The most important thing about wood ear happens during soaking. Dried wood ear is not necessarily toxic, but if it soaks too long at room temperature, bacteria in the soaking water can make bongkrekic acid, a toxin that cooking does not destroy and that can kill. So soak it briefly in cold water and cook it right away, and do not eat wood ear that soaked overnight at room temperature. If you vomit, have belly pain, or feel dizzy or weak after eating wood ear, go to the emergency department now and tell the doctor you ate wood ear. The chapter on why over-soaking can poison you explains how this toxin does its damage.
Wood ear is a fungus, in the same kingdom as mushrooms, not a plant. Once soaked it is mostly water: a low-calorie dish with a pleasant bite. The popular claim that wood ear builds iron and blood does not hold up.
Background · What wood ear is and how it is eaten
Wood ear (also called cloud ear; genus Auricularia) is a fungus. It belongs to the same kingdom as mushrooms, not to the plant kingdom. It grows on dead wood. China is its main producer and market, and dried wood ear sits in almost every household pantry.
It shows up in two common roles:
The star of cold dishes and stir-fries: wood ear salad, wood ear with pork, and the black shreds in fish-fragrant pork (yu xiang rou si)A dried good that must be soaked: dried wood ear has to be rehydrated first, and that soaking step hides a food-safety risk that is very easy to underestimate, bongkrekic acid poisoning For nutrition, split dry from wet before reading any number: dried wood ear takes up about 10 times its weight in water when soaked, so whatever looks plentiful by dry weight has to be divided by ten once it reaches your bowl. The saying that wood ear builds iron and blood is extremely widespread, but by the usual food-composition data, soaked wood ear holds little iron, and it is the poorly absorbed non-heme kind. The chapter on whether wood ear supplies iron takes that apart.
Chapter 2
Dense when dry, mostly water soaked
Wood ear numbers must split dry from wet. Dried wood ear has about 284 kcal per 100 g. Soaking makes it take up about 10 times its weight in water, so 100 g of soaked wood ear has only about 25–30 kcal.
Most of the carbohydrate in the dry weight is dietary fiber, which the small intestine can barely digest. It takes the gut-bacteria fermentation route, not the blood-sugar route. So a plate of wood ear salad gives you texture, fullness and very few calories. It is neither a protein source nor a calorie source; it is a refreshing dish you can eat freely.
A CLOSER LOOK
Soaking adds water: compare like weights
This illustrates the weighing basis, without encoding nutrition by area or count; dry-weight and hydrated-weight tables are not directly comparable.
1The dry-weight denominator
Per-100-gram figures for dried wood ear describe concentrated solids such as fibre.
2Water joins the total weight
After soaking, water occupies more weight, so the same hundred grams contains less dry matter.
Illustration for understanding; not to scale. Saved figures include explanations and sources.
Numbers · What is left dry versus soaked
By dry weight, 100 g of dried wood ear has about 9.3 g of protein, about 73 g of carbohydrate and only about 0.7 g of fat. Of that 73 g of carbohydrate, 70 g is dietary fiber, so almost none of it is sugar the small intestine can digest and absorb. Dried wood ear looks calorie-dense, but most of those calories are locked in fiber.
Once soaked, protein and fiber are diluted to about a tenth. A plate of wood ear salad uses about 100–150 g of soaked wood ear, and what it gives you is texture and fullness, not macronutrients.
Wood ear feels slippery because it contains soluble polysaccharides (glucans), in the same soluble-fiber family as the beta-glucan in oats (for the mechanism, see Oats and see Carbs & Fiber).
Chapter 3
The soluble fiber in wood ear
Wood ear has almost no fat (about 0.7 g per 100 g dry weight). What is worth talking about is its soluble polysaccharides: they form a gel in water and are fermented by bacteria in the gut, in the same soluble-fiber family as the beta-glucan in oats.
But the limits need saying. Wood ear polysaccharides have been studied in people far less than oat beta-glucan, and claims that they fight cancer or dissolve blood clots come from cell and animal experiments, with no conclusion in humans. Think of wood ear as a tasty, low-burden dish with a bit of soluble fiber. It is not a blood-vessel cleaner, and it is not an anti-cancer food.
Evidence · What wood ear polysaccharide shows in people
In terms of mechanism, soluble fiber is linked to several things: lower cholesterol, steadier blood sugar after meals, and food for gut bacteria (oat beta-glucan is where that story is best worked out). Wood ear polysaccharides belong to the same family, so the direction makes sense.
In people, though, the evidence is far apart:
Oat beta-glucan is backed by human trials and already carries an approved health claim (see Oats); wood ear polysaccharides have been studied in people much lessMarketing that says wood ear polysaccharides fight cancer or dissolve clots is extrapolated from lab-dish and animal experiments, with no conclusion in humans; it comes apart the same way as every fungal-polysaccharide product So the soluble fiber is real; the dose and the strength of the evidence are another matter. For fiber as a whole, see Carbs & Fiber and oats.
Chapter 4
Does wood ear supply iron
Wood ear's biggest nutrition myth is that wood ear builds iron and blood. The claim has three layers, and none of them holds:
Layer one · the number itself is disputed: two national food composition tables measured the same dried wood ear and got iron values 17 times apart, so the claim does not even have a settled starting pointLayer two · the dried food is not what you eat: dried wood ear takes up about 10 times its weight in water when soaked, so the plate in your bowl corresponds to only a tenth as much dried wood ear, and any dry-weight number has to be divided by 10 before you read itLayer three · eating is not absorbing: the iron in wood ear is non-heme iron, absorbed far less well than the heme iron in meat (mechanism: see Iron); wood ear also carries oxalate and phytate, which grab the iron and cut absorption further If you want iron, red meat, liver and animal blood are the well-absorbed choices. Wood ear is a textbook case of looking iron-rich while delivering little that is absorbed (the same kind of takedown: see Spinach). What it really contributes is fiber and texture.
Numbers · why the two tables do not match
The two tables differ this much, and it is worth a look on its own, because the disagreement is not across the board:
China's food composition tables give dried black wood ear 97.4 mg of iron per 100 g; USDA measured the same dried fungus at 5.88 mg, a 17-fold gapIn the same pair of entries, potassium matches almost exactly (757 vs 754 mg), and carbohydrate is close tooSelenium differs 12-fold the other way (3.7 vs 43.4 µg) The items that agree show this is not a unit or conversion error: a conversion error would push every item in the same direction by the same multiple. The disagreement sits only on particular minerals, which looks more like a difference in sample source and analytical method (wood ear grows on dead wood, and drying easily picks up soil dust).
Which figure you trust sets the starting point of the iron claim, but layers two and three hold whichever number you pick: after soaking, every 100 g corresponds to only about 10 g of dried wood ear, and non-heme iron is not absorbed any better because a table printed a larger number.
Keep the general lesson too: a number in a composition table is one measurement, not a constant. If two national tables can differ 17-fold on the same food, a lone figure quoted by a marketing account deserves the question even more: where did it come from?
Two claims with the same structure can be blocked here as well: wood ear does not build collagen (see Collagen & Bone Broth), and it does not clean the lungs, which the chapter on whether wood ear adds iron or cleans the lungs takes apart.
Chapter 5
What it lacks · how to pair
Wood ear lacks protein, fat, vitamin C and calcium: once soaked it is almost only water and soluble fiber, so the other foods on the plate have to fill those gaps. The easiest ways are wood ear stir-fried with meat, or wood ear salad with egg shreds or tofu.
One caution is the same as for spinach: wood ear contains a little oxalate. If you are prone to kidney stones, do not pile it into the same meal as spinach, amaranth and other high-oxalate vegetables (for the oxalate mechanism, see Spinach).
In practice · What to put on the plate with wood ear
A few practical pairings:
With meat (wood ear stir-fried with pork): this adds complete protein while wood ear brings texture and fiber; it is the most sensible everyday combinationWith egg or tofu: also adds protein; wood ear salad with egg shreds makes a complete corner of a mealWith vitamin C-rich vegetables and fruit: vitamin C helps the meal's non-heme iron get absorbed, although wood ear has little iron to begin with, so this pairing adds only a little For balancing the whole plate, see Fruit & vegetables and Carbs & Fiber.
Chapter 6
Why over-soaking can poison you
Bongkrekic acid is a toxin that a bacterium makes in soaking water. Dried wood ear is not necessarily contaminated; the risk lies in soaking wood ear and snow fungus (tremella). Left too long at room temperature, the bacteria have time to multiply and make the toxin. Fatal poisonings have been reported many times.
Three things make it hard to fight:
It is heat-stable: boiling does not destroy it, so cooking it through makes it safe does not applyIt attacks mitochondria: it jams the door through which the cell's power plant ships energy out. The most energy-hungry organs, the liver, brain and kidneys, run into trouble first, and the illness moves fastIt has no color, taste or smell: you cannot see it or smell it So, three habits: soak briefly in cold water and cook right away; if it has to soak a long time, soak it in the fridge; throw it out if it smells off, feels slimy or looks odd, but remember that no smell does not mean safe. Snow fungus carries the same bacterium and the same toxin, and cold rice-sheet noodles (liangpi) and wet rice noodles carry a similar risk (see Foodborne Illness).
If you feel dizzy, vomit or feel weak after eating it, seek medical care immediately.
Mechanism · which door it welds shut
First, the name: the organism that makes the toxin is Burkholderia gladioli pathovar cocovenenans (formerly called Pseudomonas cocovenenans). It grows and makes toxin when conditions are warm, nutritious and wet, and a basin of water used to soak wood ear supplies all three.
Why this toxin is so hard to fight becomes clear once you see where it strikes.
Every cell in your body runs on mitochondria. A mitochondrion is like a small power plant, and its inner membrane has a dedicated swap door: spent batteries (ADP) go in through this door to be recharged, and charged batteries () come out through the same door for the cell to use. The door turns without a pause, and the whole body's energy supply hangs on it.
Bongkrekic acid does only one thing: it welds that door shut. Once the door is closed, energy already made inside the plant cannot get out, and the places waiting outside cannot swap in a fresh charge. The plant is still there; it has just lost contact with the whole body it serves.
Three things then make sense:
Why it moves so fast: it does not slowly wear organs down; it cuts the power on the spot. The first to fail are the most energy-hungry organs, the liver, brain and kidneys, which is why the signs of poisoning cluster there from the startWhy boiling does not help: bongkrekic acid is a molecule, not a living bacterium. High heat can kill bacteria still making toxin, but it cannot change toxin already made. That is what heat-stable really means. Once the toxin is there, that basin cannot be savedWhy there is no specific antidote: an antidote usually works either by occupying the site the toxin is heading for, or by giving the body a faster way to clear it. For bongkrekic acid, neither route works at present, so treatment can only be supportive So soaking gets this much attention not because it is complicated, but because it is the one link that most needs holding. And if you have already eaten it and feel unwell, there is only one correct move: seek care immediately. Do not wait for it to pass on its own.
Chapter 7
Choosing, soaking and how much
When buying dried wood ear, pick pieces that are evenly dark, grayish-white on the back, dry and free of any moldy smell. Soak it in cold water for as short a time as you can and cook it as soon as it is ready; if it has to soak a long time, soak it in the fridge, and do not soak it for hours in warm or hot water. Refrigerate any extra and finish it the same day. Cook it thoroughly by stir-frying or blanching: heat kills the bacteria but cannot remove toxin already made, so the protection comes from the soaking step.
Soaked wood ear is low in calories and has a pleasant bite; 100–200 g (soaked weight) per meal is fine. If you are prone to kidney stones, keep oxalate in mind. Wood ear that soaked too long or has spoiled is not for anyone.
In practice · Buying, soaking and cooking in detail
Buying: take another look after soaking, and throw out anything that smells off, feels slimy or has turned mushy. No smell does not mean safe, because bongkrekic acid itself has no color, taste or smell.
Fresh wood ear: fresh wood ear contains porphyrin-type light-sensitizing compounds, and a very small number of people get a sun-triggered skin rash (solar dermatitis) after eating it and going out in the sun. Fresh wood ear also has to be cleaned properly. But the main safety risk is still in soaking dried wood ear, which is the more common situation and the one that most needs rules.
Soaking:
At room temperature, use cold water, keep the soak as short as you can, and cook it at the next mealIf it must soak overnight, soak it in the refrigeratorDo not soak it for long in warm or hot water; warmth is exactly what that bacterium likes Cooking: heat it thoroughly, whether by stir-frying, blanching or boiling. Bongkrekic acid is heat-stable, so the protection has to happen at the soaking stage; heating still kills the bacteria themselves.
Who should be careful: people prone to kidney stones (wood ear contains oxalate), and people with sensitive digestion, who should watch how much fiber they take in. Wood ear holds no major nutrition, so do not expect it to be a tonic. For personal medical questions, ask a doctor.
Chapter 8
Can wood ear add iron or clean lungs
Three claims about wood ear:
Claim one: wood ear builds iron and blood. All three layers fail: the two composition tables already disagree by an order of magnitude on dry-weight iron; soaking dilutes it about 10 times more by wet weight; and non-heme iron is poorly absorbed to begin with, with oxalate and phytate holding it back further. For iron, choose red meat, liver or animal blood.
Claim two: wood ear clears the lungs, prevents dust lung disease (pneumoconiosis) and pulls dirt out of the lungs. There is no anatomical basis for this. Food goes down the esophagus into the stomach, while the lungs connect to the windpipe, and after the two tubes split in the throat they never meet again, so food has no route into the lungs to pick up dust. Eating wood ear on a smoggy day can be a comfort, but there is no mechanism (these lung-clearing foods are close relatives of detox claims; for the takedown, see Detox & Cleanse).
Claim three: wood ear moves blood and breaks up stasis, so people with clots should eat more. The human evidence is thin. If you take an anticoagulant (warfarin and the like), eating wood ear as an ordinary cooked dish is fine; ask your doctor before taking a wood ear polysaccharide supplement.
Three questions sort out claims like these: Is there human evidence? Is the dose enough? Does it interact with a drug you take?
Clinical · Wood ear when you take blood thinners
Moving blood and breaking up stasis are concepts from traditional Chinese medicine; this page covers only the limits of the mechanism.
Wood ear polysaccharides do show activity related to blood clotting in vitro (in a lab dish). But between active in a lab dish and useful when a person swallows it sit three gates: the gut has to let it through, it has to reach an effective concentration in the blood, and it has to stay there long enough. For now the human evidence is thin and does not support using it as a treatment.
What really needs care is the other direction: if you take an anticoagulant (warfarin and the like) and casually add a high-dose wood ear polysaccharide product, you may stack extra bleeding risk on top of the drug. That is not added therapy. Anticoagulant treatment already walks a tightrope between not clotting too fast and not bleeding without stopping, and your dose is one a doctor has adjusted again and again. Adding something from the side that pushes the same way, with an unstandardized amount that nobody has counted into your dose, pushes that wire to one side. It is the same logic as the nattokinase warning in the natto story.
References · 6
U.S. Department of Agriculture, Agricultural Research Service. (2019). FoodData Central: Fungi, Cloud ears, dried (SR Legacy, FDC ID 168581). Per 100 g dried: 284 kcal, 9.25 g protein, 0.73 g fat, 73.0 g carbohydrate of which 70.1 g is dietary fibre, 5.88 mg iron, 159 mg calcium, 754 mg potassium, 83 mg magnesium, 43.4 µg selenium. Note the iron figure — it is an order of magnitude below the value carried in Chinese food composition tables. fdc.nal.usda.gov
Chinese Center for Disease Control and Prevention, National Institute for Nutrition and Health. China Food Composition Tables (6th ed.), consulted through the official food-composition query platform, entry 579: dried black wood ear (Auricularia). Per 100 g dried: 12.1 g protein, 1.5 g fat, 65.6 g carbohydrate, 97.4 mg iron, 247 mg calcium, 3.18 mg zinc, 3.70 µg selenium, 757 mg potassium, 152 mg magnesium. Compare USDA SR Legacy FDC ID 168581 for the same food: the potassium figures agree almost exactly (757 vs 754 mg) but iron differs about 17-fold (97.4 vs 5.88 mg) and selenium about 12-fold in the opposite direction (3.70 vs 43.4 µg) — a nutrient-specific divergence, not a unit error. nlc.chinanutri.cn/fq/foodinfo/579.html
Hunt, J. R. (2003). Bioavailability of iron, zinc, and other trace minerals from vegetarian diets. The American Journal of Clinical Nutrition, 78(3 Suppl), 633S-639S. Heme iron is absorbed far more efficiently than non-heme iron and is less affected by dietary inhibitors. The abstract gives NO absorption percentages and does not itself state the heme vs non-heme comparison above: it says iron and zinc absorption is lower from vegetarian than non-vegetarian diets, vegetarians have lower iron stores, adverse effects have not been shown with varied vegetarian diets in developed countries, and routine iron supplementation has no demonstrated benefit. The heme 15-35% / non-heme 2-20% figures sometimes attached to this record are not in it (abstract, PMID 12936958). 10.1093/ajcn/78.3.633S
National Institutes of Health, Office of Dietary Supplements. (2024). Iron — Fact Sheet for Health Professionals. Fact sheet (updated September 4, 2025; Wayback snapshot 21 September 2026): RDAs 8 mg/day for men and for women 51+, 18 mg women 19-50, 27 mg pregnancy; UL 45 mg/day from age 14; bioavailability about 14%-18% from mixed diets with meat, seafood and vitamin C and 5%-12% from vegetarian diets; serum ferritin below 30 mcg/L suggests iron deficiency and below 10 mcg/L IDA, but inflammation can raise ferritin; supplemental iron of 45 mg/day or more may cause nausea and constipation; people with hereditary hemochromatosis are at risk of iron overload (fact sheet). Heme vs nonheme: heme iron (lean meat and seafood are the richest sources) has higher bioavailability than nonheme iron, and other dietary components affect it less; calcium might reduce the bioavailability of both forms; heme iron is about 10%-15% of total iron intake in western populations. The sheet gives no separate heme and nonheme absorption percentages (fact sheet, Wayback 2026 snapshot). ods.od.nih.gov/factsheets/Iron-HealthProfessional
Han, D., Chen, J., Chen, W., & Wang, Y. (2023). Bongkrekic acid and Burkholderia gladioli pathovar cocovenenans: Formidable foe and ascending threat to food safety. Foods, 12(21), 3926. BKA is described as a flavorless, odorless, colorless, thermally stable, highly unsaturated tricarboxylic acid. (The paper states thermal stability without specifying a temperature or duration.) 10.3390/foods12213926
Centre for Food Safety, Government of the Hong Kong SAR. (2024). Bongkrekic acid - uncommon but fatal toxin in certain foods. Food Safety Focus, Issue 213. Advises soaking wood ear and tremella under refrigeration if prolonged soaking is necessary. www.cfs.gov.hk/english/multimedia/multimedia_pub/multimedia_pub_fsf_213_01.html