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There’s a lot of nutritional advice out there to tell us certain foods are bad. Leafy greens seem like they should be the least of our worries! Chomp down on salads, juice them, or throw them in smoothies… the more the better, right?
Believe it or not, the presence of something called oxalates in leafy greens might make us think twice about how much raw kale, spinach, or chard we should consume in some cases.
What Is Oxalate?
All plants, including leafy greens, can’t run away from predators or animals that eat them. As a result, they develop a chemical self-defense. Two of these mechanisms are alkaloids, which give them a bitter taste, and oxalates, which give some leafy greens a chalky taste.
These substances can be toxic, or even lethal, at high doses. At low doses, they could be a form of hormesis, a fancy word for a little stress that makes the body stronger and healthier.
Oxalate is a metabolic byproduct found in both animals and plants. The human body can produce oxalate internally from substances like vitamin C, certain amino acids, and glyoxylic acid. Plant sources that are high in oxalate include seeds, tubers, dark leafy greens, some teas, and sadly, even chocolate.
Is Oxalate in Foods Really Problematic?
In vegetables, oxalates are often found already in complex with minerals such as calcium so it is unlikely to steal nutritional minerals from foods.
The mineral to oxalate ratios in these foods determine how problematic the oxalates can be. Most leafy greens are rich in minerals, but these minerals are often bound to oxalate so they become less bioavailable.
Many leafy greens contain high oxalates and low calcium levels, including (per 100 g):
- Spinach contains about 890 – 1100 mg per 3 1/3 cup raw, about 4 – 5 folds oxalate to calcium
- Rhubarb contains 275 – 1336 mg, about 8 – 9 fold oxalate to calcium
- Purslane contains 910 – 1679 mg, ~5 fold oxalate to calcium
- Beet leaves 300 – 450 mg, ~2.5 fold oxalate to calcium
Many tubers and seeds also contain high levels of oxalate, including:
- Beetroots 121 – 450 mg, ~5 fold oxalate to calcium
- Taro 278 – 574 mg, ~10 – 20 fold oxalate to calcium
- Sweet potatoes 470 mg, ~30 fold oxalate to calcium
- Sesame seeds 350 – 1750 mg, ~0.3 – 2 fold oxalate to calcium
- Cacao 500 – 800 mg, ~4 – 6 fold oxalate to calcium
Typically the human digestive system can absorb around 2 – 5% of the ingested oxalate if it is consumed with a meal. On the other hand as much as 12% of the oxalate can be absorbed in a high-oxalate food or beverage such as tea consumed on an empty stomach.
In people with impaired fat absorption in the gut, the fat in the gut can bind up calcium and make more free oxalates available for absorption. As a result, these people might absorb more than 30% of dietary oxalates. People with these conditions are especially at risk:
- Irritable bowel disease
- Celiac disease
- Short bowel syndrome
- Bariatric surgery
- Medications that block fat absorption, such as Oristat (Alli)
However, typically, only about 20% of oxalate in the body comes from foods. (More on the other 80% later.)
Like phytates, free oxalate is an antinutrient that can reduce mineral absorption. The negative charge of oxalate causes it to readily bind and crystallize with many important minerals that are present in positively charged forms. These include calcium, magnesium, and iron.
Ingested oxalate can bind up these minerals in the gut and cause them to get excreted with the stool rather than absorbed and used in the body.
It’s possible then that a high consumption of oxalate-containing foods could cause mineral deficiencies. This can be a problem. For example, calcium deficiency can contribute to osteopenia and osteoporosis. Iron deficiency can cause iron-deficient anemia.
Kidney Stone Formation
Oxalates in the body, either produced internally or absorbed from foods, can form dangerous oxalate crystals if mixed with minerals elsewhere in the body. Calcium salts and oxalates may both be excreted by the kidneys and cause kidney stones. In fact about 80% of kidney stones are made of calcium oxalate.
Pain and UTI-like Symptoms
People with high oxalate levels may not necessarily develop kidney stones, but oxalate salt crystals anywhere in the urinary tract can be problematic. In some women, it causes vulvodynia or vulvar pain. About 1 in 4 women with vulvodynia find that their symptoms significantly improve when they go on a low oxalate diet.
Symptoms of urinary tract infections such as frequent urination, bladder pain, and intense urge to urinate may also be caused by high oxalate levels in the urine. Interestingly, many women with recurrent UTIs also have high oxalate levels in their urine, which could also be because their antibiotic use kills off healthy bacteria.
Possible Toxicity in Large Doses
There are a few case reports showing that ingestion of 4-5 grams of oxalate can cause death in adults. In one case, a 56-year old man who consumed 16 glasses of black tea daily for years developed kidney failure. In another case report, a man died after consuming an estimated 6 – 8 grams of oxalate by consuming 500 grams of sorrel in a soup in one sitting.
May Contribute to Other Diseases
Dr. Isabella Wentz suspects that oxalate sensitivity may contribute to hypothyroidism in some people. It may also cause joint pain, pain in the body, and depression.
In addition, oxalate injection causes breast cancer in mice, and oxalate calcium salts are also found in breast cancer cells.
Should You Go on a Low Oxalate Diet?
Although oxalate can be toxic and dangerous, it’s the dose in the body and the dose that you absorb that make the poison.
The Food Factor
Foods that are high in oxalate levels tend to be delicious and nutritious. If you are healthy you probably only absorb a very small percentage of oxalate from foods and it is unlikely to be a problem. In these cases, enjoy leafy greens, wild greens, teas, and chocolate in reasonable amounts.
You may want to try a low oxalate diet if you struggle with any of these health conditions:
- Vulva pain or pain during intercourse
- Frequent urinary tract infections or frequent urination
- Kidney stones
- Joint pain or pain the body
- Health problems due to mineral deficiencies, such as osteopenia, osteoporosis, or iron deficiency anemia
In addition, you should be careful about oxalate levels if you have a digestive problem that hinders fat absorption. This is especially the case if you can see floating stool or fat in your stool.
If you are wondering whether you have high oxalate levels, it might be worth getting a urinary oxalate test through your doctor. In addition, organic acid tests (offered through laboratories like Genova or Great Plains) also include urinary oxalate levels.
Eating a low oxalate diet might be a good way to manage your levels if you are sensitive to it. Many (but not all) patients find significant relief in their symptoms when they go on a low oxalate diet.
If you have high oxalate levels and some symptoms of oxalate sensitivity, it might be better to avoid the high oxalate greens (such as spinach) and eat more of the lower oxalate, higher calcium vegetables, such as kale, instead.
Other Factors That Raise Oxalate Levels in the Body
Aside from leafy greens, chocolate, and other high oxalate foods, it is possible to have exposure from other causes, including:
- A yeast overgrowth, as some yeast strains can produce oxalate in the body
- Genetic conditions that increase oxalate production or reduce oxalate breakdowns
- Reduced good bacteria that break down oxalates in the gut
- Dehydration, which can increase oxalate levels in the urine and increase the odds of developing kidney stones
- Vitamin C supplementation as vitamin C can be converted into oxalates inside the body
- Low dietary minerals such as calcium, magnesium, and iron. These minerals bind to oxalate in the gut and prevent absorption.
Ways to Reduce Oxalate Levels in Foods
There are several ways to reduce oxalate in food:
- For seeds and tubers, soaking or blanch before eating. For legumes and grains, soaking, sprouting, or cooking will also help.
- Cooking and blanching leafy greens will significantly reduce its oxalate levels.
- Take a calcium supplement to reduce oxalate absorption in the gut.
Have you changed your diet to avoid oxalates? Has your health improved as a result? Please share!
- https://www.theatlantic.com/health/archive/2015/04/the-man-who-almost-died-from-drinking-tea/389706/; https://apjcn.nhri.org.tw/server/APJCN/8/1/64.pdf