Calcium oxalate crystals are the most common cause of kidney stones — hard clumps of minerals and other substances that form in the kidneys. These crystals are made from oxalate — a substance found in foods like green, leafy vegetables — combined with calcium. Having too much oxalate or too little urine can cause the oxalate to crystalize and clump together into stones.
Kidney stones can be very painful. They can also cause complications like urinary tract infections. But they are often preventable with a few dietary changes.
Oxalate comes from many of the foods in our diet. The main dietary sources of oxalate are:
- spinach and other green, leafy vegetables
- wheat bran
- navy beans
- French fries and baked potatoes
- nuts and seeds
- soy products
- strawberries and raspberries
When you eat these foods, your GI tract breaks them down and absorbs the nutrients. The leftover wastes then travel to your kidneys, which remove them into your urine. The waste from broken-down oxalate is called oxalic acid. It can combine with calcium to form calcium oxalate crystals in the urine.
Kidney stones may not cause symptoms until they start to move through your urinary tract. When stones move, the pain can be intense.
The main symptoms of calcium oxalate crystals in the urine are:
- pain in your side and back that can be intense, and may come in waves
- pain when you urinate
- blood in your urine, which can look red, pink, or brown
- cloudy urine
- foul-smelling urine
- an urgent and constant need to urinate
- nausea and vomiting
- fever and chills if you have an infection
Urine contains chemicals that normally prevent oxalate from sticking together and forming crystals. However, if you have too little urine or too much oxalate, it can crystalize and form stones. Reasons for this include:
- not drinking enough fluids (being dehydrated)
- eating a diet that’s too high in oxalate, protein, or salt
In other cases, an underlying disease causes the crystals to form into stones. You’re more likely to get calcium oxalate stones if you have:
Your doctor might use these tests to find out if you have calcium oxalate stones:
- Urine test. Your doctor may request a 24-hour urine sample to check levels of oxalate in your urine. You’ll have to collect your urine throughout the day for 24 hours. A normal urine oxalate level is less than 45 milligrams (mg) per day.
- Blood test. Your doctor can test your blood for the gene mutation that causes Dent disease.
- Imaging tests. An X-ray or CT scan can show stones in your kidney.
During pregnancy, blood flow increases to nourish your growing baby. More blood gets filtered through your kidneys, which causes more oxalate to be removed into your urine. Even though the risk of kidney stones is the same during pregnancy as it is during other times of your life, extra oxalate in your urine can promote stone formation.
During pregnancy, imaging tests like a CT scan or X-ray may not be safe for your baby. Your doctor can use an ultrasound to diagnose you instead.
If you’re having severe symptoms from the kidney stone, or your pregnancy is at risk, procedures like a stent or lithotripsy can remove the stone.
Small stones may pass on their own without treatment in about four to six weeks. You can help flush out the stone by drinking extra water.
Your doctor can also prescribe an alpha-blocker like doxazosin (Cardura) or tamsulosin (Flomax). These drugs relax your ureter to help the stone pass from your kidney more quickly.
Pain relievers such as ibuprofen (Advil, Motrin) and acetaminophen (Tylenol) can help relieve your discomfort until the stone passes. However, if you’re pregnant, talk with your healthcare provider before taking non-steroidal, anti-inflammatory drugs (ibuprofen, naproxen, aspirin, and celexcoxib).
If the stone is very large or it doesn’t pass on its own, you may need one of these procedures to remove it:
- Extracorporeal shock wave lithotripsy (ESWL). ESWL delivers sound waves from outside your body to break the stone into little pieces. Within a few weeks after ESWL, you should pass the stone pieces in your urine.
- Ureteroscopy. In this procedure, your doctor passes a thin scope with a camera on the end through your bladder and into your kidney. Then the stone is either removed in a basket or broken up first with a laser or other tools and then removed. The surgeon may place a thin plastic tube called a stent in the ureter to hold it open and allow urine to drain while you heal.
- Percutaneous nephrolithotomy. This procedure occurs while you’re asleep and pain-free under general anesthesia. Your surgeon makes a small incision in your back and removes the stone using small instruments.
You can prevent calcium oxalate from forming crystals in your urine and avoid kidney stones by following these tips:
- Drink extra fluids. Some doctors recommend that people who’ve had kidney stones drink 2.6 quarts (2.5 liters) of water each day. Ask your doctor how much fluid is right for you.
- Limit the salt in your diet. A high-sodium diet can increase the amount of calcium in your urine, which can help stones form.
- Watch your protein intake. Protein is essential to a healthy diet, but don’t overdo it. Too much of this nutrient can cause stones to form. Make protein less than 30 percent of your total daily calories.
- Include the right amount of calcium in your diet. Getting too little calcium in your diet can cause oxalate levels to rise. To prevent this, be sure you’re getting the appropriate daily amount of calcium for your age. Ideally, you’ll want to get calcium from foods like milk and cheese. Some studies have linked calcium supplements (when not taken with a meal) to kidney stones.
- Cut down on foods that are high in oxalate, like rhubarb, bran, soy, beets, and nuts. When you do eat oxalate-rich foods, have them with something containing calcium, like a glass of milk. This way the oxalate will bind to calcium before it gets to your kidneys, so it won’t crystallize in your urine. Learn more about a low-oxalate diet.
If you’ve had calcium oxalate stones in the past, or you have symptoms of stones, see your primary care doctor or a urologist. Find out what changes you should make to your diet to prevent these stones from forming again.