Mushroom cell membranes contain a compound called ergosterol, which ultraviolet light turns into vitamin D2. That makes mushrooms that have been in the sun or treated with UV light one of the few vitamin D sources that do not come from animals; the chapter on how UV light makes vitamin D covers that step. Be careful with wild mushrooms: do not pick and eat any wild mushroom you cannot identify. If you feel unwell after eating wild mushrooms, with vomiting or diarrhea, go to the emergency department right away, even if you later seem to get better, and bring any leftover mushrooms with you.
A mushroom is a fungus, not a plant. It does not grow by capturing sunlight with chlorophyll; it breaks down organic matter. Its cell wall is chitin, not the cellulose of plants. That chitin wall is part of why most mushrooms should be cooked before you eat them.
Background · The mushrooms you meet at the market
Mushrooms look like vegetables and sit in the produce section, but they are fungi, a biological kingdom of their own, alongside plants and animals.
Common kinds include the button mushroom (button, cremini, and portobello are actually the same species at different stages of maturity), shiitake (more savory once dried), oyster, enoki, and king oyster mushrooms. All of them are the fruiting bodies of fungi: what you eat is the part that grows out, while the real body of the fungus, the mycelium, stays buried in the growing medium.
Chapter 2
Rich in · vitamin D made by UV light
When ultraviolet B light (UVB) hits the ergosterol in fungal membranes, it turns it into vitamin D2. It is the same kind of light-driven reaction as sunlight turning 7-dehydrocholesterol in human skin into vitamin .
Mushrooms grown in the dark have very little vitamin D. Those that have been in the sun or treated with UV light can build up substantial D2, which raises vitamin D levels in the blood when you eat them — especially useful for people who do not eat animal foods (see Vitamin D). Still, D2 raises blood vitamin D less than D3 does, which is why supplements usually use D3.
A CLOSER LOOK
Light changes ergosterol in the membrane
This shows the fungal D2 route; the light symbol stands for UVB, and mushroom appearance cannot reveal whether it was treated.
1A precursor in fungal membranes
Ergosterol is a fungal-membrane component and the starting material for this photochemical reaction.
2UVB exposure matters
Mushrooms grown in darkness can differ greatly in D2 from sun-exposed or UV-treated mushrooms.
Illustration for understanding; not to scale. Saved figures include explanations and sources.
Mechanism · How UV turns ergosterol into D2
Ergosterol is a component of fungal cell membranes, playing roughly the role that cholesterol plays in human cell membranes. When UVB hits it, a single light-driven reaction turns it into vitamin D2 (ergocalciferol). Fungi end up with D2; human skin ends up with .
The key is whether the mushrooms have seen the light. Cardwell 2018's review is clear: mushrooms grown in dark sheds are very low in vitamin D, while sun-exposed or UV-treated ones can build up substantial D2, on the order of 10 µg per 100 g, sometimes more. Keegan 2013 then confirmed the human step: this mushroom D2 raises in people's blood (25-hydroxyvitamin D, the marker on a lab report used to judge whether you have enough vitamin D).
Background · Other nutrients, and ergothioneine
Besides that light-driven pathway, mushrooms are also commonly listed as sources of riboflavin (Riboflavin), niacin (B3), pantothenic acid (B5), selenium (selenium), and copper (copper). These are the everyday base on the plate, not the mushroom's signature.
Mushrooms also contain ergothioneine, an antioxidant the body cannot make and gets mainly from fungi. Some researchers call it a possible "longevity vitamin." But the evidence so far is mostly from cells and animals, without large human trials, so it remains worth watching and not yet settled — not something to upgrade into a proven anti-aging molecule.
Chapter 3
Where umami comes from · cut salt with mushrooms
That savory taste in mushrooms comes from free glutamate, the molecule behind umami, the fifth basic taste. Guanylate (GMP, a nucleotide) in dried shiitake amplifies it, well beyond simple addition.
The glutamate in mushrooms is the same molecule as the one in . Using mushrooms to build savoriness often lets you add less salt and still have a flavorful dish; the sodium, though, still counts as whatever salt you add, and mushrooms do not make it disappear.
Mechanism · How umami lets you use less salt
When glutamate and guanylate come together, the tongue's umami receptors respond more strongly. That is why a handful of dried shiitake can make a dish taste finished almost at once.
This has a kitchen use: umami can take over part of the satisfaction that saltiness provides. Building savoriness with mushrooms, especially dried mushrooms or mushroom powder, often keeps a dish full-flavored with less added salt. For anyone who needs to watch sodium, it is a way to cook food that tastes good without leaning on heavy salt.
Myth · Is natural umami better than MSG?
The glutamate in mushrooms is the same molecule as the glutamate in the seasoning (monosodium glutamate), and the body handles them the same way. So natural umami versus MSG is not a chemical ranking.
The only difference is whether you choose the whole mushroom, and eat its fiber, B vitamins, and minerals along with it.
Chapter 4
How to cook, and wild mushroom risk
Most edible mushrooms should be cooked before eating: heat makes the chitin cell wall easier to digest, and a natural compound in raw white mushrooms called agaritine also drops markedly with cooking. For vitamin D, choose mushrooms labeled as UV-treated, or set the ones you buy out in the sun for a while.
Foraging in the wild is a different matter entirely: never pick and eat wild mushrooms on the strength of your own experience.
In practice · How to choose them and get a good sear
Choosing: pick caps that are firm and dry on the surface, with no slime or dark patches. Mushrooms labeled as UV-treated and rich in vitamin D are the better pick if vitamin D is what you are after. Dried shiitake have the strongest umami.
Cooking: mushrooms release a lot of water, so for a browned sear, do not crowd the pan and do not salt too early; let the water cook off first. Keep the soaking water from dried mushrooms, since that is where the umami is. At everyday amounts, agaritine poses little risk, and cooking lowers it markedly, so there is no need to worry about it; just cook your mushrooms.
How much: mushrooms are very low in calories and contain nothing that needs limiting, so eat them as a normal food. Their best role is a supporting one: adding umami, bulk, and a little vitamin D and B vitamins to the plate.
Red flag · Never eat a wild mushroom you cannot identify
All of the above applies only to edible mushrooms bought through proper channels. Wild mushrooms you forage yourself are a different matter entirely: some deadly species look almost exactly like edible ones, eating them by mistake can be fatal, and you should never pick and eat wild mushrooms on the strength of your own experience.
If you vomit or have diarrhea after eating wild mushrooms, go to the emergency department right away, even if you later seem to get better. Bring any leftover mushrooms, and tell the doctor you ate wild mushrooms: some mushroom poisonings bring a period when the person seems to recover, and that is exactly when liver damage is happening.
References · 7
U.S. Department of Agriculture, Agricultural Research Service. (2019). FoodData Central: Mushrooms, white / crimini / portabella, raw, cooked, and UV-exposed. Very low calorie; riboflavin, niacin, pantothenic acid, selenium, copper; UV-treated mushrooms supply vitamin D2. fdc.nal.usda.gov
Keegan, R. J. H., Lu, Z., Bogusz, J. M., Williams, J. E., & Holick, M. F. (2013). Photobiology of vitamin D in mushrooms and its bioavailability in humans. Dermato-Endocrinology, 5(1), 165-176. UV exposure converts mushroom ergosterol to vitamin D2, which raises human 25(OH)D. 10.4161/derm.23321
Cardwell, G., Bornman, J. F., James, A. P., & Black, L. J. (2018). A review of mushrooms as a potential source of dietary vitamin D. Nutrients, 10(10), 1498. UV-treated mushrooms can reach substantial vitamin D2 levels; dark-grown mushrooms are very low. 10.3390/nu10101498
National Institutes of Health, Office of Dietary Supplements. (2024). Vitamin D — Fact Sheet for Health Professionals. Fact sheet (updated June 27, 2025; Wayback snapshot 20 September 2026): 25(OH)D below 30 nmol/L (12 ng/mL) is associated with deficiency, 30 to below 50 nmol/L is generally considered inadequate, 50 nmol/L (20 ng/mL) or more is sufficient for most people, and above 125 nmol/L (50 ng/mL) can be associated with adverse effects; RDA 600 IU (15 mcg) to age 70 and 800 IU (20 mcg) above 70; adult UL 4,000 IU (100 mcg); older people and people with darker skin (more melanin) make less vitamin D from sunlight (fact sheet). ods.od.nih.gov/factsheets/VitaminD-HealthProfessional
National Institutes of Health, Office of Dietary Supplements. (2021). Selenium — Fact Sheet for Health Professionals. Table 2: Brazil nuts, 1 ounce (6-8 nuts), 544 mcg selenium (989% DV); the text says Brazil nuts contain 68-91 mcg per nut and could cause selenium toxicity if consumed regularly, and that values from other analyses vary widely. The version read carries 'Updated: April 15, 2024' (Wayback Machine snapshot of 31 December 2024), newer than the 2021 date above (fact sheet, read 2026-09-24). The September 4, 2025 update (Wayback snapshot 19 September 2026) adds: the body absorbs up to about 90% of selenium from selenomethionine, selenium-enriched yeast, selenite and selenate; selenium-only supplements typically contain 100 to 400 mcg; Keshan disease, an endemic cardiomyopathy first identified in 1935 in low-selenium parts of China, fell dramatically after selenium intervention trials in the 1970s-1990s; in 2017 the American Thyroid Association issued a weak recommendation against selenium supplements for TPOAb-positive pregnant women (fact sheet). ods.od.nih.gov/factsheets/Selenium-HealthProfessional