Mushrooms represent a unique plant-based source of vitamin D, offering particular value for individuals seeking to boost their intake through diet. Unlike most vegetables, mushrooms contain ergosterol, which converts to vitamin D2 (ergocalciferol) when exposed to ultraviolet B radiation—a process similar to vitamin D synthesis in human skin. Whilst commercially grown mushrooms typically contain negligible vitamin D unless UV-treated, both shop-bought UV-enhanced varieties and home sun-exposure methods can significantly increase their nutritional content. This article examines how mushrooms produce vitamin D, compares their contribution to other dietary sources, and provides evidence-based guidance on incorporating them into a balanced approach to meeting UK recommended intakes.

How Mushrooms Produce Vitamin D

Mushrooms occupy a unique position in the fungal kingdom as one of the few non-animal foods capable of producing vitamin D naturally. Unlike most edible plants, which provide negligible vitamin D, mushrooms contain ergosterol—a sterol compound present in their cell membranes that functions similarly to the 7-dehydrocholesterol found in human skin.

When mushrooms are exposed to ultraviolet B (UVB) radiation, either from natural sunlight or artificial UV lamps, the ergosterol undergoes photochemical conversion to ergocalciferol, also known as vitamin D2. This process mirrors the mechanism by which human skin produces cholecalciferol (vitamin D3) upon sun exposure. The conversion efficiency depends on several factors, including the intensity and duration of UV exposure, the wavelength of light (UVB at 290–315 nm is most effective), and the surface area of the mushroom exposed to radiation.

Commercially cultivated mushrooms are typically grown in dark environments and therefore contain minimal vitamin D unless deliberately exposed to UV light post-harvest. Research has demonstrated that even brief UV treatment—as little as 5 to 60 minutes—can significantly increase the vitamin D2 content of mushrooms, with levels largely remaining stable during storage and cooking, though some loss may occur. The ergosterol is concentrated in the gills and cap surface, making these areas particularly responsive to UV exposure.

It is important to note that whilst vitamin D2 from mushrooms is biologically active in humans, there is ongoing scientific discussion regarding its equivalence to vitamin D3 from animal sources. Current evidence suggests both forms effectively raise serum 25-hydroxyvitamin D levels, though some studies indicate vitamin D3 may be more potent at maintaining long-term stores.

Vitamin D Content in Different Mushroom Types

The vitamin D content of mushrooms varies considerably depending on species, growing conditions, and post-harvest treatment. Wild mushrooms that grow in sunlight naturally contain higher levels of vitamin D2 compared to commercially cultivated varieties grown in darkness. Wild mushrooms can have variable vitamin D2 content, with some species containing significant amounts when sun-exposed.

Commercially grown mushrooms such as white button mushrooms (Agaricus bisporus), chestnut mushrooms, and portobello mushrooms typically contain negligible vitamin D (less than 0.1 micrograms per 100g) when grown conventionally. However, the food industry has responded to public health needs by developing UV-treated mushrooms, which are now available in many UK supermarkets. These products are exposed to controlled UV light and can contain 10–20 micrograms of vitamin D2 per 100g serving—equivalent to 100–200% of the UK recommended daily intake.

Shiitake mushrooms deserve particular mention, as they naturally contain higher baseline levels of ergosterol and respond exceptionally well to UV treatment. Dried shiitake mushrooms exposed to sunlight can accumulate substantial vitamin D2 concentrations. Oyster mushrooms and other gourmet varieties also demonstrate good conversion rates when UV-treated.

When purchasing mushrooms for vitamin D content, consumers should look for packaging that specifically states 'high in vitamin D' or 'UV-treated'. Standard mushrooms, regardless of variety, will not provide meaningful vitamin D unless they have been deliberately exposed to UV radiation. Under UK nutrition and health claims regulations, mushrooms labelled as 'high in vitamin D' must contain at least 3 micrograms per 100g (30% of the Nutrient Reference Value).

Importantly, never consume wild mushrooms unless they have been expertly identified, as some species can be toxic. Always purchase wild varieties from reputable suppliers.

Comparing Mushrooms to Other Vitamin D Sources

Vitamin D is naturally present in relatively few foods, making dietary sources particularly valuable for individuals at risk of deficiency. The primary natural sources include oily fish (such as salmon, mackerel, sardines, and herring), which contain vitamin D3 and typically provide 7–25 micrograms per 100g serving. Egg yolks contain approximately 1.6–2 micrograms per egg, whilst red meat and liver provide smaller amounts (0.3–1 microgram per 100g).

UV-treated mushrooms can provide comparable or superior vitamin D2 levels to many animal sources, with a 100g serving potentially delivering 10–20 micrograms—significantly more than eggs or meat. However, it is important to recognise that mushrooms provide vitamin D2 (ergocalciferol) rather than the D3 (cholecalciferol) found in animal products. Both forms are converted in the liver to 25-hydroxyvitamin D, the circulating form measured in blood tests, though some evidence suggests vitamin D3 may be marginally more effective at raising and maintaining serum levels over time.

Fortified foods represent another important source in the UK diet. Many breakfast cereals, fat spreads, and some plant-based milk alternatives are fortified with vitamin D (usually D2 or D3), typically providing 1.5–2.5 micrograms per serving. Infant formula is mandatorily fortified. For individuals following vegan or vegetarian diets, UV-treated mushrooms offer a valuable whole-food source of vitamin D2, alongside fortified products and supplements.

The NHS recommends that most people can obtain adequate vitamin D through a combination of sunlight exposure (March to September in the UK) and dietary sources. However, Public Health England advises that everyone should consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D during autumn and winter months, when sunlight is insufficient for cutaneous synthesis. It's important not to exceed safe upper limits: adults and children over 11 years should not take more than 100 micrograms daily, children 1-10 years should not exceed 50 micrograms daily, and infants should not have more than 25 micrograms daily.

How to Increase Vitamin D in Mushrooms at Home

Individuals can enhance the vitamin D2 content of shop-bought mushrooms through simple UV exposure at home. Research has demonstrated that placing mushrooms in direct sunlight for as little as 15 minutes to 2 hours can increase their vitamin D levels, with longer exposure times generally producing higher concentrations, though yields vary considerably with season, latitude and weather conditions.

For optimal results, slice mushrooms to increase surface area exposure, or arrange them gill-side up, as the gills contain the highest concentration of ergosterol. Place them on a clean surface in direct midday sunlight when UVB radiation is strongest (typically between 11:00 and 15:00 during spring and summer months in the UK). Turn the mushrooms periodically to ensure even exposure. Studies suggest that summer sunlight exposure can generate meaningful amounts of vitamin D2, though exact yields will vary.

Weather and seasonal considerations are important: UVB radiation is significantly reduced during winter months in the UK (October to March), through cloud cover, and by window glass, which blocks most UVB rays. Therefore, mushrooms must be placed outdoors rather than behind windows. On overcast days, exposure times may need to be extended, though some UVB radiation does penetrate cloud cover.

The vitamin D2 produced through home UV exposure is generally stable during cooking. Sautéing, grilling, or roasting typically retains much of the vitamin D content, making UV-enhanced mushrooms a practical addition to meals. The vitamin D is also largely retained during refrigeration for several days post-exposure.

For food safety, limit the time mushrooms remain at ambient temperature, cover them to prevent contamination during sun exposure, and refrigerate promptly afterwards. Whilst this method is evidence-based and safe, it is important to note that there is no official standardisation for home UV treatment. Those requiring precise vitamin D intake for medical reasons should rely on commercially UV-treated mushrooms with verified nutritional labelling or discuss supplementation with their GP.

Daily Vitamin D Requirements and Mushroom Intake

The UK government, following advice from the Scientific Advisory Committee on Nutrition (SACN), recommends a Reference Nutrient Intake (RNI) of 10 micrograms (400 IU) of vitamin D daily for everyone aged 4 years and above, including pregnant and breastfeeding women. Infants from birth to 1 year require 8.5–10 micrograms daily. These recommendations assume minimal sunlight exposure and are designed to maintain serum 25-hydroxyvitamin D concentrations above 25 nmol/L year-round, the threshold below which bone health may be compromised.

To meet the 10 microgram daily requirement through UV-treated mushrooms alone, an individual would need to consume approximately 50–100g of commercially UV-enhanced mushrooms (depending on the specific product's vitamin D content). This is a realistic portion size—roughly equivalent to a generous serving in a meal. However, most people obtain vitamin D from multiple sources, including fortified foods, oily fish, eggs, and sunlight exposure during summer months, meaning smaller mushroom portions can contribute meaningfully to overall intake.

At-risk groups who may particularly benefit from vitamin D-rich foods include individuals with limited sun exposure (those who are housebound, cover their skin for cultural reasons, or work indoors), people with darker skin (as melanin reduces cutaneous vitamin D synthesis), older adults (whose skin becomes less efficient at producing vitamin D), and those with malabsorption conditions such as coeliac disease or Crohn's disease. NICE guidance recommends that these groups should be especially vigilant about vitamin D intake.

It is important to recognise that whilst mushrooms can contribute to vitamin D intake, supplementation remains the most reliable strategy for at-risk populations during winter months. The NHS advises that adults and children over 4 years should consider taking a daily supplement containing 10 micrograms of vitamin D between October and March. Those in high-risk groups may require supplementation year-round. Breastfed infants should receive a supplement of 8.5-10 micrograms daily, while formula-fed infants consuming at least 500ml of formula daily do not need additional supplements as formula is already fortified.

Individuals concerned about vitamin D status should consult their GP, who can arrange a serum 25-hydroxyvitamin D test if clinically indicated (not as routine screening) and provide personalised advice on dietary sources and supplementation.

Frequently Asked Questions