White Button Mushroom: Nutrition and Vitamin D

Mushrooms do something no plant food can: when exposed to ultraviolet light, they manufacture real vitamin D. The white button mushroom's membranes are rich in ergosterol, a provitamin that UV-B light converts into vitamin D2 by the very same photochemical reaction that makes vitamin D3 in human skin. A mushroom that saw almost no light in the growing house can go from nearly zero to a full day's vitamin D after a few minutes under a lamp — or on your windowsill in the sun. That makes UV-exposed mushrooms the only common vitamin-D food that is not from an animal, uniquely valuable for vegetarians and vegans. This page covers the vitamin-D story in full — the chemistry, the human trials, an honest D2-versus-D3 comparison, and a do-it-yourself method — then the whole nutrition picture: low calories, protein, fibre, B vitamins, potassium, selenium, and copper.

Nutrition Overview
The One Thing Mushrooms Do That Plants Cannot
Ergosterol to Vitamin D2: the Photochemistry
How Much Vitamin D UV Mushrooms Provide
Human Bioavailability Trials
Vitamin D2 vs D3: an Honest Comparison
Sun-Exposing Mushrooms Yourself
B Vitamins, Minerals, and Umami
Overall Place in the Diet
Key Research Papers
External Authoritative Resources
Connections
Featured Videos

Nutrition Overview

White button mushrooms are roughly 92% water, which is why they are so low in calories — about 20–22 kilocalories per 100 grams raw. Despite that, they are far from nutritionally empty. Per 100 grams (raw, USDA data), a white button mushroom provides on the order of:

Protein: around 3 g — high for a vegetable-like food, and containing all nine essential amino acids, though in modest total amount.
Fibre: about 1 g, including the fungal beta-glucans discussed on the Immune Support page.
B vitamins: a standout feature — notably riboflavin (B2), niacin (B3), and pantothenic acid (B5), with useful amounts of folate and B6. Mushrooms are one of the better whole-food sources of these among low-calorie foods.
Potassium: roughly 300–350 mg — comparable to many vegetables and useful for the sodium-potassium balance.
Selenium and copper: genuinely notable trace-mineral content (see the minerals section below).
Very little sodium, fat, or sugar — and no cholesterol.

The overall profile — very low energy density, satisfying texture, savoury umami flavour, real micronutrients — is why mushrooms are prized in weight-conscious and vegetable-forward eating. They add bulk, flavour, and nutrients for almost no calories.

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The One Thing Mushrooms Do That Plants Cannot

Vitamin D is famously scarce in food. In nature it is concentrated in fatty fish, fish-liver oils, egg yolks, and animal liver — all animal foods. Plant foods contain essentially none, which is why fortified milks and cereals exist and why vitamin-D deficiency is common, especially at higher latitudes and among people with limited sun exposure. This is a real public-health problem: vitamin D is essential for calcium absorption, bone health, muscle function, and immune regulation.

Mushrooms are the exception to the plant-food rule — and technically they are not plants at all, but fungi, which is exactly why they can do it. Like human skin, fungi carry a sterol in their membranes that ultraviolet light can convert into vitamin D. In skin the sterol is 7-dehydrocholesterol and the product is vitamin D3 (cholecalciferol); in fungi the sterol is ergosterol and the product is vitamin D2 (ergocalciferol). The reaction is the same kind of UV-driven photochemistry. This makes UV-exposed mushrooms the only common non-animal, non-fortified food that delivers a real dose of vitamin D — a genuinely valuable fact for vegetarians, vegans, and anyone wanting a food source.

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Ergosterol to Vitamin D2: the Photochemistry

Ergosterol sits in the fungal cell membrane. When a photon of ultraviolet-B light (roughly 280–315 nm) strikes it, it opens one of the sterol's rings to form previtamin D2, which then rearranges thermally into vitamin D2 (ergocalciferol). This is precisely analogous to the skin reaction that Michael Holick and others mapped for vitamin D3, and indeed much of the mushroom vitamin-D research comes from Holick's laboratory (Keegan et al., 2013).

A few practical consequences follow from the chemistry:

It needs UV-B specifically. Ordinary indoor light and most window glass block UV-B, so mushrooms grown in the dark (as most commercial ones are) contain very little vitamin D unless deliberately exposed.
Exposure is fast. Because the gills present a large surface area rich in ergosterol, even a few minutes to an hour of UV-B can dramatically raise vitamin D2 content.
Gills-up matters. Slicing mushrooms or placing them gills-up exposes more ergosterol-rich surface and increases the yield.
It is self-limiting. As with skin, prolonged UV converts some vitamin D2 into inactive photoproducts, so there is a practical ceiling — you cannot create unlimited amounts.

Some producers now sell mushrooms deliberately treated with UV light and labelled as high in vitamin D; USDA FoodData Central lists these UV-exposed products separately from ordinary ones.

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How Much Vitamin D UV Mushrooms Provide

The numbers are striking. Ordinary, dark-grown white button mushrooms contain very little vitamin D — often only a few IU per serving. After deliberate UV-B exposure, the same mushrooms can reach several hundred to well over a thousand international units (IU) of vitamin D2 per 100 grams, depending on dose and duration. Commercial UV-treated mushrooms are frequently formulated to provide roughly a full day's reference intake (the adult reference is 600–800 IU, i.e. 15–20 micrograms) in a standard serving.

Simon and colleagues (2011) directly compared button mushrooms treated postharvest with UV-B light versus sunlight and confirmed that both raise vitamin D2 substantially, with the content depending on the intensity and length of exposure. Phillips and colleagues (2011) surveyed retail mushrooms and mapped the wide range of vitamin D and sterol content across types and treatments. The upshot: a UV-exposed white button mushroom is not a trace source — it can be a meaningful, even primary, dietary contributor of vitamin D.

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Human Bioavailability Trials

Making vitamin D2 in a mushroom is only useful if the human body can absorb and use it. Two lines of evidence say it can:

Urbain et al., 2011 (European Journal of Clinical Nutrition). A randomised controlled trial gave healthy adults who were deficient in serum 25-hydroxyvitamin D a serving of UV-B-irradiated button mushrooms and measured whether their vitamin-D status improved. It did — the mushrooms raised serum 25(OH)D, demonstrating that the D2 they make is bioavailable to humans.
Keegan et al., 2013 (Dermato-Endocrinology). This Holick-laboratory paper laid out the photobiology of vitamin D formation in mushrooms and showed the resulting D2 is absorbed and raises blood levels in people, comparable in effect to a vitamin-D2 supplement.

Earlier work (Outila et al., 1999) had already shown, using wild mushrooms, that mushroom-derived vitamin D2 is bioavailable in a human bioassay. Together these studies move mushroom vitamin D from "interesting chemistry" to "a food that measurably improves vitamin-D status in people" — a well-supported, human-tested benefit, in contrast to some of the more preliminary claims on other pages in this hub.

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Vitamin D2 vs D3: an Honest Comparison

Mushrooms make vitamin D2; sunlight on skin, fish, and most supplements provide vitamin D3. Both raise blood 25-hydroxyvitamin D and both are used by the body, but they are not identical, and honesty requires noting the difference.

A well-known systematic review and meta-analysis by Tripkovic and colleagues (2012, American Journal of Clinical Nutrition) concluded that, dose for dose, vitamin D3 is somewhat more effective than D2 at raising and maintaining blood 25(OH)D levels, particularly with bolus (large, infrequent) dosing. D2 works — it is a legitimate source of vitamin D — but you may need a little more of it to achieve the same blood level, and it may not sustain levels quite as long.

What this means practically:

For a vegetarian or vegan, UV-exposed mushrooms are one of the very few whole-food vitamin-D options and a genuinely useful one — D2 is far better than no dietary vitamin D at all.
For someone with significant deficiency, mushrooms can contribute but are usually not sufficient on their own to correct a large deficit quickly; a measured supplement (often D3) guided by blood testing is the reliable route.
For general maintenance alongside sensible sun exposure and a varied diet, regular UV-exposed mushrooms are a pleasant, effective way to top up.

For the full picture of the vitamin itself, see Vitamin D3 and its Benefits deep-dive.

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Sun-Exposing Mushrooms Yourself

You do not need a commercial UV-treated product to take advantage of this chemistry — you can do it at home, and the research supports it. Because ordinary shop mushrooms are grown in the dark, they arrive low in vitamin D, but their ergosterol is still there waiting.

A simple, evidence-aligned method:

Slice the mushrooms (more surface area) or at least turn them gills-up, which exposes the ergosterol-rich underside.
Place them in direct sunlight outdoors — not behind a window, because glass filters out the UV-B that drives the reaction.
Give them time in midday sun, when UV-B is strongest. Studies have found that a period of direct sun (roughly on the order of an hour or more, depending on latitude, season, and intensity) can raise vitamin D content substantially. Longer or split exposures increase the yield up to the self-limiting ceiling.
Then cook and eat as usual. Vitamin D2 is reasonably heat-stable, so normal cooking retains most of what you have made.

The yield varies with conditions and cannot be precisely dialled in at home, so treat this as a helpful boost rather than a measured dose. It is a genuinely satisfying bit of practical food science: a windowsill (or better, a sunny step) turning a bland mushroom into a vitamin-D food.

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B Vitamins, Minerals, and Umami

Beyond vitamin D, the white button's nutrition rests on two pillars: B vitamins and trace minerals.

B vitamins. Mushrooms are among the better low-calorie whole-food sources of several B vitamins — especially riboflavin (B2), niacin (B3), and pantothenic acid (B5), with folate and B6 as well. These are the cofactors that run energy metabolism in every cell. Note the honest limit: despite persistent myths, common cultivated mushrooms are not a reliable source of active vitamin B12 — vegetarians and vegans should not rely on white button mushrooms for B12 and should use a fortified food or supplement.

Selenium and copper. White button mushrooms are a genuinely useful source of selenium (a cofactor for the body's glutathione-peroxidase antioxidant enzymes) and copper (a cofactor for energy production and for the antioxidant enzyme superoxide dismutase). Among plant-style foods, mushrooms punch above their weight for these two minerals. See the Antioxidant & Ergothioneine page for how these minerals fit the antioxidant picture.

Umami and the sodium angle. Mushrooms are rich in glutamate and 5'-nucleotides, the compounds behind savoury umami taste. Practically, this means mushrooms can add deep, satisfying flavour to a dish with little or no added salt — a useful tool for anyone reducing sodium. Blending finely-chopped mushrooms into ground-meat dishes is a well-known way to cut calories and sodium while keeping flavour and moisture.

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Overall Place in the Diet

Put together, the white button mushroom earns its place as a nutritional workhorse rather than a superfood headline. Its strongest, best-proven benefit is the one people least expect: when UV-exposed, it is a real, human-tested source of vitamin D — the only common one that is not an animal food. On top of that it delivers B vitamins, potassium, selenium, and copper; fungal fibre; a little complete protein; ergothioneine and other antioxidants; and umami flavour — all for around 20 calories per 100 grams.

The sensible way to use it: eat mushrooms cooked and often, choose or make UV-exposed mushrooms when vitamin D is a goal, and treat them as one reliable, affordable, everyday building block of a vegetable-and-whole-food diet. No single food is a shield, but few foods give back as much per calorie as this ordinary mushroom. Explore the wider range in the Mushrooms collection and the whole-food context in Food.

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Key Research Papers

Keegan RJ, Lu Z, Bogusz JM, Williams JE, Holick MF (2013). Photobiology of vitamin D in mushrooms and its bioavailability in humans. Dermato-Endocrinology. — PubMed
Urbain P, Singler F, Ihorst G, Biesalski HK, Bertz H (2011). Bioavailability of vitamin D2 from UV-B-irradiated button mushrooms in healthy adults deficient in serum 25-hydroxyvitamin D: a randomized controlled trial. European Journal of Clinical Nutrition. — PubMed
Cardwell G, Bornman JF, James AP, Black LJ (2018). A review of mushrooms as a potential source of dietary vitamin D. Nutrients. — PubMed
Simon RR, Phillips KM, Horst RL, Munro IC (2011). Vitamin D mushrooms: comparison of the composition of button mushrooms (Agaricus bisporus) treated postharvest with UVB light or sunlight. Journal of Agricultural and Food Chemistry. — PubMed
Phillips KM, Ruggio DM, Horst RL, et al. (2011). Vitamin D and sterol composition of 10 types of mushrooms from retail suppliers in the United States. Journal of Agricultural and Food Chemistry. — PubMed
Tripkovic L, Lambert H, Hart K, et al. (2012). Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. American Journal of Clinical Nutrition. — PubMed
Mattila P, Lampi AM, Ronkainen R, Toivo J, Piironen V (2002). Sterol and vitamin D2 contents in some wild and cultivated mushrooms. Food Chemistry. — PubMed
Outila TA, Mattila PH, Piironen VI, Lamberg-Allardt CJ (1999). Bioavailability of vitamin D from wild edible mushrooms as measured with a human bioassay. American Journal of Clinical Nutrition. — PubMed
Nölle N, Argyropoulos D, Müller J, Biesalski HK (2017). Vitamin D2 enrichment in mushrooms by natural or artificial UV-light during drying. LWT – Food Science and Technology. — PubMed
Holick MF (2007). Vitamin D deficiency. New England Journal of Medicine. — PubMed