The kidneys preserve a very delicate balance in the body. They compensate for your food and fluid intake to maintain this balance.
For instance, if you drink a great deal of water, there is a greater volume of urine. If you eat a lot of salt (that bag of potato chips that disappeared during the football game), more comes out in the urine. If you eat a lot of protein, more of the waste products of protein metabolism come out in your urine. The body must stay in zero balance, or you would blow up with water or become ill from an excess of salt.
The only time you don’t have zero balance is when the body is sick. For instance, if you get food poisoning, the body becomes dehydrated and loses salt and other minerals necessary for balance and good health. Patients with chronic bowel disease lose fluid and minerals through diarrhea.
Fluids and Balance
When the body is deprived of water, the kidneys conserve fluid and make a more concentrated urine (one that contains little fluid).
Concentrated urine is dangerous for the stone former, as kidney stones are much more likely to form there than in a dilute urine.
We often assume that the amount of water you take in is equal to the amount of urine. This is true if someone is not active: the fluid in equals the fluid out. At rest, the amount of water generated from the metabolism of food and drink is roughly equal to the amount you lose in stool and perspiration.
That assumption falls apart if you work in a hot office or factory, are near a hot oven, are on the beach, or for any other reason are perspiring heavily. Then you are losing a lot of water through your skin. If you are exercising strenuously, the kidneys will return more fluid to the body to enable it to cool the surface of the skin through the sweat glands. The kidney compensates by reducing the urinary volume. You may notice that you urinate less when you are sweating heavily. In these situations, the amount of fluid you are taking in does not equal the amount of urine you make. The kidneys are balancing the amount of water, as well as sodium and other substances such as potassium, that the body needs for its activity level.
How Trouble Starts
The urinary tract is really the body’s sewer system, and the kidney is the processing plant for waste disposal. The more waste the kidney must get rid of, and the less fluid volume it has to flush the waste out, the greater the chance that it will clog.
I look at kidney stones as clogs in the urinary system. My goal is to show you how to keep your sewer pipes unclogged.
John S. Rodman, M.D., R. Ernest Sosa, M.D., and Cynthia Seidman, M.S., R.D., with Rory Jones
The Experts Tell You All You Need to Know about Prevention and Treatment
John S. Rodman, M.D., is an internationally recognized expert and researcher on kidney stone disease. He is associate clinical professor of medicine at the Weill Cornell University School of Medicine in New York City and a member of the attending staffs at New York Presbyterian Hospital and Lenox Hill Hospital. He has published extensively in the professional journals and medical textbooks and has lectured worldwide on kidney stone formation and treatment.
R. Ernest Sosa, M.D., is a leading urologist in New York City who specializes in surgical techniques for kidney stones. He is associate clinical professor of urology at the Weill Cornell University School of Medicine and attending urologist at New York Presbyterian Hospital. He served for 20 years as director of the Brady Stone Center at the Department of Urology at Weil Cornell–New York Presbyterian Hospital. He has written and lectured on and taught the diagnosis and treatment of stone disease internationally.
Cynthia Seidman, M.S., R.D., is director of dietary services at the Rockefeller University Hospital in New York City. She coordinates the design and development of all research diets at Rockefeller and has published in a number of professional journals. She is cochair of the Research Committee of the National GCRC Research Dietitians Association and is a member of the American Dietetic Association.
Rory Jones, an award-winning writer and television producer, has done extensive work on health and medical topics. She has developed educational programs as well as interactive multimedia for both adults and children.