Selenium in Edible Mushrooms

Selenium is vital to human health. This article is a compendium of virtually all the published data on total selenium concentrations, its distribution in fruitbody, bioconcentration factors, and chemical forms in wild-grown, cultivated, and selenium-enriched mushrooms worldwide. Of the 190 species reviewed (belonging to 21 families and 56 genera), most are considered edible, and a few selected data relate to inedible mushrooms. Most of edible mushroom species examined until now are selenium-poor (< 1 μ g Se/g dry weight). The fruitbody of some species of wild-grown edible mushrooms is naturally rich in selenium; their occurrence data are reviewed, along with information on their suitability as a dietary source of selenium for humans, the impact of cooking and possible leaching out, the significance of traditional mushroom dishes, and the element's absorption rates and co-occurrence with some potentially problematic elements. The Goat's Foot (Albatrellus pes-caprae) with ∼ 200 μ g Se/g dw on average (maximum up to 370 μ g/g dw) is the richest one in this element among the species surveyed. Several other representatives of the genus Albatrellus are also abundant in selenium. Of the most popular edible wild-grown mushrooms, the King Bolete (Boletus edulis) is considered abundant in selenium as well; on average, it contains ∼ 20 μ g Se/g dw (maximum up to 70 μ g/g dw). Some species of the genus Boletus, such as B. pinicola, B. aereus, B. aestivalis, B. erythropus, and B. appendiculus, can also accumulate considerable amounts of selenium. Some other relatively rich sources of selenium include the European Pine Cone Lepidella (Amanita strobiliformis), which contains, on average, ∼ 20 μ g Se/g dw (up to 37 μ g/g dw); the Macrolepiota spp., with an average range of ∼ 5 to < 10 μ g/g dw (an exception is M. rhacodes with < 10 μ g/g dw); and the Lycoperdon spp., with an average of ∼ 5 μ g Se/g dw. For several wild-grown species of the genus Agaricus, the selenium content (∼ 5 μ g/g dw) is much greater than that from cultivated Champignon Mushroom; these include A. bisporus, A. bitorquis, A. campestris, A. cesarea, A. campestris, A. edulis, A. macrosporus, and A. silvaticus. A particularly rich source of selenium could be obtained from selenium-enriched mushrooms that are cultivated on a substrate fortified with selenium (as inorganic salt or selenized-yeast). The Se-enriched Champignon Mushroom could contain up to 30 or 110 μ g Se/g dw, while the Varnished Polypore (Ganoderma lucidum) could contain up to 72 μ g Se/g dw. An increasingly growing database on chemical forms of selenium of mushrooms indicates that the seleno-compounds identified in carpophore include selenocysteine, selenomethionine, Se-methylselenocysteine, selenite, and several unidentified seleno-compounds; their proportions vary widely. Some aspects of environmental selenium occurrence and human body pharmacokinetics and nutritional needs will also be briefly discussed in this review.


INTRODUCTION
Knowledge of the amounts and chemical forms of selenium compounds in food sources, foodstuffs or dietary supplements (nutraceuticals), their intake and absorption rates, as well as further human body biotransformation pathways resulting in accumulation, activity, and release of selenium is vital to understanding the complexity of maintaining human health. This is because selenium is fundamentally essential to man. Selenium is required in biosynthesis of important selenoenzymes. Examples of these important, known selenoenzymes and selenoproteins include glutathione peroxidases, iodothyronine 5deiodases, thioredoxin reductases, selenoprotein P, and selenoprotein W. Some are active as catalysts for reduction of extracellular oxidants (such as hydrogen peroxide and lipid hydroperoxides), thereby protecting cells from potential damage by these hazardous compounds. They can also play a role in some pathways of energy metabolism and gene expression. Selenoprotein P and selenoprotein W, two important extracellular glycoproteins, are particularly rich in selenocysteine residues. These and some other aspects of selenium and food and human health of selenium have been comprehensively reviewed in a number of recent articles (1)(2)(3)(4)(5)(6).
Selenium usually accompanies sulfur ores of certain metallic elements of the volcanic origin as well as some deposits of coal. Natural ores of this element do not occur. Selenates (SeO 2− 4 ) are more water soluble than selenites (SeO 2− 3 ). Therefore, selenates can more easily infiltrate from soil into soil solution as well as be transported with ground and surface water and be absorbed by plants (7).
Soil, sediment, and water are the primary sources of selenium to fungi and vascular plants and, further, to animals that are natural in the human food chain worldwide. Abundance of selenium in food sources and foodstuffs of plant and animal origin is the function of its content in soil, bioavailability, biotransformation, food web transfer, and accumulation/homeostasis potential of selenium ions and organoseleno compounds. Therefore, the selenium content of locally available food sources and foodstuffs varies spatially worldwide. This phenomenon seems more marked for terrestrially available food than seafood. For example, the selenium content of terrestrial plants depends on the amount of the element available in soils, with > 2, 0.11 and 0.005 µg Se/g dw found in whole-wheat grain from soils of high (United States), low (New Zealand), and ultra-low (China) selenium status, respectively (1).
The selenium content of surface soil horizon worldwide is usually below 0.5 mg/kg, while most soils contain from 0.1 to 2 mg Se/kg dry weight (1,8). In some regions of the world where soil selenium content is < 0.125 mg/kg dw, severe endemic Se deficiency in humans causes juvenile cardiomyopathy (Keshan disease) and chondrodystrophy (Kaschin-Beck disease). Surface soil horizon can contain 5 mg Se/kg dw or more (areas with moor soils, alkaline soils) (8,9). Soil that contains nearly 8 mg/kg may lead to highly elevated amounts of this element in regionally produced food, resulting in excessive daily intake of 3200-6690 µg Se per person (i.e., 100 times over nutritional needs) and chronic selenosis. At the seleniferous sites, soil contains up to 90 mg Se/kg dw (1). Selenium-accumulator plants (genus Astragalus, Xylorrhiza, Oonopsis, and Stanleya) caused selenium poisoning of livestock (10). The values of bioconcentration factor (BCF; a quotient of concentration in plant and substrate calculated from dry weight data) of selenium in plants of the families of Compositae, Fabaceae (Leguminosae), Cruciferae, and the genus Allium when grown at soils abundant in this element can reach as much as 1000 (7).
Soil management may result in the increase of selenium content of soil that is poor in this element and may support the growth of selenium up-regulated vegetable food. Cultivation of some species of plants and mushrooms or the culturing of yeast with substrate fortified in inorganic salts of selenium (e.g., sodium selenate; Na 2 SeO 4 ) enables production of food that is enriched in this element (e.g., selenium-enriched garlic, which contains even up to > 1000 µg Se/g dw) (11,12).
Livestock usually contains between 0.3 and 0.4 µg Se/g fresh weight in muscle meat and about 4-fold and 10-to 16-fold more in liver and kidneys, respectively, while the selenium level of feed is the only and highly regulating agent (1). The mineral feed mixtures that are fortified with selenium (sodium selenate; sodium selenite, Na 2 SeO 3 ) or sodium hydrogen selenite (NaHSeO 3 ) when fed to farm animals result in enhanced selenium content of the slaughtered animal meats (kidney, liver) from 4.0 ± 0.9 to 8.8 ± 2.3 µg/g dw (13).
The main focus of this article is to summarize data on total selenium content and its chemical forms in edible wild-grown and cultivated mushrooms in the context of body fate of this element and human nutritional needs. Selected data on selenium in a few species of inedible mushrooms are also included.

BODY FATE OF SELENIUM Absorption
Selenium occurs mostly at -2, +4, and +6 oxidation states and forms covalently bounded compounds with C-Se and Se-S bonds. The human fate Selenium in Edible Mushrooms 259 of dietary seleno-compounds can differ from other mammals. Under normal circumstances, their fate and activity depend on their original chemical forms at the time of ingestion (14). Some chemical-form-dependent variations in efficiency of dietary selenium absorption have been observed. In this context, it is important to note that the type of foodstuffs may also play a role in determining its significance as a source of selenium and other essential elements. Selenium largely occurs in natural food as selenium-containing amino acids and in foods of plant origin, mainly as selenomethionine and that of animal origin as selenocysteine. The chemical forms of selenium that are ingested are important factors that determine not only the element's bioavailability but also its metabolic fate, distribution, nutritional importance (accessibility for functional selenoproteins), accumulation, and toxicity (14,15). Selenocysteine-containing animal proteins previously mentioned are the most readily available source of this element to man (5).
Selenium as selenomethionine in solution is actively transported (mechanism shared with methionine) with a yield of above 95%, and passively as inorganic selenate or selenite, with yields of above 90% and about 60%, respectively (14). Some dietary factors can influence the absorption rate of selenium (e.g., vitamin C hampers selenite absorption). Selenium in wheat, wheat bread, fish, or meat could be retained similarly in humans, as indicated by the enhancement of glutathione peroxidase activity, even though the absorption rate may fluctuate somewhat (14).

Distribution
Inorganic selenium compounds are absorbed and excreted quickly. However, when selenium is ingested in the form of selenomethionine, its clearance rate is substantially slower. Selenomethionine can be readily incorporated into body tissues in a nonspecific and unregulated manner. Selenomethionine, when absorbed by humans or animals, is not distinguishable from methionine and can be incorporated into general body proteins (14).

Metabolism
Inorganic selenium compounds such as selenite or selenate are absorbed by mammal body and are further reduced to hydrogen selenide (H Se H), which, as the presumed key selenium intermediate, is used for the synthesis of selenoamino acid (e.g., selenocysteine) and selenoproteins (2,3). Selenium in selenocysteine is almost fully ionized; this is what enables it to be an efficient catalyst for cells. A proposed metabolic pathway for selenomethionine includes trans-selenation to selenocysteine and the β-lyase reduction to hydrogen selenide, while methylselenocysteine includes the β-lyase reduction to monomethylselenol (CH 3 -Se-H), which may be further methylated or demethylated back to selenide (1,15).
Ingested selenium can bind with toxic elements such as mercury and cadmium to prevent their toxic action and to act antagonistically with other elements (e.g., As, Ba, Cu, Zn). Absorbed inorganic mercury ion and selenium could form an equimolar complex that specifically binds to plasma protein but less to some other proteins (16). Selenoprotein P that contains histidine and up to 10 selenocysteine moieties per molecule seems to bind certain metallic elements (6).

Excretion
Selenite and other forms of selenium (except selenomethionine) that are used in the biosynthesis of functional selenoproteins appear to be under homeostatic regulation. Selenite and selenocysteine are readily excreted in the urine when in excess (14). In mammals, hydrogen selenide not used for selenoprotein synthesis is believed to be disposed after biotransformation to selenosugar and/or stepwise methylation to monomethylselenol, dimethylselenide, and trimethylselenonium ions (15). Trimethylselenonium ion is a good urinary marker of intake of toxic doses of selenium. Selenomethionine, selenite, selenate, selenoaminoacids, and selenocholine are also urinary excreted selenocompounds, and daily urinary excretion of selenium could be as much as half of its daily intake (14).

SELENIUM: HUMAN NUTRITIONAL NEEDS AND TOXICITY
The intake of selenium from food should be sufficient to support the optimal expression of the selenocysteine-enzymes. Selenium that is incorporated as selenocysteine residues in various selenoproteins of animal meats is the most desirable form of selenium in food for humans. Under normal circumstances, dietary intake of selenium rarely exceeds the needs for selenocysteine-enzymes expression. In contrast, deficient intake of selenium (less than 40 µg Se per person per day) is more widespread (1,3). Dietary allowance for selenium is 0.87 µg/kg body weight (60.9 µg/person; 70 kg body weight). The recommended daily allowance is 55 µg Se/person for healthy adults. The maximum safe, daily dietary dosage of selenium assessed is 400 µg/person. Adverse health effects may occur at daily dosage of 900-1600 µg/person, and selenosis occurs at 3200-5000 µg/person (1,3,10).
The plasma and erythrocyte selenium concentrations of man of deficient, adequate, and excessive selenium intake correlate positively with seleniumintake status. In contrast, the level of glutathione peroxidase activity is not as sensitive to selenium status. Selenocysteine in proteins such as glutathione peroxidase and selenoprotein P occurs in stoichiometric amounts, while that of selenomethionine in hemoglobin and albumin occurs on a random basis (14).
Seleno-compounds exert effects on cells, which are strictly compositional and concentration-dependent. An inverse relationship between sele-Selenium in Edible Mushrooms 261 nium intake and the incidence of certain cancers has been documented in epidemiological studies (3). Selenium (selenoproteins) at supranutritional doses may have anticancer properties. The mechanisms of impact of Se on cancer reduction remain to be explored. Selenium at greater doses can be either cytotoxic or possibly carcinogenic (10). As mentioned before, the successive methylation of hydrogen selenide (H-Se-H) to monomethylselenol (CH 3 -Se-H), dimethylselenide (CH 3 -Se-CH 3 ↑; breath), and trimethylselenonium ion (CH 3 ) 3 Se + ; urine) can detoxify excess selenium; on the other hand, oxidation of excess hydrogen selenide may lead to production of toxic superoxide and other reactive oxygen species (H 2 O 2 , O 2 ) (1, 10).

TOTAL SELENIUM AND ITS CHEMICAL FORMS IN GENERAL AND IN SELENIUM-ENRICHED FOOD
Foods of plant origin containing from 0.017 ± 0.012 to 0.12 ± 0.06 µg Se/g dw in the vegetables and from 0.0062 ± 0.0016 to 0.089 ± 0.11 µg Se/g dw in fruits rich in starch or sugar are poor in selenium. An exception is protein-abundant leguminosae or cruciferes rich in glucosinolate (mustard, caraway, cabbages) as well as young asparagus and oat flakes that are relatively abundant in selenium (13). Egg yolk and the offal meats (liver, kidney) of livestock usually did contain selenium at concentrations of magnitude greater than muscle meats. In addition, fishes and seafood are considered as foodstuffs that are relatively abundant in selenium with 0.56 ± 0.2 to 2.0 ± 0.5 µg/g dw in some products and species (1,13,17).
As mentioned earlier, at nonseleniferous regions such as Central and Eastern Europe, livestock fed with feed with added selenium (usually as sodium selenate) will contain selenium at greater concentrations in muscle and organ meats when compared with the animals fed with local feeds (13). Selenium of cattle or pork meats probably constitutes selenoproteins and selenoenzymes as major forms, which are considered highly bioavailable.
Tubes and gills are usually the morphological parts of the carpophore (fruitbody, sporocarp) that are most abundant in selenium, and the cap (pileus) contains this element in greater concentrations than the stem (stipe, stalk). However, data regarding selenium distribution in the mushrooms' fruitbody is relatively small (Table 1).                       Table 2 lists published data on the total selenium content of edible wildgrown and cultivated mushrooms worldwide. These data are presented in order based on higher mushrooms' systematic classification (36,37). Of the 188 mushroom species reviewed (belonging to 21 families and 56 genera), a few are inedible (Table 2). These few selected inedible species are included only for the purpose of comparison with the edible ones. The question of whether some species of wild-grown mushroom should be edible or inedible really depends on local tradition and the procedures of preparation and cooking.
A figure of 180 species of wild-grown and more or less edible mushroom species for which selenium data exist is not that high, especially, when compared with the number of published data for elements more frequently determined (zinc, copper, cadmium, etc.) than selenium. In addition, in many of the species reviewed in Table 2, frequently, only a single carpophore or stand was examined. In the present estimation, the number of edible species worldwide should be much greater than 200.
Another problem is analytical quality of data that are available for selenium in mushrooms, as can be drawn from Table 2. The absolute concentration values of selenium provided at least by certain investigators are evidently biased (38)(39)(40)(41). In some cases, the data reported by these authors may be as much as three orders of magnitude greater than those reported for the same mushroom species by other authors using validated procedure. Obviously, all these excessive data (shown in Table 2 and marked with a question mark in parentheses) are highly doubtful or simply wrong. For the reason mentioned, two other published sets of data on selenium in mushrooms in Latin America and in some of the European stands are not included in the present review.
The mushroom most frequently studied regarding its selenium content is King Bolete (Boletus edulis), which is relatively abundant in this element ( Table 2). Among the mushroom species examined until now, the Goat's Foot (Albatrellus pes-caprae) with ∼ 200 µg, Se/g dw on average (up to 370 µg Se/g dw), is the richest one in this element ( Table 2). In an interesting survey by Stijve et al. (42), selenium data is known also for several other representatives of the genus Albatrellus. . Species such as A. ellisii seem to be rich in selenium (∼ 100 µg/g dw), but lower levels were found in A. flettii (∼ 25 µg/g dw), A. confluens (∼ 5 µg/g dw and up to 12 µg/g dw), A. cristatus (∼ 3 µg/g), and other Albatrellus spp. (Table 2).
King Bolete, as mentioned, is considered to be abundant in selenium, and on the average contains ∼ 20 µg Se/g dw (maximum up to 70 µg/g dw) ( Table  2). Pinewood King Bolete (B. pinicola) is also considerably rich in selenium and contains on average ∼ 40 µg/g dw, but the number of specimens examined from three stands is only five ( Table 2). Some other species of the genus Boletus could be also considered as relatively abundant in selenium: B. aereus and B. aestivalis (∼ 20 µg/g dw), B. erythropus (∼ 10 µg/g dw), and B. appendiculus (∼ 5 µg/g dw) ( Table 2).

Selenium in Edible Mushrooms 289
Other relatively rich species of selenium are the European Pine Cone Lepidella (Amanita strobiliformis), which contains on average ∼ 20 µg Se/g dw (up to 37 µg/g dw), and some other edible representatives of the genus Amanita, which contains ∼1 µg/g dw ( Table 1). The cultivated Champignon Mushroom (A. bisporus) is poor in selenium, with ∼ 0.5 µg/g dw on average. For several wild-grown species of the genus Agaricus selenium, content is much greater than for cultivated Champignon (∼ 5 µg/g dw) and includes species such as A. bitorquis, A. cesarea, A. campestris, A. edulis, A. macrosporus, and A. silvaticus, and is ∼ 2 µg/g dw for A. aestivalis ( Table 2).
Certainly, several representatives of the genera of Albatrellus and Boletus characterized appear to have the specific ability to take up and accumulate selenium in the fruitbody. In most studies on selenium in mushrooms reviewed (Table 2), the data on soil (substrate) selenium content does not exist. Selenium content of soil bedrock probably can influence to some degree the selenium content of wild-grown mushroom, but this phenomenon was not studied in detail (23,24,27,28,32,(43)(44)(45).
The values of BCF of total selenium in King Bolete (B. edulis) at an area that was contaminated because of over 200 years of operation of a lead smelter varied between 8.8 and 9.2 for the caps and between 4.7 and 9.8 for the stems. The total selenium content of soil at that site was 5.9±4.2 µg/g dw (top 1-6 cm soil layer), while selenium was non detectable in 6-23 cm and deeper layers. The carpophores of Bay Bolete (Xerocomus badius) and Red-cracking Bolete (X. chrysenteron) when compared with that of King Bolete excluded (BCF < 1) selenium both in the caps and in stems (48). The value of bioconcentration factor of selenium noted for wild-grown Agaricus bitorquis is 43, 11-26 for A. campestris, 3 for A. silvicola, 8 for A. arvensis, and 12-59 for Calocybe gambosa .
Mushrooms with relatively high selenium content can also contain elevated concentrations of silver, but no significant correlation between these elements occurred for European Pine Cone Lepidella (A.strobiliformis) (47). For example, King Bolete (B. edulis) or Parasol Mushroom (M. procera) are both relatively rich in selenium and mercury (49, 50).

SELENIUM CONTENT OF SELENIUM-ENRICHED MUSHROOMS
The Champignon Mushroom (A. bisporus) cultivated on substrates that are fortified with selenium salts effectively picks up this element and accumulates in fruitbody (51)(52)(53)(54)(55). For example, selenium as sodium selenite (Na 2 SeO 3 ) that is added to a substrate was accumulated by Champignon Mushroom at concentrations of up to 3 µg Se/g fresh weight; i.e., up to 30 µg Se/g dw (assuming 90% water content) (51). These authors fortified the substrate with selenium at 2, 5, 10, 30, and 60 mg dose per cultivation box (size 40 × 60 cm; about 13 kg of substrate per box in a layer of ∼ 18 cm thick).

Wild-grown Mushrooms
In an early study of King Bolete and Champignon Mushroom, selenium was found to be mainly located in low molecular weight fraction or as inorganic compounds. In lipids it is located ∼ 10%, with a similar amount in nucleic acids, while in proteins, chitin and polysaccharides fraction is ∼ 20% (51). In a study by Slejkovec et al. (58), King Bolete and Goat's Foot (A. pes-caprae) contained mostly low-molecular weight (6 kDa) seleno-compounds. A small fraction of the extractable selenium (after proteolysis) in Goat's Foot is selenite (3.0-9.2%), while selenocysteine is minor in this species, and selenocysteine in King Bolete is at 7.5% and selenomethionine is at 1.0%. A bulk portion of seleno-compounds in these species still needs to be elucidated (58).
In King Bolete (Boletus edulis), after extraction with sodium hydroxide solution (0.05 mol/dm 3 ), hydrochloric acid solution (0.05 mol/dm 3 ), or hot water (at 60 • C), and further fractionation of the extract using size exclusion liquid chromatography, selenium found was associated primarily with low but also high molecular weight fractions (alkaline extraction) (59). Further, hot water extraction and enzymatic protein hydrolysis proved selenocysteine, selenomethionine, selenomethylselenocysteine, and two unidentified compounds as major organoseleno constituents of B. edulis (60).
Hot water extracts of the fruiting bodies of wild-grown Parasol Mushroom (Macrolepiota procera), Lurid Bolete (Boletus luridus), and Lepista luscina contained, respectively, 47, 49, and 91% of selenium found in crude mushrooms. Hot water-soluble seleno-compounds occurred in the low molecular weight fraction, and selenomethionine at varying amounts was the major unbound (not protein bound) constituent of the extracts for these three species. In addition, several others of unknown structure seleno-compounds detected were in these mushrooms (61).
The fruiting bodies of commercially cultivated Champignon Mushroom (A.bisporus) do not contain significant amounts of selenium (selenomethionine) firmly bound to proteins, but fruiting bodies of this species that have grown up (selenium-enriched) on substrates fortified with selenium could contain protein-bound selenomethionine as well as a number of unidentified seleno-compounds. Selenomethionine that is unbound with proteins in cultivated ordinary Champignon Mushroom was hot water (at 85 • C) labile, while that bound to proteins of selenium-enriched specimens was hot water stable (63). Champignon Mushroom grown up in radioselenium-fortified substrate ( 75 Se as irradiated SeO 2 ) was accumulated in the fruiting bodies selenium as biotransformed seleno-compounds. They occurred in the majority of low molecular weight compounds and were of inorganic nature and to some degree also as higher molecular weight seleno-compounds (51). A three-step extraction process using water, pepsin, and trypsin yielded up to 75% of selenium found in selenium-enriched Champignon Mushroom, and selenium (IV) and selenocysteine were the major seleno-compounds found (54).
After proteolysis using proteolitic and cell wall digestion enzymes, Lysing enzyme and Driselase for the caps of Champignon Mushroom grown up on selenized-yeast added compost; selenomethionine, which is the major seleno-compound found in selenized yeast, was also the dominant Secompound in mushroom, while selenocysteine and Se(IV) were also detected (56).
To sum up, the mushrooms' fruitbody identified until now are selenocompounds such as selenocysteine, selenomethionine, Se-methylselenocysteine, or selenite and the presence of several unidentified seleno-compounds were confirmed or indicated, while their proportions varied.

IMPACT OF COOKING ON SELENIUM CONTENT OF MUSHROOMS
In some surveys, the authors found water-soluble seleno-compounds in mushrooms (57,(59)(60)(61)63). Even breakage of the major seleno-compound to several constituents after hot water extraction of cultivated Champignon Mushroom (at 85 • C) occurs (63). Probably depending on the local gourmet tradition worldwide, there are various dishes or cooking receipts that require initial mushroom boiling. Mushrooms frequently have to be blanched (boiling water treated usually for up to 5 min.) before further use as a meal ingredient. Nevertheless, blanching is not necessary for Champignon Mushroom, the caps of Parasol Mushroom (M. procera), or Saffron Milk Cap (Lactarius deliciosus) dishes because they are cooked (roasted) directly while eating raw, unprocessed (fresh) mushrooms must be avoided. Dried, whole or crushed, the fruiting bodies of some mushroom species (e.g., of the genus Boletus, Xerocomus or Leccinum) could be added to a dish directly or together with water macerate (e.g., when cooking traditional dish named "bigos" in Poland). Mushrooms dried and further powdered are also used as ingredients to make sauces.
There are only two studies reporting on the impact of boiling selenium content of mushrooms. Complete mushroom caps (probably of the Champignon Mushroom) after being boiled for 20 min. with distilled water leaked 44% of originally present selenium (total Se content between and after experiment was 1.3-1.5 and 0.76-0.79 µg Se/g dw, respectively) (64). In Finland, Lactarius mushrooms are eaten in considerable amounts, after boiling. During the boiling, the mean selenium content of Lactarius torminosus decreased by 32% (53).

SELENIUM BIOAVAILABILITY FROM THE MUSHROOM MEAL
It is well known that chemical forms of an element as the major factor but also other dietary factors have an impact on the element's bioavailability from food as well as their biological role.
In two earlier studies, selenium compounds contained in mushrooms were poorly bioavailable to man and rat (66,67). Female university students aged 20 to 35 years were supplemented daily with 150 µg selenium per person given as mushroom (a roll with 5.95 g of dried King Bolete taken with a meal) during 28 days. They had slightly (13%) enhanced the plasma selenium concentration and significantly (26%) enhanced erythrocyte selenium concentration. By the criteria of plasma selenium concentration, the bioavailability of selenium from King Bolete was considered weak (66).

CONCLUSIONS
Most of nearly 190 species of wild-grown or cultivated edible mushrooms collected worldwide and examined until now for selenium are poor in Se; most of them contain less than 1 µg Se/g dw in caps or whole fruiting bodies. The Goat's Foot (Albatrellus pes-caprae), which contains ∼ 200 µg Se/g dw on average (up to 370 µg/g dw), could be considered to be an especially rich source of Se; however, it is one of the many unpopular species of very limited or negligible culinary use or significance. The same criterion largely applies to several other mushroom species of the genus Albatrellus.
Boletus edulis (King Bolete) and closely related species such as B. pinicola, B. aereus, B. aestivalis, B. erythropus, and B. appendiculus are some of the more popular and abundant species that are relatively rich in selenium and may represent the most important source of this element to humans from wild mushrooms. A few other wild-grown mushroom species of the genus Agaricus, Amanita, Macrolepiota, or Lycoperdon can also contain selenium at elevated concentrations (∼5-∼20 µg Se/g dw), but they are much less valued when compared with Boletus spp. because of their limited and local significance as gourmet.
Of the cultivated mushrooms, species such as Champignon Mushroom or Varnished Polypore can be enriched in selenium when grown on selenium fortified (as inorganic salt or selenized-yeast) substrate, but their commercial and nutritional/medical significance is unknown.
The seleno-compounds identified until now in edible, wild-grown and selenium-enriched cultivated mushrooms are selenocysteine, selenomethionine, Se-methylselenocysteine, or selenite, and several unidentified selenocompounds; their proportions vary widely. There is a scarcity of knowledge on the impact of mushroom cooking and traditional preserving (drying, salting, pickling) on selenium behavior as well as its availability and significance as a source of this element from mushroom meals to humans.