A little discussed, but well established, side effect of gastric bypass surgery is an increased risk of kidney stones. What can you do to prevent kidney stones if you’ve had gastric bypass surgery? This article covers everything you need to know about gastric bypass and kidney stones.
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Prevalence of Kidney Stones After Gastric Bypass Surgery
Roux-en-y Gastric Bypass Surgery
Any type of bariatric surgery that causes malabsorption increased risk of kidney stones. In particular, Roux-en-y gastric bypass surgery increases kidney stone risk by about 250%. (1)
People who have more restrictive gastric bypass surgery such as a “very long limb roux-en-y gastric bypass” or “biliopancreatic diversion/duodenal switch” have a 5 times greater risk of kidney stones. (1)
In addition, kidney stones in people who have had gastric bypass surgery may be more severe. They are more likely to need surgery to treat kidney stones and may be at higher risk of kidney disease. (1) (2)
Kidney Stones After Gastric Sleeve & Other Weight Loss Surgeries
In comparison, bariatric surgery that restricts how much food that you can eat is not associated with a higher risk of kidney stones.
The Link Between Gastric Bypass and Kidney Stones
Oxalate is a “non-nutrient” found in many healthy plant foods like almonds, spinach and beets. When there is a lot of oxalate in your urine, oxalate can bind with calcium and form a kidney stone. Calcium oxalate kidney stones are the most common type of kidney stone.(4)
Of course, most people are able to eat high oxalate foods without having kidney stones. Spinach salads are a very healthy food for most people! However, not all bodies handle oxalate the same way. Some people absorb a lot of oxalate from the food they eat, whereas others do not. Eating enough calcium is key to control oxalate absorption.
Gastric bypass surgery changes the way oxalate is absorbed in the body.
Increased Oxalate Absorption
One of the primary reasons kidney stones are so common after gastric bypass is a higher absorption of oxalate from food. Gastric bypass causes your body to absorb fewer calories and other nutrients. This poor absorption is how gastric bypass causes weight loss.
Fat is not absorbed well in people who have had gastric bypass. Extra fat in the intestine binds with calcium, making that calcium unavailable to bind with oxalate. This leaves oxalate free to be absorbed in very high amounts, leading to dangerously high urine oxalate levels and risk of kidney stones. Very high intestinal oxalate absorption is called “enteric hyperoxaluria”. (5)
In people who have not had gastric bypass surgery, most of the fat eaten is absorbed in the small intestine. In addition, calcium binds to oxalate which stops oxalate from getting absorbed. In most people, this keeps urine oxalate levels in a healthy range.(4)
Any health condition or surgery that causes fat malabsorption may cause higher oxalate absorption. For example, inflammatory bowel disease and bowel resections have been associated with secondary hyperoxaluria too. (6)(7)
Low Urine Volume
Drinking a lot of water to dilute urine is key to kidney stone prevention. After gastric bypass surgery, it can be difficult to drink a lot of water because of a smaller stomach, nausea and other side effects. Research has shown that people drink less fluid after gastric bypass.(8)
Inadequate fluid intake and very concentrated urine likely contributes to kidney stones after gastric bypass surgery.(5)
Citrate is a powerful inhibitor of kidney stones. People who have low urine citrate are more likely to form kidney stones.
It is unclear why, but people who have had gastric bypass have lower levels of citrate. A study found that 63% of people had low urine citrate after gastric bypass surgery, compared to only 5% of people who had not had gastric bypass. (9)
Low urine citrate likely contributes to kidney stones in gastric bypass patients.
Gastric bypass surgery has a big effect on the gut microbiome. In general, Lactobacillus and Bifidobacterium bacteria tend to decrease after gastric bypass. (10) Both of these bacteria may play a role in digesting oxalate before it is absorbed. The change in the gut microbiota after gastric bypass may be related to higher risk of kidney stones.
Researchers are still trying to understand the role gut microbiota play in kidney stone formation. We know that people who have kidney stones have a different bacteria profile compared to people who do not have kidney stones. (11) Certain gut microbiota also play a role in helping digest oxalate, and may reduce urine oxalate amounts. (11)
However, probiotic trials aiming to reduce urine oxalate and kidney stones have not shown consistent results. (5) This is an exciting area of research, but we need more studies before we know what kind of bacteria, and how much, will reduce the risk of kidney stones in people who have had gastric bypass.
How To Avoid Kidney Stones After Gastric Bypass Surgery
Drink Lots of Water
The most important thing you can do to prevent kidney stones is drink fluid. The American Urological Association recommends drinking 3 liters of fluid each day to make at least 2 1/2 liters of urine. (12) Drinking a lot of fluid will dilute urine, making kidney stones less likely.
Drinking this much fluid is no easy task for people who have had gastric bypass surgery. However, it is important to drink as much as you can! Even if you cannot reach the 3 liters per day goal, every extra glass will help.
Of course, plain old water is best for kidney stone prevention. Make water more exciting by adding lemon or lime juice. Try infusing water with fruit or herbs for extra flavor. Stay away from drinks that have sugar such as juice, soda, punch or sweet tea. Sugary drinks may make kidney stones worse. (13) And, lots of sugar can cause diarrhea in people who have had gastric bypass surgery.
Avoid High Oxalate Foods
The primary reason kidney stones are so common after gastric bypass is very high oxalate absorption. Therefore, avoiding very high oxalate foods may help reduce kidney stone risk. Instead of avoiding all foods high in oxalate, try to avoid just the foods that are very high in oxalate.
Very high oxalate foods:
- Almonds (and almond products like almond butter, flour and milk)
- Navy Beans
Work with your doctor or dietitian to know if you should restrict oxalate beyond these foods.
Eat Enough Calcium
Eating enough calcium will reduce how much oxalate is absorbed. This will lower urine oxalate and the risk of kidney stones.
Calcium recommendations for people who have had bariatric surgery is 1,200 – 1,500 mg of calcium per day. (14) Ideally, calcium should come from calcium-rich foods like milk, yogurt and cheese. However, dairy may be difficult to tolerate after gastric bypass surgery. If urine calcium levels are normal, calcium supplements can help people meet calcium goals to reduce kidney stones and for bone health.
Eat Fruits & Vegetables
Fruits and vegetables are a wonderful source of citrate. Citrate inhibits kidney stone formation in urine. Citrus fruits are especially high in citrate. But, all fruits and vegetables can help increase citrate in urine.
Produce also is a great source of potassium .Eating enough potassium can help make urine less acidic, which can help prevent calcium oxalate kidney stones. (5)
Aim for at least 5 servings of fruits and vegetables each day.
Get Protein From Food First
Protein from foods like meat, chicken, fish and eggs may be less likely to cause kidney stones compared to protein supplements, bars or powders. But, we need more research in this area.
Make sure to choose low oxalate nuts if you are using nuts to get protein.
Protein supplements are necessary for some people. Protein is important after bariatric surgery to protect muscles and prevent poor nutrition. Be careful to check the ingredients of any protein supplement for high oxalate ingredients such as soy, spinach or nuts. Work with your doctor or dietitian to know what is best for you.
Avoid Excess Vitamin C Supplements
The liver can make oxalate from vitamin C. High intake of vitamin C, especially from supplements, is associated with kidney stones. (15)
It is easy to eat enough vitamin C from food. One orange or bell pepper has nearly 100% of vitamin C you need in a day. Most people do not need vitamin C supplements. And, supplements present an unnecessary kidney stone risk.
However, vitamin C needs might be higher in people who have had gastric bypass. And, vitamin C is important for iron absorption. Severe dietary restrictions might make it difficult to meet vitamin C needs with food alone. Work with your doctor or dietitian to know what is best for you.
Advocate For Testing & Prevention
Testing for kidney stone urine risk factors after bariatric surgery is not routinely done. However, a 24-hour urine test can help you and your medical team know if you have a high risk of kidney stones. A 24-hour urine test will measure the amount of oxalate in your urine, along with many other kidney stone urine risk factors.
If you have had gastric bypass surgery, ask your doctor for a 24-hour urine test. Or, schedule an appointment with a urologist or nephrologist to help prevent kidney stones before they happen!