How do kidney stones form?

Early stages of growth

Kidney stones begin as tiny crystals inside the kidney. They form from substances such as calcium and oxalate that have been filtered into the urine. The salts bind together, creating a central core over which additional layers of material can begin to grow.^1,2

Not all crystals will develop into large, painful kidney stones. Some remain so small they pass out of the body unnoticed. Others, however, will continue to grow and may eventually become so large they cause obstruction in the ureter, the narrow tube leading to the bladder from the kidney.^1,2

Factors that increase risk

Supersaturation

For kidney stones to develop, several factors must be present. First, the urine must be supersaturated or have high levels of uric acid, a by-product of certain foods. Supersaturation means there are too many salts and not enough fluids in the urine. It’s the driving force behind stone formation. Insufficient fluid intake, which causes dehydration, is one of the main reasons urine can become supersaturated.^2,3

Inhibitors

The other critical factor in stone formation is low levels of stone inhibitors. Inhibitors are substances that slow the formation and growth of crystals and help to eliminate them from the kidney before they can aggregate together.^2,4

Health and diet

Certain medical conditions, being overweight, or the use of some medications can also make it easier for stones to form. Diet and lifestyle choices, such as eating a diet high in animal protein or not drinking enough water and other fluids, can further increase the risks of stone formation.^1,3

1. PDR Health [Internet]. Kidney stones [cited 2011 Jan 11]. Available at: http://www.pdrhealth.com/disease/disease-mono.aspx?contentFileName=BHG01NP15.xml&contentName=Kidney+Stones&contentId=89.
2. University of Wisconsin School of Medicine and Public Health [Internet]. How do kidney stones form? [cited 2011 Jan 11]. Available at: http://www.uwhealth.org/urology/how-do-kidney-stones-form/11210.
3. Krieg C. The role of diet in the prevention of common kidney stones. Urol Nurs. 2005;25(6):451-56.
4. Asplin JR, Coe FL, Favus MJ. Nephrolithiasis. In: Fauci AS, Braunwald E, Kasper DL, Hauser SL, Longo DL, Jameson JL, Loscalzo J, eds. Harrison’s Principles of Internal Medicine. 17th ed. New York, NY: McGraw Hill Medical; 2008:1815-20.