The alterations in the handling of calcium by the body are responsible for a large percentage of kidney stones. The changes in calcium metabolism are frequently accompanied by abnormalities in the metabolism of two compounds that are products of the chemical reactions that constantly keep us going: oxalate and uric acid. Attempts at curbing the incidences of kidney stones include control of the metabolism of calcium, oxalate, and uric acid. While all these compounds are part of the normal composition of our bodies, we also ingest them in our food, which may modify their handling by the body and contribute to stone formation.
Calcium and Dairy Products
Dairy products are rich in calcium, and their ingestion may compound the presence of too much calcium in the urine. Calcium in the diet is absorbed in the intestine and excreted in the urine. High levels of calcium in the urine predispose to stone formation. Moreover, if the volume of urine is relatively low, the concentration of the excess calcium will be higher than it ought to be and propitiate the formation of stones. It is therefore recommended that all patients who have stones should take two important measures in an attempt to reduce the incidence of stones and prevent their further formation: limit dairy product consumption and ingest enough fluids to keep urine output greater than two liters per day. Patients must themselves be willing to undertake these two simple and inexpensive measures.
Uric Acid and Purine Sources
Uric acid has been found to worsen the incidence of calcium stone episodes in as many as 30 percent of patients with calcium stones. Uric acid is the end-product of the breakdown of a group of compounds called purines. These in turn have their precursors in the components of the cell nucleus. Most animal cells have a nucleus, and meat, poultry and fish are rich in cells that have nuclei and thus are sources of purines and, ultimately of uric acid.
There are individuals who eat these foods at the exclusion of some that have fewer purines, such as bread, grain, and starches. This has been called and may be enormously lethal to stone formation. Reduction in the consumption of animal protein, particularly if it has been determined that urine uric acid excretion is higher than normal, may be a crucial step in preventing stone formation.
Oxalate production is much more an endogenous process, and close to 90 percent of its concentration in the urine comes from the activity of the liver. The rest can be traced to food we eat either directly as oxalate or ascorbic acid. Overproduction of oxalate by the liver and excess intake of oxalate-rich foods may contribute to high levels of oxalate in urine and increase the incidence of stones. Under most circumstances, however, dietary indiscretion is not as prominent a contributor to excess oxalate in the urine as is true with purines.
A series of gastrointestinal disturbances such as inflammatory bowel disease, small bowel resection, or connection of the duodenum to the colon may set up the patient for hyperabsorption of oxalate from dietary sources and increase the incidence of stones. In addition, the bowel disease may lead to loss of fluids and low urine volumes which will also compound the susceptibility for stone formation. Special care by the physician is almost invariably required in these individuals, who may also benefit from dietary modification. Preventive measures must be carefully planned and instituted after consultation with the physician and the proper diagnostic workup has been performed.
Other Stones, Other Foods
A particularly problematic type of stone is that associated with infection. These are formed when infections with urea-splitting organisms are present. These bugs take urea and transform it into ammonia, which forms a complex with magnesium and phosphate to become something called struvite which is the most important component of “infectious” stones. It is not easy to cure these, and they grow to form large “staghorn” calculi (stones) that get trapped in the pelvis of the kidney and cause a lot of problems.
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