Mushrooms are not plants at all. In the classification of organisms of the world they belong to the Kingdom of Fungi which also includes yeasts and molds. Fungi are organisms that live on alive or dead animals, plants or other fungi. Some fungi thrive in water; some float in the air. Mushrooms generally grow in soil or on trees. Fungi do not obtain their food like animals by eating it nor by harnessing the sun’s energy through photosynthesis like plants. Fungi gain their energy by absorbing it from other organisms. They grow in or around a food source and then secrete enzymes that digest the food so that they can absorb the released nutrients.
Mushrooms are valued for their compelling taste and the unusual texture they add to our meals but many people do not realize that they contain some rare bioactive compounds, constituents that can contribute to our overall health. An estimated 50% of edible mushrooms species are considered functional foods, foods that exert positive effects on health above and beyond basic nutrition (43).
Mushrooms are low in fat and high in nutrients. They contain 2% to 6% of their dry weight in fat, 50% to 65% in carbohydrate and 15% to 35% in protein depending on the species. Mushroom fat is mostly unsaturated fatty acid in the form of linoleic acid. Their protein contains all the essential amino acids but is especially rich in lysine and leucine. Mushrooms are a significant source of potassium, calcium, phosphorus, magnesium, sodium and selenium. They also contain the B vitamins thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), and biotin (B7) as well as ascorbic acid (vitamin C) (20,21,22).
BIOACTIVE COMPOUNDS FOUND IN MUSHROOMS
Mushrooms are a rich source of antioxidants, an integral part of the immune system. They neutralize the damaging free radicals that result from oxidation and play a role in preventing and fighting chronic diseases such as cardiovascular disease and cancer. White button mushrooms (which become crimini mushrooms and then portobello mushrooms as they mature) are the most common mushrooms found in stores and are second only to porcini mushrooms in antioxidant levels. Below them in antioxidant content are maitake and oyster mushrooms. The mushroom with the least number of antioxidants is the chanterelle (19).
Ergothioneine is a unique amino acid that cannot be synthesized by humans. It is so important to the human body that our bodies have a specific transporter protein for latching on to the ergothioneine offered by the foods we eat and ushering it into our cells. Ergothioneine protects cells, acting as an antioxidant in tissues most sensitive to oxidation such as blood, liver, bone marrow, semen and the lenses of our eyes. It is taken up into the cell nucleus and the mitochondria, protecting both DNA and the energy-producing machinery of the cell (28). DNA inside the mitochondria is particularly vulnerable to free radical damage due to the difficulty of many antioxidants in penetrating the mitochondrial membrane.
Because our bodies cannot make ergothioneine, we must obtain it from food. It is not produced by animals or plants but only by some bacteria and fungi such as mushrooms. In fact mushrooms are the best source of ergothioneine, containing forty times more than black or red beans, the next best source. Mushrooms contain about 0.4 to 2.0 mg/g of dry weight of ergothioneine with the highest amount found in shiitake, maitake and oyster mushrooms (1). There is a small amount of ergothioneine in organ meats such as kidney or liver and some also in vegetables, grains such as oat bran and nuts and seeds. Only tiny amounts are present in fish and eggs and there is none at all in fruit or dairy products (27).
CONJUGATED LINOLEIC ACID (CLA)
Mushrooms are a source of conjugated linoleic acid (CLA), an unusual fatty acid that can bind to and inhibit the aromatase enzyme that promotes the production of estrogen. Most breast cancers (and some prostate cancers) depend on estrogen to encourage their growth. Tumours themselves can hijack aromatase enzyme and use it to convert testosterone to the estrogen that they need. Many mushroom types, including white button, portabello, crimini and shiitake, demonstrate significant anti-aromatase influence (9,16). Assays to determine the particular component of mushrooms that exert this effect point to conjugated linoleic acid (17). Furthermore, conjugated linoleic acid seems to be able to block DNA synthesis in cancer cells and so reduce their proliferation (9,16). Other studies have shown conflicting results however and further research is needed to confirm the extent of the benefit of CLA in breast cancer.
Beta-glucan is a soluble fiber found in the cell walls of cereals, mushrooms, yeasts, seaweed and algae. Portobello mushrooms contain about 0.2% by weight of beta-glucan while most other mushrooms consist of about 0.1% (41). Beta-glucans in the small intestine exist as a thick gel that delays the absorption of nutrients. The beta-glucan in mushrooms may have a beneficial role in lowering insulin resistance and blood cholesterol, reducing obesity and boosting the immune system (35,36). A recent study suggests that beta-glucans from mushrooms have the potential to lower blood sugar levels by between 17% and 25% (37). Additionally beta-glucans have certainly demonstrated their ability to lower blood cholesterol levels (40). Many studies around the world have looked at beta-glucans and their effect on the immune system and cancer. Japan has approved the use of a beta-glucan compound derived from mushrooms for treating several types of cancer since the 1980s. Beta-glucans do not appear to directly kill cancer cells but stimulate immune responses that damage cancer cells. There is no research showing that beta-glucans can help to prevent cancer (38,39).
Mushrooms contain lectins that are able to prevent cancer cells from growing and dividing (11,23,29). In fact, lectins from many different sources are now being investigated as potential cancer therapy (30,31). Don’t worry about recent claims that lectins are dangerous. The only lectin that can cause problems in human beings is one found in raw beans, especially red kidney beans. Fortunately this lectin is completely inactivated by cooking. The rule of thumb is that when beans are cooked until they are tender enough to eat, the lectin has been completely destroyed. Eating raw beans can cause gastrointestinal distress (nausea, vomiting, diarrhea and stomach pain) that can be severe but is temporary. Most people recover quickly without requiring hospitalization.
Selenium is a trace element that is essential in small amounts but can be toxic at high levels. While Brazil nuts are the best source of selenium, actually providing more than the recommended daily allowance of 55 to 60 mcg in just one nut, mushrooms, especially portabella, shiitake and crimini mushrooms, are also a good source. One-half cup of cooked or raw mushrooms contain 10 to 21 mcg, about half the daily recommended allowance (33). The safe upper limit of selenium in adults is 400 mcg daily.
Selenium deficiency has been associated with impaired immunity and chronic inflammation. Selenium is an important component of antioxidant enzymes, acting in synergy with vitamin C and E. Its calming effect on inflammatory responses may play an important role in the prevention of chronic health conditions (34). Selenium has been associated with lower risk of cancer and cancer-related death from bladder and prostate cancer. Study results have been variable in breast cancer and no cancer risk protection was found for lung, colorectal or gastric cancer (34). Theoretically, optimal selenium levels should result in a decrease in cardiovascular risk but current research in this direction is so far inconclusive. Selenium is also important in the production of sperm and for male fertility. Research on the role of selenium in health continues. (34).
Most mushrooms available at our local supermarkets contain very little vitamin D because mushrooms are generally grown in the dark. However they do contain lots of ergosterol which can be converted into vitamin D by ultraviolet radiation. If you happen to find mushrooms that were grown outdoors or UV-irradiated they will be a good source of vitamin D2, the same type of vitamin D obtained from most supplements available on the market. Studies show that UV-irradiated mushrooms are as effective as vitamin D supplements at raising vitamin D blood level (32).
POTENTIAL HEALTH BENEFITS OF MUSHROOMS
Many studies point to mushrooms as a factor for reducing the risk of cancer. Mushrooms contain antioxidants, conjugated linoleic acid, beta-glucan, selenium and lectins, all of which may play a role in this benefit. The high levels of antioxidants in mushrooms fight against oxidative stress and boost the immune system and are being studied in relation to breast, stomach, prostate and colorectal cancers (10). White button mushrooms enhance the activity of natural killer cells in mice. (Natural killer cells are a type of white blood cell (lymphocyte) that can destroy infected or cancerous cells.) More research is needed to look at this effect in humans (7). Studies in animals show that extracts of shiitake mushrooms can trigger apoptosis (self-destruction) of cancer cells although whether this is the result of one bioactive compound alone or a combination of them is unknown (17,18). As discussed above, conjugated linoleic acid (CLA) is the most likely mushroom bioactive compound that is inhibiting aromatase, the enzyme required for the production of estrogen, a needed compound for the growth of many breast cancers. White button mushrooms can suppress aromatase activity by about 65%, more than any other food and more than any other type of mushroom too (9).
A Korean study found that females with the highest mushroom intake had a lower risk of breast cancer (2). Another study showed that adding 15 gm of dried button mushrooms per month to the diet (about one mushroom per day) reduced breast cancer risk by 47% while consumption of 30 gm of fresh mushrooms per month lowered breast cancer risk by 64% (8). Combining mushrooms with green tea produced an even greater effect. Breast cancer risk decreased by 89% in those consuming both mushrooms and green tea compared to those who ingested neither food (8). Finally, a 2014 meta-analysis looked at studies in both premenopausal and postmenopausal women concerning their intake of mushrooms of any type. The conclusion was that daily mushroom consumption of 20 gm reduced the risk of breast cancer by 33% (23).
BOOSTING IMMUNE FUNCTION
Mushrooms are rich in phytochemicals that enhance the immune system and can help the body fight off infections and tumours (13,14). About 1 cup a day of regular white mushrooms results in a 50% increase in the production of immunoglobulin A, a protective immune substance in the mucosa (mouth, nose and throat) which helps prevent respiratory infections. In this study immunoglobulin A levels remained high for a week after stopping mushroom consumption (12).
PREVENTION OF ATHEROSCLEROSIS
Once again white button mushrooms rise to the top with their effects on atherosclerosis, the build-up of fat-filled deposits on the inner walls of arteries. In an inflammatory environment, mushrooms prevent white blood cells from binding to the endothelial cells that line blood vessels. In so doing they interfere with atherosclerosis and protect against cardiovascular events such as heart attacks and strokes (26).
LOWERING BLOOD CHOLESTEROL
Mushrooms contain beta-glucans which can alter fat metabolism so that the total cholesterol content of the blood is decreased. Reductions in body weight can also occur. (40).
Mushrooms are a food with “umami” qualities, a rich taste and feel in the mouth. Umami originates with the presence of an amino acid called glutamate and creates a satisfying sense of full-bodied flavour that subtly lingers long after the food is swallowed. A small study noted that energy intake from mushroom meals was less than half that from meat meals and this lower intake was only partially compensated for during the four days following the mushroom-containing meal. Subjects reported no difference in palatability or hunger ratings between the two meals (3). Another randomized, controlled study followed two groups, one of which continued to eat meat while the other substituted 8 ounces of fresh mushrooms for 8 ounces of meat three times a week. Both ate a diet that was 500 kcal less than needed to maintain present body weight. Those eating the mushroom meals experienced greater losses in weight, waist circumference and BMI than those in the meat group and were able to maintain the losses over time. These results were not statistically significant however and further studies are needed to define the effect of substituting mushrooms for meat in weight loss (4).
BENEFITS IN NEURODEGENERATIVE DISEASES
Ergocalciferol-enriched (Vitamin D) mushrooms may protect the brain from beta-amyloid plaque toxicity and delay the onset of dementia and Alzheimer’s Disease (1). Ergothioneine is also showing promise in the prevention of neurodegenerative diseases such as Alzheimer’s Disease and Parkinson’s disease. Very recent research discovered that mushrooms contain extremely high levels of both ergothioneine and another antioxidant, glutathione, which may act synergistically to decrease the likelihood of these two diseases (42). This data is very preliminary however and further research is needed to elucidate the potential role of mushrooms in cognitive impairment and other neurodegenerative conditions.
GUT MICROBIOTA BENEFITS
Adding mushrooms to the diet results in increased gut bacterial diversity in mice including increases in the beneficial Bacteroidetes bacteria and decreases in the harmful Firmicutes type. The rate of healing from gastrointestinal injury also increased (6). A healthy gut microbiome protects against pathogens and supports the immune system.
In a randomized controlled trial, a mouthwash containing shiitake mushroom extract was found to reduce plaque and oral bacteria pathogens significantly more than either water or a leading gingivitis mouthwash (5).
A LITTLE CAUTION ….
Mushrooms contain a substance called agaritine that is a potential carcinogen. Some studies on mice have suggested an increase in risk of cancer from eating mushrooms. Recent research in humans indicated that high intake of raw mushrooms (about 100 gm per day) resulted in a possible 20% increase in total cancer risk (24). A 2013 study of 25,531 participants followed for eight years found only a small non-significant increase in cancer risk from mushroom consumption and no increased risk of type-2 diabetes or cardiovascular disease (25). Most telling though was a very comprehensive study in mice that found a significant increase in mutagenic potential only from an extract of agaritine. Ingestion of whole mushrooms resulted in slight but not significant changes. Researchers estimated that the amount of agaritine a typical mushroom eater is exposed to over a lifetime (about 4 gm per day) might potentially lead to one extra case of cancer per 50,000 lifetimes. In other words, if agaratine is indeed a carcinogen, it is a pretty weak one (44).
Conveniently the concentration of agaritine in mushrooms can be reduced by many means. Freezing and then thawing causes significant reductions in agaritine concentration. Boiling can reduce agaritine by 75% if the broth in which the mushrooms were boiled is discarded. Microwaving for one minute inactivates about 65% of agaritine content. Dry baking for 20 minutes removes 40% and 45 minutes of baking removes 89% of the arginine (15). Fortunately cooking mushrooms does not affect their antioxidant content. Taking all this information into account, it is probably prudent to limit ingestion of raw mushrooms but there is no need to fear cooked mushrooms.
Mushrooms are a culinary treat quite unlike other foods. They are naturally low in sodium and do not require the addition of salt due to their innate fulfilling taste. Best of all, their richness in unusual but remarkable components can be of great benefit for our health.
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