Cause for concern?
The most common manifestation of kidney stones is without a doubt, calcium oxalate stones – 80% of stones, to be exact. For this reason, people who are prone to stones tend to avoid high oxalate-containing foods. The most popular oxalate-rich delicacies: spinach, beets greens, chard, chocolate, tea, nuts, grains, and yes, some mushrooms. It is important to keep in mind that not all oxalate-rich foods will lead to kidney stones - chemistry is much more complex than that.
There was a recent report about an older Japanese woman with liver cancer who consumed 4-5 tsp of chaga mushroom per day for 6 months, and happened to get nephrotoxicity (1). The postulated culprit: chaga mushroom. Unfortunately, in this study, there was neither mention of the source of the chaga, if this was the canker or mycelium, nor if the chaga powder was an extract or not. This case report concluded that it must have been the high oxalate content in the chaga mushroom that induced this nephrotoxicity. Is this a true cause or simply a correlation? Should consumers be concerned about the oxalate content in in all chaga products?
My hope is that this post will be educational, and readers will walk away with some answers to these questions.
This report got me thinking, about chaga and oxalates and wondering, could this really be? I mean, they do look so similar. Could this be an example of the doctrine of signatures or is it possible that if you crush any golden substance into small enough pieces it may resemble calcium oxalate fragments? (No offense to the doctrine of signatures)
Some education about oxalates
First, not all oxalates are created equal. There are soluble and insoluble oxalates. Insoluble oxalates are already bound to minerals, for example: calcium oxalate, magnesium oxalate and iron oxalate. The insoluble oxalates in foods pass right into the GI and come out in feces. They will not be absorbed in the blood stream, and so they are not a concern for causing hyperoxaluria (high urine oxalates). Examples of soluble oxalates are potassium and sodium oxalate. Unlike soluble oxalates, these acids release free oxalate anions which can pass into the blood stream. Free oxalate can then bind with free calcium and make calcium oxalate crystals.
This distinction is important to keep in mind when considering high oxalate foods.
Mushrooms and Oxalates
White button mushroom and shiitake mushroom are both very high in oxalates, but 99% of them are insoluble! So, don’t fret about these mushrooms – the oxalate will all come out in your shiit(ake) (2).
Oyster mushrooms, on the other hand, are moderately high in oxalates, and 90% of them are soluble. They contain a comparable amount to chocolate, almonds, and grains (2). So if you can eat more oyster mushrooms than you can chocolate, you are my hero and you may want to eat them with some calcium containing foods – more about this later.
Lucky for kidney stone formers, they can continue to consume large quantities of Lion’s Mane (Hericium erinaceus), cauliflower mushroom (Sparassis spp) and reishi (Ganoderma spp). There are no soluble oxalates found in any of these mushrooms (2).
Remember, for an organism to take the time and energy to make a molecule, the molecule must serve some purpose for that organism; oxalates are not made to cause kidney stones in humans. I hope to find more on this subject in relation to mushrooms, but for now, I found one study. According to this study, calcium oxalate crystals form in response to toxic metal stress (3). So perhaps mushrooms growing in environments where there are more toxic metals will in turn have a higher calcium oxalate load than those same species growing elsewhere.
Oxalates and Chaga
Oxalate samples were assessed in chaga samples from Russia, Finland and Thailand. A hot water extract was made of the chaga canker (the growth coming out of the tree, traditionally used as medicine) and oxalate content was assessed. Researchers found that for 1 gram of the Russian chaga extract there was 97.6mg of soluble oxalic acid and 24mg of insoluble oxalic acid. The Finnish chaga extract contained 55.62mg/g of soluble oxalates and 9.5mg/g of insoluble oxalates. Japan Food Research Laboratories measured oxalate content of the mushroom powder and concentrations of 11.2g total oxalate in 100g of powder. Another test from this laboratory on a separate chaga sample was 2.8g/100g (1).
Some perspective: These amounts are higher than most edible mushrooms, similar to the amount in almonds, peanuts, cereal grains and chocolate, and much lower than what is found in high oxalate foods like spinach, rhubarb and beet greens (4).
Cooking and Oxalates
Oxalate load was compared in a 10g sample of dried mushrooms cooked versus not cooked. This study found that cooking slightly decreases oxalates, but also seems to increase them in certain cases. Interestingly, levels of soluble oxalate from forest harvested mushrooms was generally lower than cultivated mushroom (2).
Concerned about your mushroom intake and risk of kidney stones?
Here is some food for thought:
Calcium intake and oxalate absorption
People with low calcium diets can benefit from consuming calcium-containing foods along with oxalate-containing foods to decrease oxalate absorption. One study found that exogenous (dietary) calcium leads to a linear reduction of oxalate absorption within the range of daily intake of 200-1200mg of calcium. This case has been documented in numerous studies (5,6,7) and calcium supplements have been shown to lower the oxalate absorption. One study assessed this theory using 1000mg calcium supplement given simultaneously with soluble oxalates. They concluded that this technique will reduce oxalate absorption by 10%, however this is mostly effective if the person already has a low calcium diet. When calcium is supplemented to an already high calcium diet, there is only a 1% decrease in oxalate absorption.
Perhaps this is an excellent reason to add milk to tea and coffee and of course, to your favorite mushroom drink. To give some perspective, 1 cup of dairy milk has about 305mg of calcium and 1 cup of fortified almond milk has about 300mg of calcium.
The microbiome and oxalate absorption
Within the microbiome resides a bacteria known as Oxalobacter formigenes (8) This superlative gut bacteria degrades oxalates and helps to prevent hyperoxaluria and kidney stones. So, for those concerned about the oxalate content in the mushrooms discussed, perhaps this little bacteria is your ally. Studies looking at oral intake of Oxalobacter have shown less urinary oxalate excretion following administration of an oxalate heavy food load compared to when there was no Oxalobacter administered. Unfortunately, this bacteria is susceptible to antibiotic use. Specifically, clarithromycin and doxycycline have been shown to annihilate this little probiotic, leaving the human left behind to be more susceptible to kidney stones.
Representation of Oxalobacter formigenes activity. Utilizes oxalate as a source of energy through the oxalate-formate antiporter (9).
This is all very interesting and something most people do not need to worry about. Though I would say, if you are an oxalate kidney stone former: don’t eat a ton of oyster mushrooms or drink multiple cups of chaga tea per day. Do make sure to drink plenty of water and eat a diet with adequate amounts of calcium, and hope that antibiotic use hasn’t eradicated all of your Oxalobacter.
Work Cited
Kikuchi Y, Seta K, Ogawa Y, et al. Chaga mushroom-induced oxalate nephropathy. Clin Nephrol. 2014;81(6):440-444. doi:10.5414/CN107655
Savage GP, Nilzen V, Österberg K, Vanhanen L. Soluble and insoluble oxalate content of mushrooms. Int J Food Sci Nutr. 2002;53(4):293-296.doi:10.1080/09637480120057000
Jarosz-Wilkolazka A, Gadd GM. Oxalate production by wood-rotting fungi growing in toxic metal-amended medium. Chemosphere. 2003;52(3):541-547. doi:10.1016/S0045-6535(03)00235-2
Glamočlija J, Ćirić A, Nikolić M, et al. Chemical characterization and biological activity of Chaga (Inonotus obliquus), a medicinal “mushroom.” J Ethnopharmacol.2015;162:323-332. doi:10.1016/j.jep.2014.12.069
Von Unruh GE, Voss S, Sauerbruch T, Hesse A. Dependence of oxalate absorption on the daily calcium intake. J Am Soc Nephrol. 2004;15(6):1567-1573.doi:10.1097/01.ASN.0000127864.26968.7F
Bong WC, Vanhanen LP, Savage GP. Addition of calcium compounds to reduce soluble oxalate in a high oxalate food system. Food Chem. 2017;221:54-57. doi:10.1016/j.foodchem.2016.10.031
Brogren M, Savage GP. Bioavailability of soluble oxalate from spinach eaten with and without milk products. Asia Pac J Clin Nutr. 2003;12(2):219-224.
Duncan SH, Richardson AJ, Kaul P, Holmes RP, Allison MJ, Stewart CS. Oxalobacter formigenes and its potential role in human health. Appl Environ Microbiol.2002;68(8):3841-3847. doi:10.1128/AEM.68.8.3841-3847.2002
Corica, Domenico & Romano, Claudio. (2015). Renal Involvement in Inflammatory Bowel Diseases. Journal of Crohn's & colitis. 10. 10.1093/ecco-jcc/jjv138.
Oxalates in Chaga – A Potential Health Threat By Michael W. Beug, Chair NAMA Toxicology Committee
Mitchell T, Kumar P, Reddy T, et al. Dietary oxalate and kidney stone formation. Am J Physiol Renal Physiol. 2019;316(3):F409–F413. doi:10.1152/ajprenal.00373.2018
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