Causes of Kidney Stones
Although the basic cause of kidney stones is unknown, many factors contribute directly or indirectly to their formation. These factors relate to the nature of the urine itself or to the conditions of the urinary tract environment.
Concentration of urinary constituents
CALCIUM By far the majority of kidney stones-about 96%-are composed of calcium compounds. These compounds may combine calcium with phosphorus or with oxalate. Excessive urinary calcium may result from prolonged use of high-calcium foods such as milk and dairy products, from alkali therapy for peptic ulcer, or from continued use of a hard water supply. In addition, excess vitamin D may cause increased calcium absorption from the intestine, as well as increased calcium withdrawal from the bone. Prolonged immobilization such as occurs in bock casting, long-term illness, or disability may lead to withdrawal of bone calcium and increase concentrations in the urine. Excessive activity of the parathyroid gland may also cause excess excretion of both calcium and phosphorus,
URIC ACID Excess uric acid excretion may be caused by increased breakdown of purines as in gout. Purines are nucleoproteins found in a number of animal protein sources, especially organ meats. Excess uric acid may also result from rapid breakdown of tissue protein, such as in wasting diseases. It may also occur with prolonged use of fad diets for weight control that recommend high intakes of protein and restrictive amounts of carbohydrates.
CYSTINE Cystine is an amino acid that accumulates in the urine because of a hereditary metabolic defect in the renal tubular reabsorption of the acid. This accumulation in the urine is a condition called cystinuria.
Urinary Tract Conditions Several physical changes in the urine may predispose susceptible persons to stone formation. These include a concentration of urine resulting from a reduced water intake or from excess water loss such as that occurring in prolonged sweating, fever, vomiting, or diarrhea. Sometimes persons exposed to hot temperatures or climates for prolonged periods may suffer from such a concentration of urine. Also the degree of urine acidity may change to a more acid or a more alkaline state. These changes may be influenced by diet or altered by the ingestion of acid or alkaline medication.
Changes in the epithelial tissue of the urinary tract may also provide a nucleus for the formation of stones. These include bacterial masses from recurrent urinary tract infections and degeneration of tissue caused by vitamin A deficiency.
Several pain and numerous urinary symptoms may result, General weakness and sometimes fever occur. Laboratory examination of the urine and chemical analysis of any stone that is passed help to determine treatment.
Principles of diet therapy
Fluid intake. A large fluid intake produces more dilute urine and helps to prevent concentration of the stone constituents.
Elements of stone composition
CALCIUM STONES. A low-calcium diet of about 400 mg, daily is usually given. This is about half that of an average adult intake of 800 mg. This lower level of calcium intake may be achieved mainly by a removal of milk and dairy products. Sometimes a test diet of only 200 mg. of calcium may be used to rule out hyperparathyroidism as a causative factor.
In addition to calcium restriction, the material composing the calcium compound would also be restricted. For example, if the stone is of calcium oxalate composition, food sources of oxalates arc also eliminated. If the stone is of calcium phosphate composition, foods high in phosphorus are also curtailed, Since the major food sources of phosphorus are the same as those for calcium, the low-calcium diet usual suffices with one added restriction. Meat is the other major source of phosphorus. Thus it would have to be curtailed in quantity.
URIC ACID STONES About 4% of total kidney stones are uric acid stones. Since uric acid is a metabolic product of purines, dietary control of purines is indicated. Purines are found in active tissues such as glandular meat, other lean meat, meat extractives, and, in lesser amounts, in plant sources such as whole grains and legumes.
TINE STONES About 1% of the total stones produced are cystine. Their occurrence is relatively rare. Cystine is a nonessential amino acid produced from the essential amino acid methionine; thus a diet low in methionine is used, Since methionine is an essential amino acid, it is found mainly in the complete protein foods-milk, meat, and eggs. Thus the low-methionine diet restricts meat to 2 oz, daily and eliminates milk (soy milk is substituted) and eggs.
Urinary acidity An attempt to control the solubility factor is made by changing the urinary pH to an increased acidity or alkalinity, depending on the chemical composition of the stone formed. For example, since calcium stones have an alkaline chemistry, an acid ash diet also is used to create a urinary environment less conducive to precipitation of the basic stone elements. On the other hand, since uric acid stones and cystine stones are both of acid composition, an alkaline ash diet would be used for the same purpose. An acid ash diet applying these food groups is shown on the same page. Cranberry juice seems to have a urinary acidifying effect or bacteriostatic value and is frequently used as a dietary adjunct.