What are oxalates?
Oxalates are salts that form when oxalic acid anions bind with cations (positively charged ions), like calcium (Ca2+) (and in smaller amounts, sodium, potassium, or magnesium) (1, 2, 3, 4).
They are naturally occurring metabolites found in both plants and humans and have antinutrient properties (3, 5).
Note: The terms oxalate and oxalic acid are often used interchangeably.
What do oxalates do?
In plants, oxalates play a role in calcium regulation, detoxification of heavy metals, and protection against herbivores (6, 7).
In humans, oxalates are a normal byproduct of metabolism (8).
Which foods contain oxalates?
Oxalate content varies widely based on cultivar (plant variety), growing conditions, time of harvest, and the methods used for measurement (9, 10).
The highest amounts are generally found in the leaves of plants, followed by the seeds and the stem (10).
The following table lists the oxalate content of common foods:
|Category||Food (100 g)||Description||Oxalate content (mg)|
|Beet greens||Raw||300-920 (10)|
|Beetroot||Boiled||48-675 (11, 12)|
|Beetroot||Raw||64-450 (10, 12)|
|Bell pepper||Raw||40 (11)|
|Broccoli||Raw||14-190 (11, 12)|
|Brussels sprouts||Raw||15-360 (11, 12)|
|Cabbage||Raw||0-125 (10, 11)|
|Carrot||Raw||44-500 (11, 12)|
|Collard greens||Raw||450 (11)|
|Green beans||Raw||38 (11)|
|Lettuce||Raw||5-330 (10, 11)|
|Parsley||Raw||140-1700 (10, 11)|
|Potato||Raw||(20-141) (10, 12)|
|Rhubarb||Raw||532 – 800 (11, 12)|
|Spinach||Raw||320-1260 (10, 11)|
|Swiss chard||Raw||964-1167 (12)|
|Tomato||Raw||5-50 (10, 11)|
|Turnip greens||Raw||50 (11)|
|Lime & lemon peel||Raw||83-110 (11)|
|Cashews||Roasted||231-263 (x, 13)|
|Macadamia nuts||Raw||40-43 (13)|
|Pistachio nuts||Roasted||46-51 (13)|
|Soy flour||Raw||183 (11)|
|Grains||Barley flour||56 (11)|
|Brown rice flour||37 (11)|
|Buckwheat flour||269 (11)|
|Corn meal||54 (11)|
|Enriched wheat flour||40 (11)|
|Rye flour||51 (11)|
|Wheat germ||269 (11)|
|Whole wheat flour||67 (11)|
|Other||Black pepper||419 (11)|
|Cocoa powder||500-900 (10)|
|Dark chocolate||155-585 (13)|
*Higher in black tea than green or herbal tea (14)
How are oxalates processed by the body?
1. Dietary oxalate absorption
As much as half of the oxalate found in the body comes from food (15).
Absorption occurs throughout the GI tract, but mainly in the small intestine, via both paracellular (between cells) and transcellular (through cells) uptake (16).
Free oxalic acid (not bound to any minerals) is soluble, making it more readily absorbed than oxalate that has become insoluble by binding to minerals (17, 18).
Factors that increase oxalate absorption include the following:
- Fat malabsorption (fatty acids in the colon increases oxalate absorption and competitively binds to calcium, increasing the amount of free oxalate available) (19, 20)
Factors that decrease oxalate absorption include the following:
- High calcium intake (calcium binds to oxalic acid, decreasing its absorption) (21)
- High magnesium intake (magnesium binds to oxalic acid, decreasing its absorption) (22)
- Oxalate-degrading bacteria in the gut (Oxalobacter formigenes) (23)
Any unabsorbed oxalate from food is excreted in the feces (16, 23).
2. Endogenous oxalate synthesis
At least half of the oxalate in circulation is synthesized in the liver as a byproduct of metabolism (15, 24).
It can be formed from the breakdown of other compounds, such as ascorbate (vitamin C) or glyoxalate (derived from amino acids like glycine and hydroxyproline) (10, 24, 25, 26, 27).
Because the human body is incapable of metabolizing it, newly formed oxalate that enters circulation is simply filtered out by the kidneys and excreted in the urine (28, 29).
There is also some evidence that the small intestine and the proximal colon have the ability to secrete oxalate, which may be expelled in the feces, but the overall significance of this finding is not clear (16).
Are there tests to measure oxalate levels in humans?
Yes, there are several different methods.
1. Urinary oxalate
The most commonly used test is a 24-hour urine collection to evaluate urinary oxalate excretion (30).
Normal levels range from 20-40 mg oxalate excreted per 24 hours. Higher levels indicate hyperoxaluria (excess oxalate) (7, 11).
2. Urinary oxalate metabolites
Glyceric acid and glycolic acid are metabolites of oxalate that can be measured in the urine and used to detect inherited metabolic disorders such as primary hyperoxaluria, in which excess oxalate is produced within the body (31, 32).
They are included (along with oxalic acid) in the organic acids test from Great Plains Laboratory.
3. Plasma oxalate
In patients with kidney disease, urinary oxalate excretion decreases and becomes less accurate as GFR decreases, so plasma oxalate levels are often used instead (27).
Normal levels range from 1-5 μmol/L, while patients with hyperoxaluria can reach levels as high as 80 μmol/L (27, 33).
How do oxalates negatively impact health?
1. Can impair mineral absorption.
When oxalic acid is present in the gut alongside calcium or magnesium, they can bind to form calcium oxalate and magnesium oxalate (34, 35).
Calcium oxalate and magnesium oxalate are very poorly absorbed, reducing the bioavailability of calcium and magnesium from food and supplements (36).
Calcium and magnesium found in foods that naturally contain oxalates (like spinach) are only partially available for absorption. One study found that only 35% of the calcium in spinach was absorbed in calcium-deficient rats (34).
These decreases in mineral absorption may be even more significant when a high fiber diet is consumed, since fiber can also bind minerals and reduce absorption (37, 38, 39).
Although some websites claim that iron and zinc absorption are also inhibited by oxalates, the research does not support this (40, 41, 42, 43)
2. Promotes the formation of kidney stones.
Most kidney stones are made of calcium oxalate crystals that form when the urine contains more calcium oxalate than the fluid in the urine is able to dilute (44).
As urinary oxalate levels increase, the likelihood of kidney stone formation increases, because there is more oxalate available to bind with calcium (45).
3. Can be deposited in organs and other tissues.
Oxalate crystals have been found in a variety of tissues throughout the body, including the kidneys, heart, thyroid, bones, joints, eyes, and skin (46, 47)
Over time, the accumulation of these deposits can lead to pain and even organ dysfunction (47).
This occurs most often in patients with primary hyperoxaluria, but can also occur in people who have oxalate metabolism disorders and poor kidney function, since the body cannot effectively eliminate the high levels of oxalates (27).
4. May play a role in autism spectrum disorders (ASD)?
There is very weak evidence that oxalates may be involved in the pathogenesis of ASD.
This is based on one study, which found that plasma oxalate levels were three times higher in children with ASD compared with controls (48, 49).
However, it is unclear whether this increase is due to poor renal clearance, increased absorption, or both.
It is also unclear whether higher levels of oxalates are clinically significant in the etiology of ASD.
5. May increase pain in vulvodynia.
There is also very weak evidence that oxalates may aggravate vulvodynia (vulvar pain syndrome).
One study found that 24% of women with vulvodynia who followed a low oxalate diet and took 400 mg of calcium citrate 3x per day, for 3 months, experienced decreased pain levels (50).
However, other studies have failed to find any correlation (51, 52).
Do oxalates have any health benefits?
There are no known health benefits in humans (7).
What is a low-oxalate diet?
There is no universal definition for what constitutes a low-oxalate diet, but <100 mg/day is generally considered to be a reasonable goal (53, 54).
A stricter 50 mg/day limit is also sometimes used and can reduce urinary oxalate excretion by 25 to 30% (55, 56).
Because it’s so difficult to know exactly how much oxalate is in certain foods, a simpler recommendation is to avoid or limit high-oxalate foods (57).
What is the best way to reduce oxalates in food?
Boiling oxalate-rich vegetables reduces the oxalate content by 30-87% (as long as the cooking water is not consumed), while steaming reduces by 5-53% (12, 58).
Baking is the least effective method, eliminating 0-15% of oxalates (12, 58).
What are some strategies for reducing oxalate levels in the body?
1. Ensure adequate calcium intake.
In the intestines, calcium binds with oxalate, reducing its absorption and causing it to be excreted in the feces (59).
Strong evidence shows that both dietary calcium and calcium supplements (at least 500 mg/day) significantly reduce urinary oxalate levels (59, 60, 61, 62, 63, 64, 65, 66).
2. Stay hydrated.
While drinking water doesn’t technically reduce oxalate levels, it does dilute the urine and decrease the likelihood that calcium oxalate crystals will form kidney stones.
It is recommended to drink enough water to produce 2.5 liters of urine per day (67). Clear or pale yellow urine is a good sign of adequate hydration.
3. Avoid excess vitamin C intake.
A large body of research indicates that large doses of supplemental vitamin C (500-2000 mg/day) significantly increase urinary oxalate levels in both healthy subjects and those with a history of kidney stones, resulting in an increased risk of stone formation (68, 69, 70, 71, 72, 73).
More research is needed to fully explain why this happens, but we do know that oxalate is formed from the breakdown of the oxidized form of ascorbic acid (74).
4. Ensure adequate magnesium intake.
Research shows that supplementation with various forms of magnesium (250-300 mg/day, with food) significantly lowers oxalate absorption and urinary oxalate levels, although calcium tends to be more effective (22, 75, 76).
5. Ensure adequate vitamin b6 intake.
Some studies show that vitamin B6 (in doses ranging from 10-500 mg/day) may decrease urinary oxalate excretion by decreasing endogenous oxalate production, although other studies have found conflicting results (45, 77, 78, 79, 80, 81).
6. Avoid gelatin and collagen supplements
Very limited evidence suggests that gelatin consumption (5-10 grams per day) significantly increases urinary oxalate excretion in healthy subjects (26).
It is thought that this might occur because hydroxyproline, an amino acid found in collagen and gelatin, is a precursor to glyoxylate, which can be converted to oxalate in the liver (26).
7. Consider a fish oil supplement.
Limited evidence suggests that fish oil supplementation (900 mg EPA + 600 mg DHA/day) alters oxalate transporter activity, leading to a decrease in urinary oxalate excretion, except in patients following very low-oxalate diets (<50 mg/day) (56, 82, 83, 84)
8. Optimize gut microbiota.
Certain strains of bacteria produce enzymes that degrade oxalates in the gut, preventing them from being absorbed (85).
The most studied of these strains is Oxalobacter formigenes, which reduces urinary oxalate levels in animal studies, although the research from clinical trials is conflicting (86, 87, 88, 89, 90, 91, 92).
Currently, there are no commercially available Oxalobacter probiotics, but a formulation called “Oxabact” is being developed and might be available in the future.
Are there supplements to help degrade oxalates?
“Nephure” is a supplement designed to degrade oxalate in food and is sometimes used in addition to a low-oxalate diet (93).
It contains an enzyme found naturally in fungi, called oxalate decarboxylase, which is responsible for breaking down oxalate into carbon dioxide and formate (93).
A clinical trial evaluating the effects of Nephure on oxalate excretion was performed in 2018, but the results were never published in a peer-reviewed journal (94).
Which conditions might benefit from a low-oxalate diet?
Any patients with primary (inherited) or secondary (caused by diet or other factors) hyperoxaluria might benefit from a low-oxalate diet as one of many strategies to reduce oxalate levels (27, 95).
2. Nephrolithiasis (kidney stones)
It is recommended that patients with a history of calcium-oxalate kidney stones limit dietary oxalate only if hyperoxaluria is also present (96).
In one study, 24% of women with vulvodynia who followed a low-oxalate diet experienced decreased pain levels, although other studies found conflicting results (50, 51, 52).
Although some websites promote low oxalate diets for autism, fibromyalgia, candida, or cystic fibrosis, there is not enough evidence to support these recommendations at this time.