Mushrooms

TL;DR. Not plants. Fungi are a separate kingdom, more closely related to animals than to plants, and their cell walls are made of chitin, the same material as insect shells. That makes mushrooms nutritionally distinctive: meaningful protein, B vitamins, potassium, and a unique fibre. The most useful practical fact is that mushrooms contain ergosterol and will make vitamin D when exposed to ultraviolet light, including sunlight on your windowsill, in quantities that are nutritionally significant. The most important safety fact is that a handful of deadly species look like edible ones and that app-based identification has killed people.

1. What they are

The fruiting body of a fungus. The organism itself is the mycelium, a network of fine threads through soil or wood; the mushroom is the reproductive structure it pushes up, equivalent to an apple on a tree.

That last row explains the price. Porcini, chanterelles, and truffles form partnerships with tree roots and will not fruit without the living tree, so they are foraged, seasonal, and expensive. Truffles can be semi-cultivated by inoculating tree seedlings, with a lag of 5 to 10 years and no guarantee.

2. Where they come from

Cultivation of Agaricus began in France in the seventeenth century, reportedly in the caves and quarries around Paris, which gave the "champignon de Paris" its name and provided the stable cool humid conditions the fungus wants. Shiitake cultivation on logs in China and Japan is far older, documented for around a thousand years.

Production: around 12 to 15 million tonnes a year, with China producing the large majority. The Netherlands, Poland, the US, and Ireland are significant in Agaricus.

3. How they are grown

Mushroom growing is unlike any other agriculture: no light required, no soil, no photosynthesis. Fungi are heterotrophs; they digest organic matter externally by secreting enzymes and absorbing the products, which is much closer to what an animal does than a plant.

The Agaricus process:

1. Compost preparation. Straw, poultry manure, and gypsum are composted in a controlled two-phase process, then pasteurised at around 60 °C to kill competitors while retaining beneficial thermophilic organisms.
2. Spawn. Sterile grain colonised with mycelium is mixed in.
3. Colonisation, 2 to 3 weeks in the dark at 24 to 25 °C.
4. Casing. A layer of peat and lime is applied, which is what triggers fruiting.
5. Fruiting, induced by dropping temperature and raising fresh air. Mushrooms appear in flushes every 7 to 10 days for 3 to 4 flushes.
6. Harvest, entirely by hand, twisting rather than cutting.

The whole cycle takes 10 to 12 weeks and happens in windowless climate-controlled rooms. Mushroom growing is a genuine circular-economy story: it converts agricultural waste into food, and the spent compost is sold as a soil improver.

Shiitake and oyster are grown on sterilised sawdust or straw blocks, or on inoculated hardwood logs for a slower, better-flavoured product. Oyster mushrooms will grow on used coffee grounds, which is the basis of several urban food ventures.

4. What is inside them

Per 100 g raw:

The distinctive features:

• B vitamins, particularly riboflavin, niacin, and pantothenic acid, at levels unusual for a low-calorie food.
• Selenium and copper, both often under-supplied in plant-heavy diets.
• Protein that is modest in absolute terms and high relative to the calorie content, with a reasonably complete amino acid profile.
• Beta-glucans, a distinctive soluble fibre found in fungal cell walls, which is the compound class behind most of the immune claims.
• Ergothioneine, an unusual sulphur-containing amino acid that humans have a dedicated transporter for (which is suggestive) and which mushrooms are essentially the only significant dietary source of. Its function in humans is not established.
• Glutamate and nucleotides, which is why mushrooms taste savoury and why dried shiitake and porcini are among the most powerful umami ingredients available. Drying concentrates them substantially, so a small quantity of dried mushroom outperforms a large quantity of fresh.

5. The vitamin D trick

In short: Mushrooms contain ergosterol, the fungal equivalent of the cholesterol precursor in your skin, and UV light converts it to vitamin D2.

Your skin makes vitamin D3 from 7-dehydrocholesterol under UVB. Mushrooms make vitamin D2 (ergocalciferol) from ergosterol under exactly the same wavelengths. Mushrooms grown in the dark, which is essentially all commercial mushrooms, contain almost none.

Exposing them to UV changes that dramatically:

• Commercial UV-treated mushrooms are sold in several countries and can supply 10 to 25 µg (400 to 1,000 IU) of vitamin D per 100 g, which is a full day's requirement or more.
• You can do it at home. Placing sliced mushrooms gill-side up in direct sunlight for 15 to 60 minutes produces substantial vitamin D2. Multiple studies have measured this, with results in the range of 10 µg per 100 g and higher. Slicing helps because it exposes the gills, where ergosterol concentrates. It works with dried mushrooms too, and the vitamin is stable in storage.

The caveat: vitamin D2 is somewhat less effective than D3 at raising and maintaining blood 25(OH)D levels, particularly at intermittent high doses. It does work, and human trials have shown UV-treated mushrooms raise vitamin D status comparably to a D2 supplement.

For anyone at high latitude in winter, or eating a vegan diet where D3 sources are limited, this is a genuinely useful and almost cost-free intervention.

6. What the evidence actually shows

Established: mushrooms are good sources of B vitamins, selenium, copper, and potassium, and UV exposure generates nutritionally meaningful vitamin D2.

Strong: replacing meat with mushrooms reduces calorie intake without reducing satiety in controlled feeding studies, which is a real and useful finding for anyone trying to eat less meat or fewer calories.

Mixed: the immune effects of beta-glucans, where laboratory and animal evidence is substantial and human clinical outcomes are limited. Observational associations between mushroom intake and lower risk of cognitive decline (a Singaporean cohort) and of some cancers, particularly breast cancer, where a 2021 meta-analysis found a modest inverse association. All observational.

Thin: the extensive medicinal mushroom supplement market (reishi, lion's mane, cordyceps, chaga, turkey tail). Some have real pharmacological interest and a few (a polysaccharide-K preparation from turkey tail) are used adjunctively in cancer care in Japan. The consumer supplements are largely unstandardised, frequently mislabelled in independent testing, often contain mycelium grown on grain rather than fruiting body, and have limited clinical evidence for the broad claims made. Lion's mane for cognition is the most-hyped and rests on a small number of small trials.

7. Safety: the part that matters

Foraging. This is not a hobby to approach casually.

• Amanita phalloides, the death cap, accounts for the large majority of fatal mushroom poisonings worldwide. It resembles several edible species, tastes pleasant, and its amatoxins are not destroyed by cooking, drying, or freezing.
• The characteristic pattern is the dangerous part: symptoms (vomiting, diarrhoea) begin 6 to 24 hours after eating, then subside for a day, giving false reassurance, while the liver fails. By the time jaundice appears, the damage may require transplantation.
• Destroying angels (A. virosa, A. bisporigera) are similar. False morels (Gyromitra) contain gyromitrin, which metabolises to a rocket-fuel component and is toxic.
• Do not use identification apps. Several studies and multiple poison centre warnings have found them unreliable, and there are documented poisonings following app misidentification. The same is true of AI image identification.
• "Try a little and see" does not work for the deadly species, because the toxins act slowly and irreversibly.
• Learn from an experienced forager, in person, one species at a time, and start with unmistakable species with no dangerous lookalikes.

Cultivated mushrooms are safe, with three notes:

• Agaricus contains small amounts of agaritine, a naturally occurring hydrazine derivative that is carcinogenic in high-dose rodent studies. Cooking reduces it substantially, and human dietary exposure is far below levels of concern. It is a reasonable additional argument for cooking mushrooms rather than eating them raw.
• Raw mushrooms are poorly digested. Chitin cell walls resist digestion, so cooking releases considerably more of the nutrients.
• Morels must be cooked, and thoroughly. Raw morels cause gastrointestinal illness.
• Shiitake dermatitis is a distinctive, intensely itchy, linear whip-like rash caused by lentinan in undercooked shiitake. It is not an allergy, is well documented, and resolves in a week or two. Cook shiitake fully.
• Alcohol and Coprinopsis atramentaria (common ink cap) produce a disulfiram-like reaction: flushing, palpitations, nausea. Relevant only to foragers.

Who should be careful: people with gout, since mushrooms are moderately high in purines (evidence for vegetable purines raising gout risk is weak); people with IBS, since Agaricus mushrooms are high in mannitol and a common trigger while oyster mushrooms are low-FODMAP; and people on immunosuppressants considering medicinal mushroom supplements, which have plausible immune-modulating interactions.

8. How to buy, store, and cook

• Choose: dry, firm, unbruised, with closed or barely open gills for button types. Slimy, wet, or darkening mushrooms are past it. A strong ammonia smell means spoiled.
• Store in paper, not plastic. Plastic traps moisture and they go slimy within a day or two. A paper bag in the fridge gives about a week.
• Do not soak them. The old advice that mushrooms absorb water like sponges is overstated (Harold McGee measured the uptake as small), and a quick rinse or a wipe is fine. What matters far more is drying them before cooking, because surface water prevents browning.
• Cook them hot and do not crowd the pan. Mushrooms are 90 percent water. Crowd them and they steam in their own liquid and go grey and rubbery. Give them space, let the water evaporate, then let them brown.
• Salt late. Salting early draws out water and prevents browning.
• Fat carries mushroom flavour, which is why butter works so well.
• Dried mushrooms are a concentrated umami ingredient. Soak them and use the soaking liquid, which is where much of the flavour went; strain it to remove grit.
• UV them. Slice, gills up, on a windowsill in direct sun for 30 to 60 minutes, then cook or dry as normal.
• They replace meat well by texture and savouriness, and not by protein content. A 50:50 blend of mushroom and minced meat is a well-studied approach that reduces calories and sodium while keeping people equally satisfied.

9. Myths and confusions

Don't be confused: button, chestnut/cremini, and portobello mushrooms are the same species at different ages. Agaricus bisporus harvested young and white is a button; the brown strain harvested young is a chestnut or cremini; left to open fully it is a portobello. The flavour intensifies with age as water content falls.

• "Mushrooms have no nutritional value." They are among the better sources of B vitamins, selenium, and copper, and the only meaningful dietary source of ergothioneine.
• "Mushrooms are a good protein source." They are protein-dense per calorie and low in absolute protein, at 2 to 3 g per 100 g.
• "Never wash mushrooms." A quick rinse is fine; drying them before cooking is what matters.
• "Wild mushrooms are safe if you cook them." Amatoxins are heat-stable. Cooking does not help.
• "You can identify mushrooms with an app." Documented poisonings say otherwise.
• "Medicinal mushroom supplements boost immunity." Beta-glucans have real biological activity; the consumer products are largely unstandardised and the human outcome evidence is thin.

10. The bottom line

• Mushrooms are fungi, not plants, and their chitin cell walls mean they need cooking to release their nutrients.
• They are unusually good sources of riboflavin, niacin, pantothenic acid, selenium, and copper, for very few calories, plus beta-glucan fibre and ergothioneine.
• Put sliced mushrooms in sunlight for half an hour and they generate a day's vitamin D. This is free, well documented, and almost nobody does it.
• Cook them hot, in an uncrowded pan, salted late, and dry them first.
• Buttons, chestnuts, and portobellos are the same species at different ages.
• Foraging kills people, the deadly species look edible, the toxins survive cooking, and apps get it wrong. Learn in person or buy cultivated.

Sources and notes

Composition figures are from USDA FoodData Central; production figures are approximate FAOSTAT values for the early 2020s. Fungal biology, chitin cell walls, and the phylogenetic position of fungi closer to animals than plants follow standard mycology texts. Agaricus commercial production stages follow the Penn State mushroom extension programme. Ergosterol conversion to vitamin D2 under UV follows Simon et al. and Cardwell et al.'s review in Nutrients, 2018; UV-exposed mushrooms raising serum 25(OH)D follows Urbain et al.'s trials. Ergothioneine and its dedicated human transporter follow Cheah and Halliwell's reviews. Meat-mushroom blending and calorie reduction follows the Culinary Institute of America and UC Davis blendability research. Mushroom intake and cognitive decline follows Feng et al.'s Singapore cohort; breast cancer association follows Ba et al.'s 2021 meta-analysis. Agaritine content and cooking reduction follow Schulzova et al. Shiitake dermatitis follows dermatology case series. Amanita phalloides toxicity, the deceptive symptom-free interval, and mortality follow standard toxicology; app misidentification warnings have been issued by multiple poison centres and mycological societies.

Open questions. Medicinal mushroom supplement claims rest overwhelmingly on preclinical work and on unstandardised products, and independent testing repeatedly finds mycelium-on-grain sold as fruiting body. Ergothioneine's function in humans is suggested by the existence of a dedicated transporter and has not been established.

👉 Next: legumes, the highest-fibre food you can buy and the crop that fixes its own nitrogen.