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.
What is oxalate?
Oxalic acid is an organic compound found in many plants, including leafy greens, vegetables, fruits, cocoa, nuts, and seeds.
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.
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 occur in the kidneys and other parts of the urinary tract.
In most people, these compounds are then eliminated in the stool or urine.
However, in sensitive individuals, high oxalate diets have been linked to an increased risk of kidney stones and other health problems.
Summary: Oxalate is an organic acid found in plants, but your body can also synthesize it. It binds minerals and has been linked to kidney stones and other health problems.
Oxalate can reduce mineral absorption
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.
For example, spinach is high in calcium and oxalate, which prevents a lot of the calcium from being absorbed into the body.
Eating fiber and oxalate together may further hinder nutrient absorption.
Nevertheless, it’s important to remember that only some of the minerals in the food will bind to oxalate.
Even though calcium absorption from spinach is reduced, calcium absorption from milk is not affected when milk and spinach are consumed together.
Summary: Oxalate can bind to minerals in your gut and prevent some of them from being absorbed, particularly when combined with fiber.
Oxalate may contribute to kidney stones
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 for every person with kidney stones. This is because half of the oxalate found in urine is produced by the body rather than absorbed from food.
Most urologists now prescribe a strict low oxalate diet (less than 100 milligrams per day) only 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.
Summary: High oxalate foods may increase the risk of kidney stones in susceptible people. Recommendations for limiting oxalate consumption are based on urinary levels.
Does oxalate cause any other problems?
Some people 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.
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Based on study results, researchers believe neither of these conditions are likely triggered by dietary oxalates.
However, in a 1997 study, when 59 women with vulvodynia were treated with a low oxalate diet and calcium supplements, nearly a quarter experienced improvements in symptoms.
The authors of that study concluded that dietary oxalate might worsen, rather than cause, the condition.
Several online anecdotes do link oxalates with autism or vulvodynia, but only a few studies have looked into possible connections. Further research is needed.
Summary: Some people have suggested that consuming foods high in oxalate may lead to autism or vulvodynia, but at this point, the research does not support these claims.
Most foods with oxalates are very healthy
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 foods are healthy, containing important antioxidants, fiber, and other nutrients.
Therefore, it’s not a good idea for most people to completely stop eating high oxalate foods.
Summary: 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.
Your gut determines oxalate absorption
Some of the oxalates you eat can be broken down by bacteria in your gut before they can bind to minerals.
One of these bacteria, Oxalobacter formigenes, actually uses oxalate as an energy source. This significantly reduces the amount of oxalate your body absorbs.
However, some people don’t have much of this bacteria in their gut, because antibiotics decrease the number of O. formigenes colonies.
What’s more, studies have found that people with inflammatory bowel disease have an increased risk of developing kidney stones.
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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 people who have had gastric bypass surgery or other surgeries that alter gut function.
This suggests that people who have taken antibiotics or experience gut dysfunction may benefit more from a low oxalate diet.
Summary: Most healthy people can consume oxalate-rich foods without problems, but those with altered gut function may need to limit their intake.
Foods high in oxalate
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.
Serving sizes may vary, which means some “high oxalate” foods, such as endive, can be considered low oxalate if the portion size is small enough. Below is a list of foods high in oxalate (greater than 50 mg per 100-mg serving):
- beet greens
- Swiss chard
- cocoa powder
- sweet potatoes
- turnip greens
- star fruit
Summary: The amounts of oxalates in plants vary from extremely high to very low. “High oxalate” is classified as greater than 50 mg per serving.
How to do a low oxalate diet
People who are placed on low oxalate diets for kidney stones are usually instructed to eat less than 50 mg of oxalate 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.
- 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 your gut and reduces the amount your body absorbs, so try to get 800–1,200 mg per day.
Foods high in calcium and low in oxalate include:
- plain yogurt
- canned fish with bones
- bok choy
Summary: Diets with less than 50 mg of oxalate per day can be balanced and nutritious. Calcium also helps reduce the absorption of oxalate.
Should you avoid oxalate?
People who tend to form kidney stones may benefit from a low oxalate diet.
However, healthy people trying to stay healthy do NOT need to avoid nutrient-dense foods just because they are high in oxalates.
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Oxalate is simply not a nutrient of concern for most people.