The chemical composition of stones depends on the chemical imbalance in the urine. The four most common types of stones are comprised of calcium, uric acid, struvite, and cystine.

• Calcium Stones #

  Approximately 85% of stones are composed predominantly of calcium compounds. The most common cause of calcium stone production is excess calcium in the urine (hypercalciuria). Excess calcium is normally removed from the blood by the kidneys and excreted in the urine. In hypercalciuria, excess calcium builds up in the kidneys and urine, where it combines with other waste products to form stones. Low levels of citrate, high levels of oxalate and uric acid, and inadequate urinary volume may also cause calcium stone formation.
  
  Calcium stones are composed of calcium that is chemically bound to oxalate (calcium oxalate) or phosphate (calcium phosphate). Of these, calcium oxalate is more common. Calcium phosphate stones typically occur in patients with metabolic or hormonal disorders such as hyperparathyroidism and renal tubular acidosis.
  
  Increased intestinal absorption of calcium (absorptive hypercalciuria), excessive hormone levels (hyperparathyroidism), and renal calcium leak (kidney defect that causes excessive calcium to enter the urine) can cause hypercalciuria. Prolonged inactivity also increases urinary calcium and may cause stones.
  
  Renal tubular acidosis (inherited condition in which the kidneys are unable to excrete acid) significantly reduces urinary citrate and total acid levels and can lead to stone formation, usually calcium phosphate.

• Uric Acid Stones #
  
  Digestion produces uric acid. If the acid level in the urine is high or too much acid is excreted, the uric acid may not dissolve and uric acid stones may form. Genetics may play a role in the development of uric acid stones, which are more common in men. Approximately 10% of patients with kidney stone disease develop this type of stone.
  
  
• Struvite Stones #

  This type of stone, also called an infection stone, develops when a urinary tract infection (e.g., cystitis) affects the chemical balance of the urine. Bacteria in the urinary tract release chemicals that neutralize acid in the urine, which enables bacteria to grow more quickly and promotes struvite stone development.
  
  Struvite stones are more common in women because they have urinary tract infections more often. The stones usually develop as jagged structures called "staghorns" and can grow to be quite large.

• Cystine Stones #

  Cystine is an amino acid in protein that does not dissolve well. Some people inherit a rare, congenital (i.e., present at birth) condition that results in large amounts of cystine in the urine. This condition (called cystinuria) causes cystine stones that are difficult to treat and requires life-long therapy.
  

• Incidence and Prevalence #

  People who live near large bodies of water (e.g., Great Lakes, Gulf of Mexico), those who live in "soft" water areas, and those who have a sibling or parent with the condition experience a higher incidence of renal stone disease. According to the U.S. National Institutes of Health, 1 person in 10 develops kidney stones during their lifetime and renal stone disease accounts for 7–10 of every 1000 hospital admissions. Kidney stones are most prevalent in patients between the ages of 30 and 45, and the incidence declines after age 50.
  

Several factors increase the risk for developing kidney stones, including inadequate fluid intake and dehydration, reduced urinary flow and volume, certain chemical levels in the urine that are too high (e.g., calcium, oxalate, uric acid) or too low (e.g., citrate), and several medical conditions. Anything that blocks or reduces the flow of urine (e.g., urinary obstruction, genetic abnormality) also increases the risk.

Chemical risk factors include high levels of the following in the urine:

• Calcium (hypercalciuria)
• Cystine (cystinuria; caused by a genetic disorder)
• Oxalate (hyperoxaluria)
• Uric acid (hyperuricosuria)
• Sodium (hypernatremia)

A low level of citrate is a risk factor for hypocitraturia.

The following medical conditions are also risk factors:

• Congenital kidney defect that may increase urinary calcium loss and stone formation (medullary sponge kidney)
• Excessive parathyroid hormone, which causes calcium loss (hyperparathyroidism)
• Gout (caused by excessive uric acid in the blood)
• High blood pressure (hypertension)
• Inflammation of the colon that causes chronic diarrhea, dehydration, and chemical imbalances (colitis)
• Inherited condition in which the kidneys are unable to excrete acid (renal tubular acidosis)
• Intestinal disorder that causes chronic diarrhea, dehydration, and low citrate (Crohn's disease)
• Painful joint inflammation (arthritis)
• Urinary tract infections (affect kidney function)

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.