About 200,000 Americans have bariatric surgery each year in an attempt to overcome obesity after diet and exercise have failed. The surgery, in effect, changes the anatomy of the digestive system to limit the amount of food that can be consumed and the amount of food that the body can digest. Two procedures are most commonly used in the United States: gastric banding and the Roux-en-Y gastric bypass (named after the surgeon who developed it).
In gastric banding, an adjustable band is placed around the top portion of the stomach, limiting food intake. In the gastric bypass procedure, a stomach pouch is created out of a small portion of the stomach and attached directly to the small intestine, bypassing a large part of the stomach and duodenum. This not only reduces the amount of food that can be consumed but, by going around the duodenum, the absorption of calories, especially from fats, is significantly reduced as well.
There are many lifestyle and dietary adjustments that must be made by people who have undergone bariatric surgery but the positive effects on cardiovascular risk, diabetes, sleep apnea and other health problems related to obesity are considered to make it worthwhile. We do not yet have a complete understanding of the long-term effects of bariatric surgery, but urologists have taken note of a significant short-term effect of the gastric bypass. Within six months of this surgery, a patient’s risk of developing kidney stones nearly doubles.
“As the procedure has become more popular in our region we have been seeing some increase in kidney stones as a result of the gastric bypass,” says Dr. Paul Pietrow of Premier’s Urology Division. “The whole digestive process changes for people who have had Roux-en-Y gastric bypass surgery. It’s the planned malabsorption caused by this procedure—which significantly affects the composition of urine— that drives most of the stone risk. Many of the factors that we measure and try to control to prevent stones, such as high oxalate levels and low citrate levels, become less than optimal.
Especially in patients who experience diarrhea, and the resulting fluid loss, bariatric surgery can create the perfect storm for stone disease.”
Preventing stones after bariatric surgery
The main factor in stone prevention after bariatric surgery—not so different from stone prevention for everyone—is to maintain excellent hydration. “There has to be plenty of fluids going through the kidneys in order to dilute the crystals and the stone-forming risk factors. So it’s a matter of fluids, always fluids,” says Pietrow. It’s also important to keep to a moderate intake of animal protein, which tends to lower the urine PH and increase uric acid, putting these patients more at risk for stones.
Most patients, after bariatric surgery, could do with getting more citrates into their diet to make up for the reduced levels that are part of the kidney’s physiologic response to malabsorption of nutrients. That can be accomplished, in most cases, simply by adding lemon or orange juice to the water they’re staying hydrated with. “We also urge consuming dairy products for those patients that can tolerate it,” says Pietrow. “Dairy has a fair amount of calcium in it, which binds to the oxalate in the gut so calcium oxalate can be excreted rather than reaching high concentration in the kidneys and crystalizing.”
Being aware that gastric bypass surgery involves a heightened risk for developing kidney stones should make it possible to develop dietary strategies to reduce that risk, while enjoying the health benefits of weight loss