Mushrooms: sunlight turns them into a vitamin D food

A mushroom contains ergosterol, and ultraviolet light converts it into vitamin D2. Industry does this deliberately, and the optimum has been calculated: for sliced button mushrooms, 16664 joules per square metre at 3.2 watts yields about 16 micrograms of vitamin D2 per gram. Moderate exposure raises it and excessive exposure destroys nutrients instead. Human trials confirm that treated mushrooms raise the vitamin in blood. Across every food anyone has tried fortifying with vitamin D, ultraviolet mushrooms are the second most studied vehicle after milk, which puts them well past the novelty stage.

The numbers, per 100 g

How much is actually in it

• Ultraviolet light turns a mushroom into a vitamin D food, and the dose that does it has now been calculated. — published studies of ultraviolet treatment of edible mushrooms, plus human and animal trials of the treated product Evidence A source
  For sliced Agaricus bisporus the modelled optimum was 16664 J/m2 at 3.2 W/m2, yielding 16.27 micrograms of vitamin D2 per gram (95% CI 13.69 to 18.85); moderate ultraviolet exposure raised vitamin D2 while excessive exposure degraded nutrients or plateaued; human and animal trials showed treated mushrooms raise serum 25-hydroxyvitamin D2 without adverse effects

What your body absorbs

• Shiitake holds real vitamin B12 and the white button mushroom holds essentially none, a difference of several hundredfold. — 25 studies measuring B12 content in seaweeds, mushrooms, plants and fermented foods (n = 25 studies) Evidence C source
  Measured with mass spectrometry, real B12 ranged from 2 x 10-3 micrograms per 20 g dry weight for white button mushroom cap to 0.79 to 1.12 micrograms per 20 g dry weight for shiitake; seaweed ranged from hijiki at 3 x 10-3 per 7 g to nori at 1.03 to 2.68 per sheet

What cooking and storage change

• Among all the foods anyone has tried fortifying with vitamin D, ultraviolet mushrooms are one of the best studied. — 49 papers describing randomised controlled trials of fortified bread or savoury biscuits (5), orange juice (5), ultraviolet mushrooms (8), cheese (3), yogurt (5), fluid milk (13), powdered milk (5), eggs (2), edible oils (4) and breakfast cereal (1) (n = 49 trials) Evidence A source
  Ultraviolet mushrooms were the second most studied vehicle after fluid milk; no suitable trials existed for rice, maize flour, butter, margarine, dairy spreads or plant-based milk and yogurt alternatives

What it does to your health

• Pooling eleven studies, people who ate the most mushrooms had about a fifth less stomach cancer. — 3900 gastric cancer cases and 7792 controls from 11 studies in the pooled analysis, plus four published studies, 15 studies in total (n = 11692 in the pooled analysis) Evidence C source
  Highest against lowest mushroom consumption gave relative risk 0.82 (95% CI 0.71-0.95) in the pooled analysis, 0.59 (0.26-1.33) in the meta-analysis of four published studies and 0.77 (0.63-0.95) for all 15 combined; in Western Pacific countries 0.59 (0.40-0.87)

Who this works differently for

• Of 2349 papers on medicinal mushrooms in cancer care, 39 involved patients and most were small and not randomised. — 39 studies meeting criteria out of 136 clinical studies identified from 2349 screened, covering 12 different mushroom preparations (n = 39 studies) Evidence C source
  A survival benefit was reported with Huaier granules in two hepatocellular carcinoma studies and one breast cancer study, and with polysaccharide-K in four adjuvant gastric cancer studies; 11 studies reported a positive immunological response and 14 reported better quality of life or fewer symptoms; adverse effects were mostly grade 2 or lower, chiefly nausea, vomiting, diarrhoea and muscle pain

What is still unknown

• The lion's mane evidence people cite for memory amounts to five randomised trials. — five randomised controlled trials and 15 laboratory studies (n = 5 randomised trials) Evidence C source
  The review covers neural stimulation, apoptotic activity, abundance of short-chain fatty acid producing microbiota and mood regulation, with the aim of establishing a safety profile for the supplement

The bottom line

Species matters more than anything else here, and averages mislead. Niacin across 28 USDA records runs from 0.2 to 14 mg per 100 grams. For vitamin B12, measured properly by mass spectrometry, white button mushrooms hold essentially none while shiitake holds 0.8 to 1.1 micrograms per 20 grams dry weight, which is a difference of several hundredfold. Advice to eat mushrooms for B12 without naming the species is useless, and even shiitake is a modest source at a large portion. Nobody has shown that B12 from mushrooms corrects a deficiency in anyone, so if you avoid animal foods this is a supplement question for a doctor rather than a shopping one. The cancer association, about a fifth less stomach cancer, comes from case-control studies where people with cancer recall old diets. And of 2349 papers on medicinal mushrooms in cancer care, 39 involved patients, most of them small and not randomised.

Sources

1. msh-dr-01 · review with weighted meta-analysis and response surface modelling
   For sliced Agaricus bisporus, an optimum was identified at 16,664 J/m2 and 3.2 W/m2, yielding 16.27 μg/g vitamin D₂ (95% CI: 13.69-18.85).
   Food Res Int, 2026 · checked 2026-09-22
2. msh-dr-03 · systematic review of food composition studies
   Real B12 content varied from mean (SD) mcg/portion size of seaweed hijiki 3 × 10-3/7 g to nori 1.03 - 2.68/sheet; mushroom white button cap 2 × 10-3(7 × 10-4)/20 g dry weight (dw) to shiitake 0.79(0.67)-1.12 (0.78)/20 g dw
   Crit Rev Food Sci Nutr, 2023 · checked 2026-09-22
3. msh-dr-02 · systematic review and meta-analysis of randomised controlled trials
   The searches identified 49 papers which described suitable RCTs using vitamin D-fortified bread/savoury biscuits (n = 5), orange juice (n = 5), UV-mushrooms (n = 8), cheese (n = 3), yogurt (n = 5), fluid milk (n = 13), powdered milk (n = 5), eggs (n = 2), edible oils (n = 4), or breakfast cereal (n = 1).
   J Steroid Biochem Mol Biol, 2024 · checked 2026-09-22
4. msh-dr-04 · pooled analysis of case-control studies with meta-analysis
   Higher mushroom consumption was associated with a lower risk of gastric cancer [relative risk (RR) for the highest vs. lowest consumption categories, 0.82; 95% confidence interval (CI), 0.71-0.95].
   Eur J Cancer Prev, 2023 · checked 2026-09-22
5. msh-dr-05 · systematic review of clinical studies
   Of 136 clinical studies identified by screening 2349, 39 met inclusion criteria.
   Curr Oncol Rep, 2023 · checked 2026-09-22
6. msh-dr-06 · systematic review
   This review includes results from five randomized controlled trials (RCTs), 15 laboratory
   Front Nutr, 2025 · checked 2026-09-22