What are the risk factors for kidney stones?
Kidney stones affect about 12% of men and 5% of women by the time they are 70 years old. About 80% of kidney stone sufferers are men between the ages of 20 and 50 years old. Caucasian men are at higher risk than other groups. Kidney stones that strike women are more apt to occur during pregnancy, usually in the late stages. During pregnancy, women tend to have a higher calcium intake and at the same time their kidneys handle calcium less efficiently. (Kidney stones are still a rare occurrence, however, affecting only 1 in 1500 pregnancies.)
Anatomic abnormalities in the urinary tract pose a significant risk for kidney stones in children. Children with low birth weight who need to be fed intravenously are also at risk for stones. Inherited factors account for 45% of all cases of kidney stones. People with a family history of kidney stones are at higher risk than those without relatives with stones. In one Italian analysis of kidney stone patients, 22% of their parents and 14% of their siblings also had one. A family history of gout may also predispose a person to stones. There appears to be a higher occurrence of kidney stones in the southern region of the US compared to other areas. One study suggested that the higher risk may be due to a higher rate of hypertension and certain dietary habits, particularly lower intake of magnesium and low use of calcium supplements.
In general, certain foods increase the risk for stones only in people who have genetic or medical susceptibility. People whose diets are high in animal protein and low in fiber and fluids may be at higher risk for stones. A number of foods contain oxalic acid, but there is no proof that such foods make any major contribution to calcium oxalate stones in people without other risk factors. Dietary calcium appears to be protective. People who are overweight may be at higher risk than thinner people, although the evidence is weak. In a 2000 study, obesity was associated with abnormal blood and urine chemistries that appeared to increase the risk for stones in overweight women but not in overweight men. In any case, obesity poses particular difficulties in elimination of kidney stones. One study reported that people who had a major, stressful life experience were more likely to develop stones than those who had not. Some experts speculate that this increased risk may be due to a hormone called vasopressin, which is released during stress. Among its other functions, vasopressin increases the concentration of urine. Sleeping in the same position consistently may influence risk. A 2001 study reported that in people who had a history of kidney stones, recurrences tended to occur on the same side that people favored. An earlier study suggested that people who had kidney stones were more apt to sleep on their stomachs. Movement during sleep did not appear to affect the risk.
Any medical or physical condition that results in a patient being immobilized or bedridden increases blood levels of calcium from bone breakdown, thereby posing a risk for stone formation. Hypertensive people are up to three times more likely to develop kidney stones. It is not entirely clear whether having high blood pressure increases the risk for a stone, whether stones lead to hypertension, or if there is a mechanism common to both. Some experts suggest that imbalances between uric acid levels in the blood and urine and sodium excretion may put hypertensive patients at higher risk. People with gout are at risk for uric acid stones. Struvite stones are almost always caused by urinary tract infections. Many other medical conditions, including but not limited to hyperparathyroidism, kidney disease, chronic diarrhea, certain cancers (eg, leukemia and lymphomas), and sarcoidosis, put people at higher risk for stones.
AIDS patients are at high risk for stones, mainly because of medications. Over 10% of AIDS patients who take indinavir develop stones, and the risk is even higher in AIDS patients who have hepatitis B or C or hemophilia, who are very thin, or who are receiving the antibiotic combination TMP-SMX. In one study of AIDS patients taking a combination of indinavir, zidovudine, and lamivudine, 36% developed kidney stones. Many drugs increase the risk for kidney stones, including cancer chemotherapy, thyroid hormones, and diuretics. Taking medications for long periods that change the acidic content of urine, such as antacids, may increase susceptibility for kidney stones.
Diet plays an important role in the development of kidney stones, especially in patients who are predisposed to the condition. A diet high in sodium, fats, meat, and sugar, and low in fiber, vegetable protein, and unrefined carbohydrates increases the risk for renal stone disease. Recurrent kidney stones may form in patients who are sensitive to the chemical byproducts of animal protein and who consume large amounts of meat.
High doses of vitamin C (i.e., more than 500 mg per day) can result in high levels of oxalate in the urine (hyperoxaluria) and increase the risk for kidney stones. Oxalate is found in berries, vegetables (e.g., green beans, beets, spinach, squash, tomatoes), nuts, chocolate, and tea. Stone formers should limit their intake of cranberries, which contain a moderate amount of oxalate.