Kidney Stones - Causes

Excess Calcium in the Urine (Hypercalciuria). About 70% of calcium-containing stones are caused by hypercalciuria, in which there is too much calcium in the urine. A number of conditions may produce hypercalciuria. Many are due to genetic factors, but most cases are idiopathic, or due to unknown causes.

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The following can lead to hypercalciuria and calcium stones.

Overly efficient intestinal absorption of calcium. In many cases, the source of calcium overload in urine is the intestine, not the kidney. In most of these conditions, genetic factors conspire to increase calcium absorption in the intestine. Researchers are investigating a number of suspects, including a possible defective gene that regulates calcitriol, a form of vitamin D, which, in excess levels, may increase intestinal absorption of calcium. (This is the situation in which restricting calcium in the diet can help prevent stones.)
Excessive chloride. Chloride has a negative charge and calcium a positive one, so they are often used by the body to balance each other. Excess chloride, then, may lead to excess calcium. A gene known as CLCN5, which regulates chloride in the urine, is defective in many patients with calcium stones.
Renal calcium leak. This is a condition in which the filtering processes in the kidney fail, causing an increase of calcium in the urine.
Excessive sodium. Calcium absorption in the kidney tubules follows the absorption of sodium and water. High urinary levels of sodium then result in increased levels of calcium. Defects in the kidney tubules transport system can cause imbalances in sodium and phosphate that result in elevated calcium in the urine. A high salt diet can also produce this effect.

Excess Oxalate in the Urine (Hyperoxaluria). Oxalate, also called oxalic acid combines with calcium to form calcium oxalate, which is the most common stone-forming compound. Excessive oxalate in the urine (hyperoxaluria) is responsible for about 30% of calcium stones and is a more common cause of stones than too much calcium in the urine.

Hyperoxaluria is defined as either primary or secondary.

Primary hyperoxaluria is an inherited disorder in which excess oxalate in the urine is the primary problem.
Secondary hyperoxaluria is caused by specific conditions that result in excess urinary oxalate.

Secondary hyperoxaluria is usually caused by excessive intake of dietary oxalates (found in a number of common vegetables, fruits, and grains) or by abnormalities in the metabolism of oxalates. Such defects may be due to various factors, such as the following:

Deficiencies of pyridoxine (vitamin B6). Severe vitamin B6 deficiencies (usually due to genetic disorders) can result in overproduction of oxalic acid.
Deficiencies in Oxalobacter formigene. Deficiency in an intestinal bacterium called Oxalobacter formigenes is now a suspect in some cases. This bacterium degrades oxalate and low levels in the intestine increase the risk for oxalate absorption and stone formation.
Short bowel syndrome. Short bowel syndrome, which may result from surgery in the small intestine, is marked by the inability of the intestines to absorb fat and nutrients properly (malabsorption). In such cases, calcium may bind to unabsorbed fat instead of to oxalates. This leaves excess oxalate, which is absorbed by the intestine and excreted into the kidney.
Hormones. Some studies have suggested that androgens (male hormones) are associated with a higher risk for the formation of calcium oxalate crystals while estrogens (female hormones) are linked to a lower risk. Estrogen may help prevent the formation of calcium stones by keeping urine alkaline and raising protective citrate levels.