Introduction

We are eating mushrooms all the time and in fact they are cultivated all over the world (production reached 8.99 million tons in 2018 and China is the largest global producer) (Kumla et al., 2018). We have been known to consume mushrooms and using them for their medicinal properties from pre-historic years (Tran and Juergens, 2020) (reports from India (5,000 years ago), in China (during the Ming Dynasty), Japan and used by African shamans and native Americans) (Money et al., 2016; Vetvicka et al., 2019).

Even though we have been eating and using mushrooms for thousands of years globally, do we really know what they are? Here, I did some research regarding their biology and nutritional value.

Mushroom anatomy

Mushrooms are not plants nor animals, but fungi. Plants make their food by using the sun, whereas mushrooms do not require sunlight to grow. Animals and fungi need a food source either a plant or an animal, alive or dead. In contrast to plants and animals, fungi are not made of cells, but they are comprised of thin tubes called hyphae. Fungi (in contrast to animals) digest their food externally. When the hyphae find a food source, they secrete enzymes to digest it into simple nutrients that are easier absorbed by the hyphae. The mesh of growing hypha produces a web of threads called mycelium. Mycelium reproduces by forming fruitbodies that they form spores. The term ‘mushroom’ refers to this fungal fruiting body. The rest of the fungus is hidden from view (e.g. underground, inside a stump etc) and might live for hundreds of years whereas the fruitbody can exist only a few days. Anatomically, the mushroom comprises of the stem and a cap with gills on the underside. The gills produce spores, important for the propagation of the fungus (Carroll, 1989).

Figure 1: Anatomy of a mushroom.

The number of species of mushrooms on Earth is estimated to be 140,000 and only 10% (=14,000) of which are known. From these, about 3,000 are edible, with approximately 700 exhibiting medicinal properties and 1% being poisonous (Ayeka, 2018, Akramiene et al., 2007). Mushroom poisonings can occur either due to misidentification of poisonous species as edible or due to intentional ingestions (over half of the exposures are in children under six years). Mushroom poisonings may range from a gastrointestinal upset to potentially liver failure, kidney failure, and neurologic sequelae (complications involving the central nervous system). The symptoms depend to the kind of toxin ingested (e.g. amatoxin, psilocybin, muscarine, coprine, allenic norleucine, gyromitrin, etc.) (Tran and Juergens, 2020).

Psilocybin and its active metabolite, psilocin (psilocybin is quickly converted to psilocin in the body) are found in mushroom species known as ‘magic mushrooms’. These agents bind (=act as agonists) to a subtype of serotonin receptors (5-hydroxytryptamine (5-HT) subtype receptors (5-HT2AR)]. Stimulation of these cerebral serotonin receptors account for the psychedelic effects of psilocybin (Madsen et al., 2019). These include the sensation of euphoria, visual and mental hallucinations, changes in perception, a distorted sense of time, and spiritual experiences, and can include possible adverse reactions such as nausea and panic attacks for 30 minutes to 2 hours after ingestion and can last 4–12 hours depending on the amount (Tran and Juergens, 2020). The range of the consequences are diverse and this is not surprising as the serotonergic system (=neurons secreting serotonin) plays a key role in regulating vital processes such as mood, sexual behaviour, appetite, pain, sleep, memory etc

Figure 2: The effects of psilocin on the human brain. When ‘magic’ mushrooms are ingested, psilocin makes its way to the brain where it binds to serotonin receptors due to its similarity with serotonin, overstimulating the brain.

Mushroom nutritional value

Mushrooms are highly nutritious and administration of extract of medicinal mushrooms can help patients struggling with number of diseases including: diabetes, cardiovascular disease, obesity and even cancer (Chaturvedi et al., 2018).

They are rich in protein that can be compared with eggs, milk and meat. They can be digested easily providing high quantities of amino acids (many of them essential i.e. cannot be made by the body) such as phenylalanine and threonine (Kulshreshtha et al., 2014). They are also rich in many essential unsaturated fatty acids, such as linoleic and oleic acids (Chaturvedi et al., 2018). In addition, mushrooms contain minerals, such as calcium, potassium, magnesium, iron, and zinc (Jayachadran et al. 2017; Chaturvedi et al., 2018). Finally, they also have laarge amounts of fibre and at the same time they are low in calories, in sugar and lack cholesterol (Kulshreshtha et al., 2014).

Maybe the most important nutritional elements of mushrooms are their polysaccharides (=glucose polymers). In particular, 𝛽-glucans which are present in edible mushrooms [such as lentinan (shiitake mushroom), grifolan, (maitake mushroom), and schizophyllan glucan (suehirotake mushroom)] have been shown to have antitumor activities. 𝛽-glucans are water-soluble polysaccharides found in the cell walls of mushrooms (see Box1 for details) (Jayachadran et al., 2017; Ayeka, 2018). 𝛽-glucans are one of the best  characterised immunomodulating factors i.e. substances capable of regulating the immune system (Chaturvedi et al., 2018). Their ability to modulate immune responses are due to their conserved structures [named pathogen-associated molecular patterns (PAMPs)] which are recognised by humans as ‘non-self’ and as a consequence immune cells get activated (Figure 3)(Ayeka, 2018, Chaturvedi et al., 2018). Their ability to stimulate the immune system contributes to their anti-tumour activity. This ability in combination with their negative effects on angiogenesis (=growth of blood vessels) and their ability to interfere with particular cellular responses of tumour cells e.g. their proliferation (e.g. Pleurotus ostreatus  blocks the cell proliferation of breast and colon cancer), makes them strong anti-tumour agents (Murphy et al., 2020, Akramiene et al., 2007).

Box 1: Structure of 𝛽-glucans. 𝛽-glucans are nonstarch polysaccharides that consist of D-glucose units linked via 𝛽-glycosidic bonds (see image below). Human consume dietary 𝛽-glucans from cereal grains (especially oats and barley), mushrooms, seaweeds, and yeast.

The 𝛽-glucans present in edible mushrooms consist of a linear 𝛽-1,3-glucan backbone with 𝛽-1,6-linked glucose branches (left image). The length and branches of the β-glucan from various fungi are widely different. In contrast, the 𝛽-glucans present in cereals include a mixture of 𝛽-1,3 and 𝛽-1,4 glycosidic bonds, without any 𝛽-1,6 branching (right image)(Nakashima et al., 2018).

Is causing immune responses good for human health? When humans consume β-glucans, these molecules reach the intestine intact since they are resistant to stomach acid (=non-digestible due to the absence of the appropriate enzymes) (Batbayar et al., 2012; Ayeka, 2018). In the first part of small intestine (=duodenum), they interact with the cells lining the intestine and they get across the cell barrier (=epithelium) to the lymphatic tissue on the other side (Peyer’s patch) where cells of our immune system (especially innate immune system e.g. macrophages) reside. β-glucans can also get across the cellular barrier by interacting with projections of immune cells (e.g. dendritic cells) crossing the epithelia and projecting through to the lumen. β-glucans in both cases are binding to receptors found on immune cells (e.g. dectin-1) and they get engulfed and fragmented. Fragments of β-glucans are then used to activate more cells of the immune system (e.g. NK cells), triggering further immune reactions (Figure 3).

Activation of immune cells in the gut by 𝛽 -glucans, have a positive influence on gut health and can reduce immune disorders in the rest of the body. For example, 𝛽-glucans can play a role in the balanced immune homeostasis i.e. activating specific cells of the immune system, shifting the equilibrium and resulting in alleviating the symptoms of allergic or autoimmune diseases. Finally, ingestion of 𝛽-glucans can prevent age-related diseases due to reduction of immunological function (Nakashima et al., 2018)

Figure 3:Uptake of β-glucans by the gut and activation of the immune system. When β-glucans are indigested they arrive in the gut where they can be either absorbed through M cells or through binding to the projected tips of dendritic cells. The β-glucans subsequently bind to receptors (e.g. dectin-1) on the surfaces of macrophages or dendritic cells which they engulf them and process them to fragmented β-glucans. These fragments activate NK cells by binding to specific receptors (e.g. CR3) and drive them to secrete cytotoxic agents that could target tumour cells (adapted by Batbayar et al., 2012).

Furthermore, β-glucans not only affect the immune system but can also reduce cholesterol levels. They do that due to their ability to act as dietary fibers and can entrap aggregates containing fats (bile acid micelles), leading them to excretion instead of absorption (Figure 4). The cholesterol-lowering property has only been reported for the β-glucans derived from oats and barley. However, β-glucan-containing mushrooms have been shown to lower cholesterol levels in animal models (Nakashima et al., 2018, Sima et al., 2018).

Figure 4: β-glucans effect on cholesterol balance. β-glucans disrupt the interaction of the micelles with the intestinal epithelium, increasing the fecal output of fat, bile acids, and cholesterol (adapted from Sima et al., 2018).

Discussion

Mushrooms can be quite beneficial to humans through different mechanisms. As discussed before, they have a number of medicinal properties that can benefit the human body in different ways (reduction of cholesterol levels, prevention of diabetes, regulation of gut microflora, anti-oxidant, anti-tumor and anti-microbial activities etc) (Kulshreshtha et al., 2014; Nakashima et al., 2018).

In culinary terms, mushrooms contain substances related to umami’s flavour so they can be used to improve the taste of other ingredients (Santos et al., 2019). Miller et al., showed that mushrooms can successfully be used as a healthy substitute to meat, without compromising the overall taste in the presence of reduced salt (Miller et al., 2014).

Finally, mushrooms can be used for environmental purposes such as bioremediation (or mycoremediation). Mushrooms produce enzymes that can degrade pollutants and waste (Kulshreshtha et al., 2014). Their decomposing ability, converting low-quality waste into high quality food, can be fundamental for agricultural sustainability and the use of agro-industrial waste.

In conclusion, producing mushrooms has many advantages considering their high nutritional value, their cultivation can be done sustainably all over the world and even though they are delicious on their own, they can work as flavour enhancers .

References

Akramiene D, Kondrotas A, Didziapetriene J, Kevelaitis E. Effects of beta-glucans on the immune system. Medicina (Kaunas). 2007;43(8):597-606. https://pubmed.ncbi.nlm.nih.gov/17895634/

Ayeka PA. Potential of Mushroom Compounds as Immunomodulators in Cancer Immunotherapy: A Review. Evid Based Complement Alternat Med. 2018 Apr 22;2018:7271509. https://pubmed.ncbi.nlm.nih.gov/29849725/

Batbayar S, Lee DH, Kim HW. Immunomodulation of Fungal β-Glucan in Host Defense Signaling by Dectin-1. Biomol Ther (Seoul). 2012 Sep;20(5):433-45. https://pubmed.ncbi.nlm.nih.gov/24009832/

Caroll J.E. (1989). Button mushroom. 4-H Members’ guide M-11-6. Cornell Cooperative Extension Service.

Chaturvedi VK, Agarwal S, Gupta KK, Ramteke PW, Singh MP. Medicinal mushroom: boon for therapeutic applications. 3 Biotech. 2018 Aug;8(8):334. https://pubmed.ncbi.nlm.nih.gov/30073119/

Jayachandran M, Xiao J, Xu B. A Critical Review on Health Promoting Benefits of Edible Mushrooms through Gut Microbiota. Int J Mol Sci. 2017 Sep 8;18(9):1934. doi: 10.3390/ijms18091934. PMID: 28885559; PMCID: PMC5618583. https://pubmed.ncbi.nlm.nih.gov/28885559/

Kulshreshtha S, Mathur N, Bhatnagar P. Mushroom as a product and their role in mycoremediation. AMB Express. 2014 Apr 1;4:29. https://pubmed.ncbi.nlm.nih.gov/24949264/

Kumla J, Suwannarach N, Sujarit K, Penkhrue W, Kakumyan P, Jatuwong K, Vadthanarat S, Lumyong S. Cultivation of Mushrooms and Their Lignocellulolytic Enzyme Production Through the Utilization of Agro-Industrial Waste. Molecules. 2020; 25(12):2811. https://doi.org/10.3390/molecules25122811

Nakashima A, Yamada K, Iwata O, Sugimoto R, Atsuji K, Ogawa T, Ishibashi-Ohgo N, Suzuki K. β-Glucan in Foods and Its Physiological Functions. J Nutr Sci Vitaminol (Tokyo). 2018;64(1):8-17. https://pubmed.ncbi.nlm.nih.gov/29491277/

Money NP. Are mushrooms medicinal? Fungal Biol. 2016 Apr;120(4):449-453. https://pubmed.ncbi.nlm.nih.gov/27020147/

Murphy EJ, Rezoagli E, Major I, Rowan NJ, Laffey JG. β-Glucan Metabolic and Immunomodulatory Properties and Potential for Clinical Application. J Fungi (Basel). 2020 Dec 10;6(4):356. https://pubmed.ncbi.nlm.nih.gov/33322069/

Myrdal Miller A, Mills K, Wong T, Drescher G, Lee SM, Sirimuangmoon C, Schaefer S, Langstaff S, Minor B, Guinard JX. Flavor-enhancing properties of mushrooms in meat-based dishes in which sodium has been reduced and meat has been partially substituted with mushrooms. J Food Sci. 2014 Sep;79(9):S1795-804. doi: 10.1111/1750-3841.12549. Epub 2014 Aug 14. PMID: 25124478. https://pubmed.ncbi.nlm.nih.gov/25124478/

Santos F. F., Dantas N. M., Simoni N. K., Pontes L. S., Pinto-e-Silva M. E. M. (2020). Are foods naturally rich in glutamic acid an alternative to sodium reduction?. Food Science and Technology, 40(Suppl. 1), 190-196. Epub November 28, 2019.https://doi.org/10.1590/fst.08819

Sima P, Vannucci L, Vetvicka V. β-glucans and cholesterol (Review). Int J Mol Med. 2018 Apr;41(4):1799-1808. https://pubmed.ncbi.nlm.nih.gov/29393350/

Tran HH, Juergens AL. Mushroom Toxicity. [Updated 2020 Aug 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK537111/

Vannucci L, Šíma P, Vetvicka V., Křižan, J. (2017). Lentinan properties in anticancer therapy: A review on the last 12-year literature. American Journal of Immunology. 13. 50-61. https://thescipub.com/abstract/10.3844/ajisp.2017.50.61

Vetvicka V, Vannucci L, Sima P, Richter J. Beta Glucan: Supplement or Drug? From Laboratory to Clinical Trials. Molecules. 2019;24(7):1251. https://doi.org/10.3390/molecules24071251

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