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Eating a nutritious diet should be as simple as consuming more colorful, plant foods, especially fruits and vegetables. But then, certain “antinutrients” have made headlines, with experts saying certain plant foods may not be as nutritious as once thought due to some of the chemicals in them restricting the bioavailability of key nutrients.

One of these purported antinutrients is oxalates. Due to their binding to certain minerals, especially calcium, they restrict our ability to absorb essential minerals. Thus, oxalates have become one of the maligned substances in certain plant foods.

Do oxalates really reduce the nutrition enough to where you should restrict how much you consume? Let’s see what the literature has to say.

What Are Oxalates?

Oxalate, or oxalic acid, is a substance that can bind with minerals, including sodium, potassium, calcium, iron, and magnesium. Insoluble oxalates are excreted in the feces along with the bound minerals, whereas soluble oxalate (or free, unbound oxalate) can be absorbed in the intestines and colon before going into the bloodstream. Once in the bloodstream, soluble oxalates like calcium oxalate are excreted via the kidneys and can increase urinary oxalates or chelate with calcium, which can lead to kidney stones.

In addition to dietary oxalates, humans also create endogenous oxalates. Oxalate is the byproduct of the metabolism of three main molecules: ascorbate, glyoxylate (which comes from the breakdown of several different amino acids), and glycine. Most urinary oxalate comes from endogenous metabolism rather than dietary oxalate. Although oxalates have a bad reputation, some research is pointing to their use as a possible chelator, or binder of toxins for removal.

Some of the foods highest in oxalates include:

• Beets
• Berries, especially strawberries
• Chocolate
• Legumes (including soy)
• Nuts
• Parsley
• Rhubarb
• Spinach
• Swiss chard
• Sweet potatoes
• Tea
• Wheat bran
• Whole grains

Healthy individuals should be able to regularly consume high-oxalate foods without complications if daily oxalate intake is between 50-200 mg. For those susceptible to kidney stone formation, it may be best to limit oxalates to 40-50 mg per day.

Oxalates and Kidney Stones

Oxalates are frequently studied in connection with kidney stones because calcium oxalate stones make up more than 80% of kidney stones. Hypercalciuria and hyperoxaluria are risk factors for calcium oxalate stone formation. Increasing the concentration of oxalate in urine promotes calcium oxalate crystal formation as well as oxidative stress, which facilitates crystal adhesion.

Though 60-80% of plasma oxalate is endogenously produced, 20-40% of plasma oxalate from the diet is considered a more controllable factor. It is estimated that the oxalate absorption rate for ingested oxalates is between 5-10%. One study found that oxalate intake over a range of 50-750 mg increased urinary oxalate excretion, which leads to the question of whether a low-oxalate diet could also help decrease the risk of kidney stones, especially in those who are susceptible.

However, factors other than dietary oxalate intake may play a role in stone formation. Low calcium intake increases the amount of free oxalates that can be absorbed and may promote stone formation. A recent meta-analysis showed a positive association between non-dairy animal protein and kidney stone risk, whereas a negative association was seen with dairy protein. Hyperabsorption of oxalates, or intestinal absorption greater than 10%, may occur because of fat malabsorption, as fatty acids bind calcium and thus increase oxalates available for absorption. A recent study suggested that fresh fruit was a protective factor against kidney stone formation, despite the potential oxalate content.

Thus, although there is a connection between calcium oxalate excretion, dietary oxalate, and stone risk, it may not be as significant an association as historically believed. Although a low-oxalate diet may still be prescribed for the prevention of kidney stones, many doctors are realizing it is more complex and may consider other factors, such as the role of endogenous oxalate formation and gut health.

Oxalates and Gut Health 

With the amount of research detailing the importance of gut health and microbiome on many seemingly unrelated components of health, it should come as no surprise that gut health also plays a role in oxalate levels and associated risk factors.

Gut dysbiosis influences oxalate metabolism and may contribute to stone formation. Oxalate-degrading bacteria, such as Oxalobacter formigenes, use oxalate as an energy source and can reduce the risk of oxalate stone formation. The presence of this bacterium can be assessed from a stool test. Lactobacillus and Bifidobacterium species are also shown to degrade oxalates. An older study using a probiotic product called VSL#3, which contains both Lactobacillus and Bifidobacterium species, found that the product effectively reduced urinary oxalate. However, studies have been mixed as to whether probiotics or having certain bacteria species, such as O. formigenes, in the microbiome help reduce urinary oxalates and/or kidney stone formation.

Fat malabsorption also contributes to increased enteric hyperoxaluria due to calcium binding to free fatty acids, leading to more soluble oxalate in the intestinal lumen. This also increases gut permeability, leading to more oxalate absorption.

Though some evidence indicates a relationship between irritable bowel disease (IBD) and kidney stone formation, a meta-analysis of more than 13 million individuals found that the prevalence of kidney stones in those with IBD and the general population was similar. Nonetheless, a history of intestinal surgery was associated with a higher risk of developing kidney stones. Findings from another study suggest that hyperabsorption of oxalate follows extensive ileal resection in those with Crohn’s disease, causing enteric hyperoxaluria and stone formation.

Reducing the Impact of Oxalates

Cooking Methods. Most traditional food preparation methods, including soaking, boiling, and steaming, have been shown to reduce oxalate content. Because oxalates are soluble in water, wet methods are most efficient in their removal. Fermentation was also shown to reduce the oxalate content of spinach while maintaining its antioxidant potential.

In one study comparing cooking methods of the highly nutritious “weed” fat hen, also known as goosefoot and lamb’s quarter, the soluble calcium percentage out of the total calcium was 65.9% in raw and 78.14% in boiled, while wok-fried leaves were 20.78% due to an increase in insoluble oxalates. Another study involving red and white beans found there was a reduction in oxalates after boiling or soaking and boiling.

On the other hand, roasting, grilling, and baking foods high in oxalates tend to increase oxalate content, so those who wish to decrease their oxalate intake may want to carefully choose food preparation methods.

Diet. To offset soluble oxalate absorption, it may help to pair high-oxalate foods with calcium-rich foods as calcium binds oxalate and reduces its absorption. Though calcium restriction has been recommended for stone formers in the past, a low calcium diet may increase the risk of stone formation. Therefore, adults should consume the recommended dietary allowance for calcium (1000-1200 mg/day for non-pregnant adults) regardless of their predisposition to stone formation unless instructed otherwise.

Following the Dietary Approaches to Stop Hypertension (DASH) diet might reduce urinary risk factors that contribute to kidney stone formation. The DASH diet promotes fruit and vegetable intake, and interestingly, a recent study found that high intake of carotenoids like beta-carotene, lutein, zeaxanthin, and beta-cryptoxanthin was associated with decreased kidney stone prevalence.

Finally, high sodium intake has the potential to increase urinary calcium, and stone formers may benefit from a low sodium diet.

Hydration. Drinking fluids is important to maintain adequate urine output and reduce urine saturation and staying hydrated can reduce the risk of stone formation. It’s recommended to drink enough fluids to create at least 2 L of urine per day, but a more personalized fluid recommendation is 30 mL/kg per day. Though evidence is conflicting, some fruit juices, such as orange and lemon, may reduce stone formation. Tea intake may reduce the risk of kidney stones.

Vitamin C. Vitamin C supplementation may increase urinary oxalate levels. Conversely, one study found that vitamin C can reverse the impact oxalate and other antinutrients have on iron absorption. A study on multivitamins found that vitamin C supplementation decreases and then increases the prevalence of kidney stones as intake increases, suggesting that balance is important for this nutrient. A more recent study pointed to gender differences; therefore, more research is needed.

So, Should You Be Concerned About Oxalates?

For decades, a low-oxalate diet has been a mainstay for kidney stone prevention in those at highest risk, but doctors are starting to question whether it is truly necessary, even for known stone formers. Certain populations are at a greater risk of increased oxalate excretion in the urine, which puts them at risk of kidney stones, and consuming oxalate-rich food may play a role, among other factors.

Although the term antinutrient may be too strong, depending on whom you talk to, oxalates can bind to minerals, reducing their bioavailability in food. Certain preparation methods, as well as certain combinations of foods, may reduce the soluble oxalate, allowing for enhanced bioavailability of the minerals.

In many cases, traditional cooking and preparation methods of foods mitigate many of the problems associated with oxalates. Furthermore, many of the foods containing oxalates have health benefits beyond their mineral content; retaining them as part of a varied, nutrient-dense, colorful diet can potentially bring out their other benefits.

Final Remarks

If you are concerned about the oxalate content of your diet, it is best to speak to your doctor, nutritionist, or another healthcare practitioner, especially if you are at a greater risk of kidney stones, have gastrointestinal problems, or otherwise may be more sensitive to issues relating to oxalates.