Leafy green plant foods like spinach and kale are among the most popular foods with health-conscious consumers.
The benefits of including these foods in the diet are often mentioned without reference to the fact that they are high in oxalates, or oxalic acid, an anti-nutrient that can negatively impact the body’s ability to absorb minerals.
Besides leafy greens, other foods high in oxalates include certain vegetables, seeds and nuts, cocoa and some fruits. (1)
Oxalates are organic acids that can be produced by the body or obtained through eating foods; when vitamin C is metabolized, it can also be converted into oxalic acid. (2)
When we eat foods rich in oxalates, these substances can bind with minerals, creating compounds such as iron oxalate and calcium oxalate.
While this process usually occurs in the colon, it can also take place in parts of the urinary tract, such as the kidneys.
Normally oxalates will be expelled in urine or stools, but some people are sensitive to a high concentration of oxalates in the diet, which can result in the formation of kidney stones as well as other health issues.
Reduced Mineral Absorption
When oxalates are bound to minerals in the gut, the result can be a reduced ability to assimilate nutrients.
So even though spinach has high calcium content, the oxalate levels prevent much of this important mineral from being absorbed; but eating spinach in the same meal with milk does not hinder the body’s ability to absorb the calcium present in milk. (3)
Eating fiber-rich foods with oxalates appears to reduce mineral absorption at an even greater level. (4) It’s important to note that only some of the minerals we get through our diets are affected by the presence of oxalates.
It’s normal for both oxalates and calcium to be found in the urinary tract, but as long as they remain dissolved, this isn’t a problem.
When these compounds bind to form crystals in people sensitive to high oxalate levels, it can lead to the development of stones that may affect the flow of urine. An estimated 80% of these kidney stones are composed of calcium oxalate. (5)
If stones are small, people don’t usually experience problems as they move through the urinary tract, but larger stones may cause nausea, extreme pain, and blood in the urine.
People who have had problems with kidney stones are often advised to cut back on consumption of foods with high oxalate content. (6)
But since most oxalate in urine is actually produced by the body, this type of dietary restriction is no longer recommended to all patients who have an episode of kidney stones. Instead, urologists restrict foods high in oxalate only for patients with extremely elevated oxalate levels in their urine. (7)
Testing can help determine how much, if any, restriction of these foods is necessary.
Some proponents of a low-oxalate diet believe that these compounds may contribute to the development of autism, as well as a condition called vulvodynia, which is characterized by unexplained, chronic pain in the vagina.
But one study that placed 59 women suffering from vulvodynia on a low-oxalate diet combined with supplemental calcium found that a full quarter of the test subjects experienced significant improvements in symptoms. (10)
This may indicate that a high-oxalate diet could make this condition worse, as opposed to causing it.
Anecdotal material found online may link the disorders with high oxalate consumption, but further study is needed in order to provide clear answers.
Should You Limit Foods High in Oxalates?
Many healthy foods abundant in important nutrients are also high in oxalates. Most people are not sensitive to oxalate consumption, and could lose a significant amount of vital antioxidants, minerals and vitamins if they were to cut these foods out of their diets.
Some of the dietary oxalate we take in is broken down in the gut before mineral binding takes place. A bacteria found in the gut called Oxilobacter formigenes uses oxalates as a source of energy, as well as limiting the amount of oxalates absorbed by the body. (11)
These O. formigenes bacteria can be destroyed by antibiotics, which may be one reason why some people don’t have as many of the colonies formed by this bacteria in their guts. (12)
Patients who have undergone gastric bypass surgery, as well as those who have had other surgeries that change gut function, have more oxalate in the urine; this could lead to an increased chance of problems with kidney stones. (15)
Anyone who has taken antibiotics or suffers from some form of gut dysfunction may want to consider limiting foods high in oxalates.
Only very small amounts of oxalates are found in animal foods, while plant foods contain them in varying amounts, from low to high.
A low-oxalate diet is usually capped at 50 mg of dietary oxalate daily.
This list includes foods considered high in oxalates, between 100 mg and 900 mg per serving:
- Swiss chard
- Beets and beet greens
- Turnip greens
- Sweet potatoes
- Star fruit
- Cocoa powder
You can find a more comprehensive list of the oxalate content in many commonly consumed foods by clicking here.
If you’re following a low-oxalate diet, consider these strategies:
- Keep dietary oxalate under 50 mg by choosing foods from this low-oxalate list.
- Boil vegetables rich in oxalate to decrease oxalate content by 30% to nearly 90%, depending on the food. (16)
- Drink at least two liters of water daily.
- Get plenty of calcium, which binds oxalate in the gut, cutting the amount absorbed by the body; shoot for 800 mg to 1,200 mg daily. (17)
Foods rich in calcium that are also low in oxalates include bok choy, broccoli, cheese, plain yogurt, and whole, canned fish with bones.
Summary: For healthy people, the oxalate content of foods should not be a concern, but those who have certain medical conditions, including a history of kidney stones, may want to limit foods high in oxalates.