Kidney stones are hard masses that form when the crystals separate from the urine and accumulate in the hollow parts (pelvis or calyces) that drain the kidneys. The stones often pass out of the body in the urine, but can be incredibly painful during the passage. Sometimes the stones settle in the urinary system and continue to grow. blocking the flow of urine, making urination painful or impossible. Urinary tract infections, certain kidney and metabolic disorders and excessive vitamin D intake are among the triggers of stone formation. But often, it is unclear why a person develops kidney stones. Most kidney stones contain calcium oxalate, which has led to the mistaken belief that stone sufferers must avoid all dairy products to keep urinary calcium levels low. Too often, serious calcium restriction can result in bone loss and the fractures that accompany osteoporosis.
In some cases stones are formed due to urinary tract infections, obstructions to the normal outflow of urine, misuse of medications, the result of rare genetic disorders, such as cystinuria, or in connection with metabolic disturbances such as gout. However, none of these factors necessarily results in the formation of stones, and research is ongoing to determine why similar conditions cause stone formation in one individual and not in another. The most common type of stone contains calcium. In most people, the kidneys flush out the extra calcium with the rest of the urine. People who have calcium stones keep the calcium in their kidneys. A struvite stone may form after an infection in the urinary system. These stones contain the mineral magnesium and the waste product ammonia. A uric acid stone may form when there is too much acid in the urine. Cystine stones are rare. Cystine is one of the building blocks that make up muscles, nerves, and other parts of the body. The disease that causes cystine stones runs in families.
Nearly everyone has high levels of calcium in the urine, yet relatively few people form calcium stones. Those who do either absorb far more calcium than normal or send more calcium than normal from the kidneys into an excretory pathway. Sometimes an underlying disease like an overactive parathyroid gland or an overdose of vitamin D disrupts calcium absorption or metabolism, and some attacks of calcium oxalate stones are triggered by megadoses of vitamin C. Stones can also form when the urine is overloaded with uric acid, which may result from a diet overly rich in animal protein. In under 10% of cases, kidney stones result from infection by bacteria that destroy a stone-inhibiting enzyme. But most cases have no obvious cause.
Most afflicted are adults living in affluent, industrialized countries. Stones are associated with a diet rich in animal protein and low in liquids. A typical kidney stone patient is a man 20 to 50 years old, as men are three to four times as likely to form stones as are women. One woman in 20 and one man in 7 in the USA can expect to experience the pain of kidney stones at least once before reaching 70. Particularly at risk are joggers and other active people who do not drink enough, women who deliberately limit their fluid intake, and travellers who are afraid of the local water supply.
Many patients form kidney stones on one side of the body. If the components of urine were the major factor in their stone formation, stones would logically occur on both sides of the body. In a 2-year study of 110 patients who suffered from stones on one side of the body, researchers found that patients' stones frequently recurred on the side they favoured during sleep. Of the 93 patients who consistently slept on one side, three-quarters had their kidney stones on that side. This study suggests that sleep position plays a role - possibly by affecting blood flow to the kidneys. Temporary slow-downs in circulation to the kidneys may affect the organs' ability to clear itself of various substances, the report indicates.