How to reduce risk of kidney stones after bariatric surgery

The Centers for Disease Control and Prevention (CDC) reports that 22.1 percent of adults in Washington, D.C., are obese. Though that’s lower than the national obesity rate of 36.5 percent, it still means more than one in five adults in our area has a serious weight problem.  

Many people who can’t lose enough weight through diet and exercise turn to weight loss surgery, or bariatric surgery. Bariatric surgery is a great option to help people lose weight. But we’ve noticed that patients who have bariatric surgery have a greater risk for kidney stones than people who haven’t had this procedure.  

Kidney stones are a significant medical problem, and they can increase your chance of developing serious issues like kidney damage and recurrent infections. We can work to identify and lower your risk for having kidney stones after your weight loss surgery.  

The relationship between bariatric surgery and kidney stones

In one study, researchers looked at a group of obese patients who had bariatric surgery and compared them to a group of obese patients who didn’t have surgery. These two groups had similar rates of developing kidney stones at the start of the study.  

But afterward, 11 percent of the patients who had surgery developed new kidney stones within six years of their surgery, compared to 4.3 percent of the patients who didn’t have surgery. Put another way, the patients who had bariatric surgery were over 255 percent more likely to develop new kidney stones than the patients who didn’t have surgery.  

This happens because bariatric surgery causes changes to patients’ urine that make kidney stones more likely. The kidneys filter waste products from the bloodstream that are absorbed in the food we eat. Some of the urinary changes we commonly see after bariatric surgery include:

  • Decreased amounts of urine, which is caused by dehydration and can concentrate waste products to form kidney stones more easily
  • Increased levels of a compound called oxalates
  • Increased levels of calcium, which enters the bloodstream when a diet is too high in salt, or sodium  

#BariatricSurgery causes changes to patients’ urine that make #kidneystones more likely. via @MedStarWHC

Having high levels of calcium and oxalates in concentrated urine increases the risk of developing calcium oxalate kidney stones, which are the most common type of kidney stone.

Reduce your risk of kidney stones after weight loss surgery

Reducing the risk of kidney stones is important for everyone, but it’s especially important for patients who have had bariatric surgery. Drinking enough water, decreasing salt intake and eating less animal protein are a few things everyone can do to lower their risk.

Related reading: Kidney stone diagnosis and treatment

But there are specific steps I recommend that patients who have had bariatric surgery take to reduce their risk. These steps relate to the levels of oxalates and calcium in a patient’s urine.


People who have had bariatric surgery need to reduce the levels of oxalates in their diets to avoid kidney stones. Oxalates are found in a wide variety of fruits, vegetables and grains. Oxalate levels in food can vary depending on many factors, including where they’re grown and when they’re harvested.

People who have had #bariatricsurgery need to reduce the levels of oxalates in their diets to avoid #kidneystones. via @MedStarWHC

Your doctor can help you determine which foods to avoid. But in general, the following foods tend to be high in oxalates:

  • Spinach
  • Tea
  • Cola
  • Soy
  • Nuts

I also recommend that you talk to your doctor if you take a vitamin C supplement. One study showed that men who took a 1,000-milligram vitamin C supplement on a regular basis had double the risk of calcium oxalate kidney stones over men who didn’t take a supplement. This may be because the body disposes of excess vitamin C in the form of oxalates, which end up in urine.  


Oxalate levels in the urine aren’t just tied to oxalate-rich foods. Calcium levels also play a role in the amount of oxalates that end up in urine. A lack of calcium in your diet can increase the amount of oxalates that make their way to the kidneys.

All of our bariatric surgery patients are instructed to take calcium supplements following their procedures. We recommend that patients take at least 1,500 milligrams of calcium per day. Taking calcium citrate supplements as opposed to calcium carbonate may help, as people with low levels of citrate are at increased risk for kidney stones. Citrate is a compound found in citrus fruits like lemons and oranges.  

Getting the calcium you need in your diet is the best option. Foods that are rich in calcium include:

  • Watercress, kale and arugula
  • Green beans 
  • Low-fat cheeses, milk and other dairy products

Reducing salt is important for maintaining healthy calcium levels. Salt, or sodium, causes the body to lose more calcium than normal. That calcium ends up in urine. Most people get far too much salt in their diets in the form of processed foods, so watch for sodium levels on nutrition information when you shop for groceries.  

What to watch out for after bariatric surgery

It’s a good idea to ask your bariatric surgeon about the risk of kidney stones. We work closely with our bariatric surgeons to minimize patients’ risk after surgery.

Make sure to watch for the signs of kidney stones if you’ve had bariatric surgery. These symptoms include:

  • Blood in your urine
  • Loss of appetite
  • Nausea or vomiting
  • Sudden, severe pain in the side or mid-back  

You may also notice a fever or chills along with any of these symptoms. This can be a sign of an infection. Request an appointment with one of our urologists if you notice any of these symptoms.  

Bariatric surgery can be life-changing—not to mention lifesaving—for many patients who struggle with obesity. But it can increase your risk of kidney stones. Making smart decisions about your diet after surgery can help you reduce this risk so you can focus on living a healthier life. 

Today’s stone age: How to eat to prevent kidney stones

Kidney stones are very common: Men have a nearly one in five chance of developing kidney stones over their lifetime, and women have a nearly one in 10 chance. And as part of what is known as the “Kidney Stone Belt,” D.C.-area residents are at even higher risk of developing kidney stones.  

This is a major problem, and it’s one that’s only gotten worse with time. The overall risk for kidney stones in the late 1970s was calculated at 3.8 percent. In the late 2000s, that number had jumped to 8.8 percent. That’s more than a 231 percent increase in the overall risk of kidney stones in just 30 years.

If you have one kidney stone, the chances of developing a second one or more over your lifetime are more than 50 percent. And having kidney stones also can put you at greater risk for long-lasting problems such as kidney failure, recurrent infections, and multiple other conditions.  

Fortunately, making smart dietary choices can prevent kidney stones from developing in the first place. And if you develop a kidney stone, we can determine what’s causing it and lower your risk for developing another one.

A formula for kidney stone protection: Drink enough water

Dehydration is the biggest risk factor for kidney stones that most people face. Our kidneys filter blood to remove waste products. These waste products enter our bloodstream from what we eat and drink. When urine is more concentrated, it’s more likely that the waste products filtered out by the kidneys will form a stone. The darker yellow urine is, the more concentrated it is. Urine should be light yellow or clear if you’re getting enough to drink.  

#Dehydration is the biggest risk factor for #kidneystones that most people face. via @MedStarWHC

Sadly, most of us don’t get enough to drink. Kidney stones tend to be more common in areas of the world with warmer temperatures, where the heat makes it easier to get dehydrated. We call this the “Kidney Stone Belt,” and it includes Maryland, Virginia and Washington, D.C., among other places.  

The simple answer for dehydration is to drink more. Water is best, of course. Citrus fruits like lemon can lower the risk for kidney stones because they contain a compound called citrate. Therefore, adding lemon to water or drinking lemonade also can help.  

Of course, drinking more water is easier for some people than others. I’ve seen patients who come in with kidney stones, and they tell me, “I have no idea what my risk factor is.” Then I find out they don’t drink water all day because they can’t go to the bathroom during the workday.  

Avoid soda and other drinks that are high in sugar when possible. Sugar alters the way the body absorbs minerals and can increase the risk for kidney stones. Sugar also increases the acid levels of urine, which makes stones more likely to form.  

A diet to prevent kidney stones: They are what we eat

It’s not just the amount of fluid in urine that determines a person’s risk of kidney stones. This risk also is based on the amount of waste products the kidneys have to filter from the blood. When there are more of certain types of waste products relative to the amount of fluid in urine, the risk of kidney stones goes up.

One of the reasons our risk for kidney stones has increased so much since the 1970s is because our diets have changed since that time. Two of the main dietary culprits for kidney stones are proteins and salt.


The modern American diet contains a lot more protein than it did in the 1970s, especially animal-based protein. We consume animal-based protein in the form of:

  • Chicken and other poultry
  • Eggs
  • Red meat
  • Seafood

I tell my patients to limit their protein intake to two servings per day. That takes some planning and thought about what you eat. If you know you’re having fish for dinner, and you had sausage with breakfast, avoiding that chicken-salad sandwich for lunch is one way to control your protein.


The salt, or sodium, that we eat plays a huge role in the formation of kidney stones. Since the 1970s, a lot more of the food we eat is processed and packaged, and that means more salt. Plus, people are eating saltier foods and more of them.

For example, just 1 ounce of potato chips can have about seven to 10 percent of the average person’s daily recommended maximum serving of salt. And as the U.S. Department of Agriculture notes, an average single-patty plain cheeseburger at a fast-food restaurant has 469 milligrams of salt—more than 20 percent of the maximum daily serving of 2,300 milligrams per day.  

That would be bad enough if people stuck to the upper limits of salt intake per day. But most people eat much more salt than the recommended maximum amount. As noted by the U.S. Department of Health and Human Services, the average woman consumes 2,980 milligrams of salt per day. And the average man consumes 4,240 milligrams of salt per day—nearly twice the recommended maximum. So people are putting tremendous loads of salt into their systems, and the kidneys have to filter all of that salt out. 

The average man consumes 4,240 mg of #salt per day—nearly twice the recommended maximum. via @MedStarWHC

Everyone can decrease the amount of salt in their diet. Salt is such a prevalent ingredient in food, even when you might not expect it. Most of the salt we eat comes from commercial food processing operations. And it can be found in unexpected places: ketchup, cereal, tomato sauce, bread and canned vegetables have some of the highest salt contents of our everyday food. Pay attention to the nutrition information on packaged foods, and choose low-sodium or no-sodium options when possible.

When you cook, limit the amount of salt you use in your recipes. Fresh herbs, pepper and other ingredients can add flavor to dishes without the need for a lot of salt.  

When to treat kidney stones and reducing future risk

It’s not always possible to avoid kidney stones. Request an appointment with one of our urologists if you notice the following symptoms:

  • Changes in your urine’s color or appearance
  • Nausea or vomiting that doesn’t go away
  • Sharp pain in the side or mid-back

Related reading: Kidney stone diagnosis and treatment

When I see patients for kidney stones, I use imaging studies to get a look at where the stone is and how big it is. Not every stone needs surgery. If you have a small stone, it’s probably one we can keep a close watch on with regular visits to make sure it passes on its own. If it’s a large stone that’s blocking the ureter (the tube that lets urine pass from the kidney to the bladder), it probably needs to be treated. Without treatment, those kinds of stones can lead to permanent kidney damage if they don’t pass on their own.  

For some doctors, treatment stops as soon as the stone is gone. But that’s not how we should treat patients with kidney stones. As we know, more than half of the patients who develop a stone will have another stone in their lifetime. Therefore, as with any other chronic disease, we work to figure out why the patient has the problem and what we can do to fix it, and that’s how I treat kidney stones.  

The process starts with a basic blood draw, called a BMP, which stands for basic metabolic panel. This shows us the basic chemistry of your blood, including levels of sodium, potassium, calcium and other substances.  

Then we do a 24-hour urine collection, which involves taking all of the urine a patient produces for 24 hours and sending it to a lab for analysis. We look for the same factors in the patient’s urine as we do in the blood and compare the levels. If a patient has low levels of citrate in their urine, we know to increase the patient’s citrate levels. Chances are, that’s going to help you not make more kidney stones in the future. This is a personalized process based on each patient’s unique factors.

Kidney stones may be little, but they’re a big problem for our country. Making smart choices about what goes into your body can reduce your risk for the pain and consequences of kidney diseases down the road.