If you are trying to eat healthily, you are probably eating greens. Lots and lots of leafy greens! But what about oxalates in foods like spinach and Swiss chard? Can they interfere with calcium absorption and affect bone health?
This is a question we hear often. The good news is: spinach and other leafy greens are loaded with vitamins and minerals your bones need and need not be limited for fear of their oxalate content.
Oxalates and Health
Oxalates, or oxalic acids, occur naturally in the body. We produce oxalates in our liver from amino acids (protein) and also when we metabolize vitamin C. In fact, oxalates produced by us account for 60-80% of the oxalates in our bloodstream! The remaining 20-40% are derived from the foods we eat, so our diet can contribute to higher oxalate levels in our blood and urine. Oxalates are found in a wide variety of foods and play a supportive role in plant, animal, and human metabolism.
The food sources with higher concentrations of oxalates include nuts, fruits, vegetables, grains, and legumes. While we only obtain 10-15% of all blood oxalates from the food we eat (the other 85-90% is a metabolic end product generated by the liver), the diet can contribute to higher levels in the blood and urine. In other words, the food you eat could potentially play a role in the ratio of oxalate concentrations in your blood and urine.
Oxalic acids can combine with calcium and iron to form calcium oxalate and iron oxalate. For the majority of people, the body can break down these substances and eliminate them via the kidney or colon. But in some people, diets high in oxalates can accumulate in the body.
The two key health concerns that are linked to oxalates are kidney stone formation and mineral deficiencies.
- Kidney Stone Formation
The frequency of kidney stones is increasing globally and is seen across sex, race, and age. In the United States, kidney stone formation is exceeding 12% in men and 6% in women. Of these, calcium oxalate makes up 60% of all stones formed.
For people with a healthy gut (a healthy digestive system), oxalates will pass through the stool or urine without causing problems. But if oxalate concentrations become too high they can accumulate in the kidney. If calcium concentrations also increase, our kidneys may be at risk of calcium oxalate stone formation. However, our gut bacteria play an important role in this process by breaking down oxalates. For example, the bacteria Oxalobacter formigenes can reduce the recurrence of kidney stones by 70%.
But age plays a role in how much of this bacteria you have. One study showed that almost all 6 to 8 year-olds possess Oxalobacter formigenes, but it’s only detected in 60-80% of adults. The use of drugs and antibiotics may contribute to this loss. (For more on this bacteria and its role in gut health, read the section below “8 Ways to Control Kidney Stone Formation“.)
What needs to be highlighted here is that oxalates in food alone do not cause kidney stones. Rather it is caused by imbalances in mineral and vitamin absorption and intake (e.g., low magnesium intake, insufficient vitamin D and K2) that lead to calcium metabolism dysfunction.
Let’s take a look at that relationship…
Vitamin D helps the body absorb calcium. Once absorbed, vitamin K2 plays an essential role by turning on different proteins that guide calcium into our bones and teeth where it belongs. You can see here how important it becomes to ensure adequate proportions of not only calcium, but also vitamins D and K2 in the diet to avoid calcium accumulation in the blood that can result in kidney stones.
- Calcium and Iron Deficiencies
Another health concern linked to oxalate accumulation is mineral deficiency. Oxalates can bind to minerals and prevent them from being absorbed in the body. The high oxalate content in spinach, for example, can inhibit calcium absorption.
But studies have found that if you eat the recommended daily amount of calcium, this effect isn’t seen. The take-home is that the calcium to oxalate ratio is more important than the high levels of oxalate. Keep eating your calcium and you have nothing to worry about!
The evidence for iron deficiency related to oxalates, however, is contradictory. One study showed a potential for calcium oxalate to decrease iron absorption whereas another suggests oxalates from fruit and vegetables are of minor relevance in iron nutrition.
The take-home here is that eating spinach and other leafy greens as part of a whole-foods, balanced diet should not be of concern when it comes to mineral deficiencies. If anything, eating leafy greens like spinach is increasing your mineral density and providing a protective effect against diseases like osteoporosis and other illnesses.
Food Sources of Oxalates
Oxalates can be found in plants and animals, but are at their highest levels in vegetables, nuts, and legumes. They’re mostly found in the leaves of plants, as opposed to roots, stems, and stalks, and their amounts vary with planting and harvesting conditions.
Most people hear a lot about oxalates in spinach but high amounts are also found in grains, legumes, nuts, chocolate, and coffee! Generally, some foods with the highest concentrations include:
|Food Type||Oxalate (mg/100gFW) |
|Oxalate (mg/100gFW) |
|Calcium (mg/100gFW) |
|Calcium (mg/100gFW) |
Oxalate (mg/100g fresh weight (FW)), calcium (mg/100g FW) and oxalate calcium (mEq) ratio of common foods. The table above divides the foods into three main group on the basis of their oxalate/calcium ratio. Group 1, >2.0; Group 2, 1.0-2.0; Group 3, <1.0. Source.
Those Who Should Avoid Oxalate Containing Foods
For most individuals, oxalate-containing foods are not a health concern and there is no reason to limit your intake of these highly nutritious foods!
If you are at high risk of calcium oxalate kidney stone formation, in addition to the approaches below, you may want to limit oxalate-rich foods to 40-50mg per day.
However, your risk of kidney stone formation should always be weighed against the risk of excluding extremely nutrient dense foods.
The only people who should consider this daily limit include those with a history of kidney stones, people with diseases that raise urine oxalate levels like primary hyperoxaluria (sometimes called Bird’s disease), and people with nutrient absorption disorders or issues like inflammatory bowel disease.
8 Ways to Control Kidney Stone Formation From Oxalates
If you are among the majority, you do not have to worry about kidney stone formation from high oxalate-containing foods. However, if you do have a predisposition for kidney stones or are unable to break down oxalates efficiently, there are ways to prevent calcium oxalate stone formation. The following are therapies for kidney stone prevention:
- Stay Hydrated!
Drinking adequate amount of fluids can reduce the risk of kidney stones by diluting waste products in the urine. Recommended fluid intake can range from 2 to 3.5L/day for kidney stone prevention and includes all liquids including coffee, tea, and fruit juices. The only fluids that should be avoided are tomato, grapefruit, and cranberry juices because of their high sodium or oxalate content. Soda intake should also be limited because it can increase the recurrence of kidney stones due to its acidifying effect in the urine. The perfect environment for kidney stones!
- Drink Lemon Water
Citric acid (lemon or lime juice) is a great addition to the diet to help reduce kidney stones. It acts by breaking up small stones and binding to calcium in the urine, both of which reduces the risk of calcium oxalate stone formation. Citric acid can also prevent small calcium oxalate crystals, already in the kidneys, from growing and massing together into larger stones.
To increase citric acid in your diet add 4 oz of fresh lemon or lime juice to water and drink throughout the day. This will provide you with the same pharmacological dose as taking a potassium citrate pill for kidney stone treatment.
Ironically, restricting calcium intake does not prevent kidney stone formation. In fact, high dietary calcium intake decreases the risk of kidney stones.
People prone to kidney stones should aim for 800-1,200 mg of calcium daily.
Calcium-rich foods like sardines, sesame seeds and a plant-based calcium supplement with no side effects such as kidney stones can be considered.
Some research suggests magnesium can lower the risk of stone formation, particularly in men. While more research is needed, as many as 68% of Americans don’t get enough magnesium – and some experts put that closer to 80%, so it may be beneficial to increase magnesium nonetheless.
Magnesium-rich foods include pumpkin seeds, spinach, Swiss chard, almonds, and cashews.
- Fish Oil
Omega-6 fatty acids are the main source of polyunsaturated fats in Western diets and are implicated in kidney stone formation. This is primarily because their breakdown product, prostaglandins, can increase excretion of calcium and oxalates. Supplementing with omega-3 fatty acids, can help offset this reaction and may decrease urinary calcium and oxalate.
Taking 1,200 mg per day of fish oil or eating whole foods rich in omega-3 fatty acids like wild salmon, sardines, walnuts and flaxseed oil may reduce the risk of kidney stones and have the additional benefits of being protective against heart disease and fight inflammation.
- Balance Fat-Soluble Vitamins
Fat soluble vitamins A, D, and K2 are essential in metabolizing calcium in our diets and blood. Deficiencies in vitamins A and K2, especially combined with an excess of vitamin D are all possible suspects potentially contributing to kidney stones.
Vitamins A and K2 are critical for balancing the calcium-absorbing effects of vitamin D and making sure the calcium from our diet gets deposited into our bones and not into our arteries. A vitamin D excess, relative to A and K2 can, therefore, be a risk factor in kidney stone formation with some studies showing that people exposed to higher levels of sunlight are at greater risk. However, studies on the effects of vitamin D supplementation on blood calcium or kidney stones formation have been inconsistent.
If supplementing with vitamin D3, make sure adequate levels of vitamins A and K2 are also taken. Food-based sources of vitamin A and K2 include organ meats like liver and full-fat dairy products (pastured butter, cheese). If choosing to meet your vitamin A needs by eating liver, it’s especially important that it be organic since the liver is the primary site of detoxification in the body.
- Support Gut Health
A healthy gut microbiome can have a protective effect on reducing the risk of kidney stone formation. Data suggest that the Oxalobacter formigenes bacteria is most effective in degrading oxalates in the gut (98% reduction).
This is still emerging research, but Oxalobacter formigenes look promising as a probiotic therapy for the treatment of hyperoxaluria (excessive urinary excretion of oxalate). However, probiotic supplements with this strain are not currently available, yet. And only one biopharmaceutical company (OxThera) is in the late stages of developing a product with this strain. So, until there is more research to confirm its safety and beneficial effects, we’ll have to wait on our final verdict for this one.
Other species like Lactobacillus and Bifidobacterium can also be protective, albeit less so (11–68%). Although more research needs to be done to confirm this beneficial effect as well, these species are easily available in supplemental form and are also found in some cultured dairy foods, such as yogurt and cottage cheese. If they are present, this will be advertised on the label.
What we do know is that in addition to the development of resistance, antibiotics affect the integrity of your gut microbiome and these bacteria are no exception.
So avoiding or limiting antibiotic use, whenever possible, and reducing refined sugar and processed foods that may also devastate your gut health is a strategy to keep in mind.
While research is yet to confirm the effectiveness of Lactobacillus and Bifidobacterium on degrading oxalates, probiotics have been shown to be an effective strategy to restore and improve dysbiotic microbiota (microbial imbalance).
You can accomplish this with daily intake of prebiotic and probiotic foods like kefir, yogurt, cottage cheese and kimchi, which can boost the healthy bacteria in your gut. And of course in supplementation form, if you see fit.
For those that do get kidney stones, it is common to develop urinary tract infections, where antibiotics are prescribed, which is something to keep in mind.
- Utilize Cooking Preparations Like Steaming
There is some evidence that limiting foods high in oxalates, such as spinach and nuts, may reduce your risk of kidney stones. But this approach was also found to be an ineffective intervention to reduce oxalate in urine. For most people, decreasing foods high oxalates like spinach will not protect against kidney stones and may result in a net decrease in micronutrients. If you are thinking of limiting foods high in oxalates, it’s best to discuss this with your physician before doing so.
What could be considered is the preparation of these high nutritional foods to minimize kidney stone formation. Cooking methods may reduce oxalate content. Some studies claim a reduction of up to 5-53% for steaming and 30-87% for boiling; however, other studies that evaluated the effect of blanching or boiling green leafy vegetables showed little to no decrease in oxalate content.
Putting all the research together, it’s reasonable to expect approximately 5-15% decrease in oxalate content from cooking a high-oxalate food. However, the loss of minerals caused by boiling will greatly lessen the food’s nutrient value. You’ll be better served by rotating greens with lower oxalate vegetables like kale, bok choy and broccoli with greens higher in oxalates, like spinach and beet greens.
The above therapies for kidney stone prevention may prevent calcium oxalate stone formation. Do you have any additional science-backed therapies you follow? Let us know in the comments below.
The Bottom Line on Oxalates and Bone Health
Eating bone-healthy foods like spinach and Swiss chard is part of a well-balanced and nutritious diet and should not be avoided because of their oxalate concentrations. The nutrient density in these foods (e.g., folate, magnesium, calcium, pro-vitamin A [beta-carotene], C and K1) far outweighs any concerns around oxalates and should be consumed in abundance!
If you have a history or predisposition to kidney stones, then consider discussing a low-oxalate diet with your physician, as one part of a holistic approach. But minimizing exposure does not necessarily mean omission of all these foods. Rotating high oxalate greens for lower oxalate greens, like bok choy and kale, and lightly steaming can be part of an approach that will maximize your nutrient density for optimal health.