietary Approaches to Stop Hypertension) is primarily known for its ability to decrease hypertension (high blood pressure). The diet is comprised of foods low in saturated fat, total fat, cholesterol, and sodium. It is rich in protein and fiber and limits foods that are high in salt and added sugars.
According to a large-scale study by researchers at Brigham and Women’s Hospital, those who followed a DASH diet were 40% to 45% less likely to develop kidney stones. The results of the study were consistent among all participants regardless of gender, age, BMI, or diabetes. All participants experienced fewer kidney stones than their counterparts who did not follow a DASH diet. Those who followed a DASH diet excreted greater amounts of urine than those who did not follow the diet, even though both groups consumed similar amounts of fluid. This is believed to be the case because of the higher water content in DASH diet foods.
According to the study’s authors, “Consumption of a DASH-style diet is associated with a marked decrease in kidney stone risk… The impact was similar in men and women, in older and younger individuals, and in participants with both low and high body-mass index.”
Researchers suspect that the decrease in kidney stone risk is due to the high fruit and vegetable intake of the DASH diet, which increases urinary citrate, inhibiting the formation of calcium-based kidney stones. Additionally, normal or high calcium intake and low animal protein and sodium intake, all features of the DASH diet, are known factors for lowering the risk of kidney stones.
If an individual does develop kidney stones, there are several methods of treatment. Surgery is often not necessary because most kidney stones can pass through the urinary tract with plenty of water, usually 2 to 3 quarts per day. A person susceptible to stone formation should try to drink enough fluids throughout the day to produce at least 2 quarts of urine in every 24-hour period. Some people may be told to avoid certain foods. Also, certain medications may be prescribed to help prevent calcium and uric acid stones. These medications control the amount of acid or alkali in the urine, which are crucial factors in crystal formation.
If surgery is necessary to remove a kidney stone, several different procedures can be utilized. Extracorporeal shock wave lithotripsy (ESWL), percutaneous nephrolithotomy, and ureteroscopic stone removal are common procedures, with ESWL being the most frequently used procedure for the treatment of kidney stones. Fortunately, surgical treatment has greatly improved, and many procedures do not require major open surgery and can be performed in an outpatient setting.
— Amanda Zell, Doctor of Pharmacy Candidate, Mercer University
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