Kidney stones are crystallized mineral deposits that form from waste products in your urine. If they grow large enough, kidney stones can become lodged in your urinary tract and cause excruciating pain or obstruction of urinary flow. Kidney stones are more common in men, but they occur in both sexes. You are more likely to get kidney stones if someone else in your family has had them. The composition of kidney stones varies considerably, and dietary modifications can help to prevent some of them.
Kidney Stone Formation
A 1996 review in "Seminars in Nephrology" describes the multi-stage formation of a typical kidney stone. First, your urine becomes saturated with a material such as calcium oxalate, which is capable of crystallization. If your urine is concentrated due to insufficient fluid intake, a microscopic crystal forms and adheres to the lining of your urinary tract. Much like rock candy in a sugar solution, the crystal gradually grows until it is large enough to either begin moving through your urinary tract or interfere with the normal flow of urine.
Types of Kidney Stones
According to the July 2006 "American Family Physician," kidney stones are classified according to their composition, with 70 percent of stones made of calcium oxalate. Smaller percentages are composed of calcium phosphate, uric acid, cystine or struvite, which is a conglomerate of phosphate, magnesium and ammonia. Many stones contain a variable mixture of materials. Because all of these substances are normally present in urine, anyone can form a stone if conditions in their urinary tract permit it.
Oxalates and Kidney Stones
The oxalic acid, or oxalate, that is absorbed from your diet is eventually filtered into your urine and eliminated when you empty your bladder. However, if your urine becomes concentrated or if it becomes too acidic, the oxalates dissolved therein can precipitate with calcium ions, and the nucleus of a stone is formed. Progressive layering of other minerals over this nucleus creates a larger and larger stone. If you develop a kidney stone, your doctor will ask you to strain your urine so you can capture the stone for laboratory analysis.
Avoiding Dietary Oxalates
Many foods contain oxalates, including rhubarb, spinach, chard, parsley and other greens, and beets, tea, nuts, soy, chocolate, wheat bran and strawberries. Many scientists have questioned the role of dietary oxalates in kidney stone formation, but a 2001 "Kidney International" study showed that significant amounts of dietary oxalate are absorbed and eventually find their way into your urine. Therefore, some doctors ask patients with any kind of kidney stone to avoid eating oxalates, particularly oxalate-rich greens.
References
- "Seminars in Nephrology"; "Mechanism of Stone Formation"; N. Mandel; September 1996
- "American Family Physician"; "Medical Management of Common Urinary Calculi"; Paul Pietrow, Michael Karellas; July 2006
- "Kidney International"; "Contribution of Dietary Oxalate to Urinary Oxalate"; R.P. Holmes et al; January 2001
