Leafy greens and other plant foods are very popular among the health-conscious.
However, many of these foods also contain an antinutrient called oxalate (oxalic acid).
This is a detailed article about oxalate and its health effects.
Oxalic acid is an organic compound found in many plants.
In plants, it’s usually bound to minerals, forming oxalate. The terms “oxalic acid” and “oxalate” are used interchangeably in nutrition science.
Your body can produce oxalate on its own or obtain it from food. Vitamin C can also be converted into oxalate when it's metabolized (2).
Once consumed, oxalate can bind to minerals to form compounds, including calcium oxalate and iron oxalate. This mostly occurs in the colon, but can also take place in the kidneys and other parts of the urinary tract.
For most people, these compounds are then eliminated in the stool or urine.
However, for sensitive individuals, high-oxalate diets have been linked to an increased risk of kidney stones and other health problems.
Bottom Line: Oxalate is an organic acid found in plants, but can also be synthesized by the body. It binds minerals, and has been linked to kidney stones and other health problems.
One of the main health concerns about oxalate is that it can bind to minerals in the gut and prevent the body from absorbing them.
Nevertheless, it's important to remember that only some of the minerals in our food will bind to oxalate.
Bottom Line: Oxalate can bind to minerals in the gut and prevent some of them from being absorbed, particularly when combined with fiber.
Normally, calcium and small amounts of oxalate are present in the urinary tract at the same time, but they remain dissolved and cause no problems.
However, sometimes they bind to form crystals. In some people, these crystals can lead to the formation of stones, especially when oxalate is high and urine volume is low ().
Small stones often don't cause any problems, but large stones can cause severe pain, nausea and blood in the urine as they move through the urinary tract.
Although there are other types of kidney stones, about 80% are made up of calcium oxalate ().
For this reason, people who have had one episode of kidney stones may be advised to minimize their consumption of foods high in oxalate (, ).
However, across-the-board oxalate restriction is no longer recommended to every person with kidney stones. This is because most of the oxalate found in urine is produced by the body, rather than absorbed from food (7).
Most urologists now only prescribe a strict low-oxalate diet (less than 50 milligrams per day) for patients who have high levels of oxalate in their urine ().
Therefore, it's important to be tested from time to time to figure out how much restriction is necessary.
Bottom Line: High-oxalate foods may increase the risk of kidney stones in susceptible people, and recommendations for patients are based on urinary levels.
Some claim that a high oxalate intake may be linked to the development of autism.
Others say oxalates may be linked to vulvodynia, which is characterized by chronic, unexplained vaginal pain.
However, when 59 women with vulvodynia were treated with a low-oxalate diet and calcium supplements, nearly a quarter experienced improvements in symptoms (10).
The authors of that study concluded that dietary oxalate might worsen, rather than cause, the condition.
Several online anecdotes do link oxalates with autism and vulvodynia, but only a few studies have looked into possible connections. Further research is needed.
Bottom Line: Some people have suggested that consuming foods high in oxalate may lead to autism and vulvodynia, but at this point the research does not support these claims.
Some proponents of low-oxalate diets say people are better off not consuming foods rich in oxalates, since they may have negative health effects.
However, it's not that simple. Many of these are healthy foods that contain important antioxidants, fiber and other nutrients.
Therefore, it's not a good idea for most people to completely stop eating high-oxalate foods.
Bottom Line: Many foods that contain oxalates are delicious and provide many health benefits. Avoiding them is not necessary for most people, and may even be detrimental.
Some of the oxalate you eat can be broken down by bacteria in the gut, which happens before it can bind to minerals.
One of them, Oxalobacter formigenes, actually uses it as an energy source. It significantly reduces the amount your body absorbs ().
However, some people don't have much of this bacteria in their gut, as antibiotics decrease the number of O. formigenes colonies (12).
This is partly because they are unable to regulate the amount of oxalate they absorb.
Similarly, elevated levels of oxalate have been found in the urine of patients who have had gastric bypass surgery or other surgeries that alter gut function ().
This suggests that people who have taken antibiotics or suffer from gut dysfunction may benefit more from a low-oxalate diet.
Bottom Line: Most healthy people can consume oxalate-rich foods without problems, but those with altered gut function may need to limit their intake.
Oxalates are found in almost all plants, but some plants contain very high amounts while others have very little. Animal foods contain only trace amounts.
Foods high in oxalate (100–900 mg per serving) include:
- Beet greens
- Swiss chard
- Cocoa powder
- Sweet potatoes
- Turnip greens
- Star fruit
To learn more, this comprehensive list provides the oxalate content of many foods.
Bottom Line: The amount of oxalates in plants varies from extremely high to very low, and "high-oxalate" is classified as 100–900 mg per serving.
People who are placed on low-oxalate diets for kidney stones are usually instructed to eat less than 50 mg of it each day.
Here are a few tips on how to follow a low-oxalate diet:
- Limit oxalate to 50 mg per day: Choose a variety of nutrient-dense animal and plant sources from this list of foods very low in oxalate.
- Boil oxalate-rich vegetables: Boiling vegetables can reduce their oxalate content from 30% to almost 90%, depending on the vegetable (17).
- Drink plenty of water: Aim for a minimum of 2 liters daily. If you have kidney stones, drink enough to produce at least 2.5 liters of urine a day ().
- Get enough calcium: Calcium binds to oxalate in the gut and reduces the amount your body absorbs, so try to get about 800–1,200 mg per day (1, 16).
Foods high in calcium and low in oxalate include:
Bottom Line: Diets with less than 50 mg of oxalate per day can be balanced and nutritious. Calcium also helps reduce its absorption.