In this topic we want to present toxins that naturally occur in plant foods. See also: Plant Food Contamination, Pesticides.

Toxins in Raw Food

Introduction

Plant defense against herbivores

Plants use several strategies to defend against damage caused by herbivores. Many plants produce  allelochemicals, that influence the behavior, growth, or survival of herbivores. These chemical defenses can act as repellents or toxins to herbivores, or reduce plant digestibility. While most plant defenses are directed against insects, the most significant herbivores, other defenses have evolved that are aimed at vertebrate herbivores, such as birds and mammals.

Antiherbivory compounds and can be classified into three sub-groups: nitrogen compounds (including alkaloids, cyanogenic glycosides and glucosinolates), terpenoids, and phenolics.

Alkaloids are derived from various amino acids. Over 3000 known alkaloids exist, examples include nicotine, caffeine, morphine, colchicine, ergolines, strychnine, and quinine. Alkaloids have pharmacological effects on humans and other animals.

Cyanogenic glycosides are stored in inactive forms in plant vacuoles. They become toxic when herbivores eat the plant and break cell membranes allowing the glycosides to come into contact with enzymes in the cytoplasm releasing hydrogen cyanide which blocks cellular respiration. The products can cause gastroenteritis, salivation, diarrhea, and irritation of the mouth.

The terpenoids, sometimes referred to as isoprenoids, are organic chemicals similar to terpenes. There are over 10,000 known types of terpenoids. Monoterpenoids are volatile essential oils such as citronella, limonene, menthol, camphor, and pinene. Diterpenoids are widely distributed in latex and resins, and can be quite toxic. Plant steroids and sterols are also produced from terpenoid precursors, including vitamin D, glycosides (such as digitalis) and saponins (which lyse red blood cells of herbivores).

Phenolics, sometimes called phenols. Some phenols have antiseptic properties, while others disrupt endocrine activity. Phenolics range from simple tannins to the more complex flavonoids that give plants much of their red, blue, yellow, and white pigments. Complex phenolics called polyphenols are capable of producing many different types of effects on humans, including antioxidant properties. Condensed tannins inhibit herbivore digestion by binding to consumed plant proteins and making them more difficult for animals to digest, and by interfering with protein absorption and digestive enzymes. Silica and lignins, which are completely indigestible to animals, grind down insect mandibles.

In addition to the three larger groups, fatty acid derivates, amino acids and even peptides are used as defence. 

Plant regulation of fruit consumption

Plants invest energy into the production of fruits. Plants have evolved to encourage mutualist frugivores to consume their fruit for seed dispersal but also evolved mechanisms to decrease consumption of fruits when unripe and from non-seed dispersing predators. Plants have chemical and physical adaptations.

Chemical deterrents in plants are called secondary metabolites. Secondary metabolites are compounds produced by the plant that are not essential for the primary processes such as growth and reproduction. Toxins might have evolved to prevent consumption by animals that disperse seeds into unsuitable habitats, to prevent too many fruits from being eaten per feeding bout by preventing too many seeds being deposited in one site, or to prevent digestion of the seeds in the gut of the animal.

Physical deterrents:

• Cryptic coloration (e.g. green fruits blend in with the plant leaves)
• Unpalatable textures (e.g. thick skins made of anti-nutritive substances)
• Resins and saps (e.g. prevent animals from swallowing)
• Repellent substances, hard outer coats, spines, thorns.

Common toxins

 Goitrogens

Goitrogens are substances that suppress the function of the thyroid gland by interfering with iodine uptake, which can, as a result, cause an enlargement of the thyroid. Certain raw foods  have been identified as lightly goitrogenic (cooking inactivates the goitrogens).

These foods include:

• soybeans, pine nuts, peanuts,
• strawberries, pears, peaches,
• spinach, bamboo shoots, , bok choy, broccoli, brussels sprouts, cabbage, canola, cauliflower, collard greens, horseradish, kale, kohlrabi, mustard greens,
• sweet potatoes, radishes, turnips,
• millet.

Fruit

Night­shades

The tox­ins found in Night­shades (scopo­lamine , atropine, sola­nine, nico­tine and more) may be the lead­ing cause of Arthritic inflam­ma­tion, can­cer,  Alzheimer’s, Migraine headaches, birth defects, mis­car­riages,  inflam­ma­tory bowel dis­or­der,  and Crohn’s Disease.

Then, in the 1600s and 1700s food and drug crops based on nightshades were imported from the Americas and for the past 400 years have penetrated and become ubiquitous in the Western diet. These include tobacco, tomatoes, potatoes and chili peppers. It is not surprising that these novel foods, being nightshades, were regarded with suspicion at first and were slow to take hold in the European diet. They all contain solanine in some form, named as solanine (potatoes), tomatine (tomatoes), alpha-solanine (aubergine) or solanadine (chillies and capsicums). They also contain nicotine in small amounts. Nicotine has a synergistic action with solanine -it stimulates the production of acetylcholine.

It is now apparent that there are groups of people who cannot tolerate nightshades in their diets, wish to avoid them anyway or find that eliminating them helps alleviate a variety of mental, emotional and physical problems.

Tomato

The leaves, stems, and green unripe fruit of the tomato plant contain small amounts of the poisonous alkaloid tomatine.

Greens

Celery

Celery is known to contain psoralens, a group of substances that cause a toxic dermal reaction on exposure to ultraviolet A rays (UVA).

Parsnip

Wild parsnip contains furanocoumarins, which deter herbivores from eating its foliage.

Roots

Sweet potato

Lin et al. (1985) have established that sweet potato shows trypsin inhibitor activity (TIA) ranging from 90 percent inhibition in some varieties to 20 percent in others. There is a significant correlation between the trypsin inhibitor content and the protein content of the sweet potato variety. Heating to 90°C for several minutes inactivates trypsin inhibitors.

In response to injury, or exposure to infectious agents, in reaction to physiological stimulation or on exposure of wounded tissue to fungal contamination, sweet potato will produce certain metabolites. Some of these compounds, especially the furano-terpenoids are known to be toxic (Uritani, 1967). Fungal contamination of sweet potato tubers by Ceratocystis fimbriata and several Fusarium species leads to the production of ipomeamarone, a hepatoxin, while other metabolites like 4-ipomeanol are pulmonary toxins. Baking destroys only 40 percent of these toxins.

Potato

Potato contains the glycoalkaloids alpha-solanine and alpha-chaconine (Maya, 1980), concentrated mainly in the flowers and sprouts (200 to 500 mg/100 g). In healthy potato tubers the concentration of the glycoalkaloids is usually less than 10 mg/100 g and this can normally be reduced by peeling (Wood and Young, 1974; Bushway et al., 1983). In bitter varieties the alkaloid concentration can go up to 80 mg/100 g in the tuber as a whole and up to 150220 mg/100 g in the peel. The presence of these glycoalkaloids is not perceptible to the taste buds until they reach a concentration of 20 ma/100 g when they taste bitter. At higher concentrations they cause a burning and persistent irritation similar to hot pepper. At these concentrations solanine and other potato glycoalkaloids are toxic. They are not destroyed during normal cooking because the decomposition temperature of solanine is about 243 C.

Levels of glycoalkaloids may build up in potatoes which are exposed to bright light for long periods. They may also result from wounding during harvest or during post-harvest handling and storage, especially at temperatures below 10°C (Jadhav and Salunkhe, 1975). Glycoalkaloids are inhibitors of choline esterase and cause haemorrhagic damage to the gastrointestinal tract as well as to the retina (Ahmed, 1982). Solanine poisoning has been known to cause severe illness but it is rarely fatal (Jadhav and Salunkhe, 1975).

Potato also contains proteinase inhibitors which act as an effective defense against insects and micro-organisms but are no problem to humans because they are destroyed by heat.

Lectins or haemogglutenins are also present in potato. These toxins are capable of agglutinating the erythocytes of several mammalian species including humans (Goldstein and Hayes, 1978), but this is of minimal nutritional significance as haemogglutenins are also destroyed by heat, and potatoes are normally cooked before they are eaten.

The modern practice of washing potatoes and packing them in plastic bags allows light to affect the potato and stimulate its production of solanine, the nightshade alkaloid that, in nature, sickens animals that might dig up potatoes for food. In 1976 the Department of Health, concerned about high levels of anencephaly and spina bifida, urged pregnant mothers to wear rubber gloves when preparing potatoes and to discard in their entirety any potatoes that showed signs of greening or of blight (black streaks in the potato) It is not enough to simply remove the discoloured part - the entire potato should not be eaten. The solanine in potatoes is 4 times greater in the skin than in the rest of the potato.

The fatal dose of solanine for an adult is 200-250 mg depending on body weight. Potatoes should not contain more than 20 mg of solanine per 100g, so it would take at least 1 Kg of potatoes (2.2 lbs) to be fatal. Potato peels have been found to contain up to 180 mg of solanine per 100g, so a person consuming 150-200g of deep fried potato peels with a high solanine content could be at considerable risk. Potatoes that have been properly stored and are from low solanine varieties will only contain 7 mg/100g. In 1996 the Committee on Toxicity stated that potatoes should not be eaten if they still taste bitter after the green parts and sprouts have been removed. However, few people taste-test a raw potato once it is peeled to assess its bitterness. Although spina bifida prevention now focuses on preconceptual consumption of folic acid, the world’s highest incidence of spina bifida is in Ireland, where the wet climate encourages late potato blight. A study in Belfast showed that mothers who had given birth to a child with spina bifida or anencephaly could reduce the risk of a similar defect in the second child by 50% if they maintained a potato-free diet.

Solanine and chaconine in potatoes are not very dangerous unless big quantities are eaten. They don't accumulate in the body, but they are not destroyed by heat.

Cassava

Though not too widely used in the United States, cassava is a woody shrub that is generally found in the Caribbean and South America. When using cassava, it can either be made to be sweet or bitter. The taste, as well as the smell, all depends on the amount of cyanogenic glucosides, which are in fact, extremely poisonous. Most who prepare it like it to be bitter, as it keeps away insects and even animals. If cassava is prepared incorrectly, it can be deadly. Cassava poisoning, due to high levels of cyanide, is known as Konzo. Cassava poisoning leads to irreversible paralysis.

Mushrooms

Indeed, I recommend that no mushrooms be eaten raw. In addition to the heat labile hemolysin, some mushrooms contain potentially carcinogenic hydrazines that are deactivated on cooking. Others contain an enzyme that blocks protein uptake. Mushrooms can be very nutritious after cooking but are generally hard to digest or worse when eaten raw. Paradoxically, several strains of the common commercial button mushroom including the now popular Portobello, contain significant levels of Agaritine, a potentially carcinogenic hydrazine that is bioactivated by cooking.

Many hydrazines are known to be strong carcinogens and can be found in a lot of edible mushrooms. Cooking destroys some or all of the hydrazines, but the steam given off during cooking has been known to make some cooks ill. Besides this fact, the structural material or cell walls in mushrooms is made of chitin, and humans don’t have the ability to digest this derivative of cellulose. The body can do several things to this undigested chitin. It can expel it by vomiting or send it the other way with diarrhea. Small amounts may pass through the gut with other food and go unnoticed, or it may stay in the gut where bacteria will work on it causing bloating, gas, and other discomfort. Cooking does not destroy chitin but may ease its effect.

Samples of Shiitake, verified as being produced without any formaldehyde treatments, were found to produce similar levels of formaldehyde. In humans, it is known to have a number of damaging effects and there is limited evidence of a carcinogenic effect.

Flagellate skin lesions occur in some patients after eating the mushroom Lentinus edodes, and they are called shiitake dermatitis in Japan.

Seeds and sprouts

Seeds are generally made to be indigestible. Plants "want" that animals swallow the fruit and "deposit" the seeds in the midst of some "fertilizer" on a new territory. The seed have more chances then to sprout into a new plant. Sprouts are normally protected from been eaten by toxins in them.

Alfalfa

Small-seeded legumes like alfalfa and broom have developed a protective chemical called canavanine. Alfalfa seeds are about 0.5% canavanine, compared with 13% in seeds of the tropical legume Dioclea megacarpa. Just 0.02% canavanine can harm insect larvae. Any animal that ingests canavanine makes incorrect proteins that malfunction as enzymes. The damage is non-specific and widespread, affecting RNA and DNA metabolism, as well as a key enzyme for destroying alcohol. Because it messes up so many aspects of metabolism, canavanine is a highly toxic chemical to animals. Pigs refuse to eat feed containing too much canavanine. Although we humans are not immune to canavanine, we don't seem to taste it.

Canavanine is a natural inhibitor of cell growth and is excreted by germinating alfalfa seeds. It was proved that the canavanine present during germination of alfalfa both in the dark and in the light does not decrease and in fact increases a little, despite earlier papers having described the disappearance of canavanine during germination of alfalfa. The presence of canavanine in alfalfa sprouts was confirmed by isolation and comparison with an authentic specimen.

Cashews

The so-called raw cashews sold in natural food stores are not exactly raw, but instead are steamed. It is the case that the double shell surrounding the raw cashew, which is technically a seed and not a nut, contains urushiol, a resin that can create significant skin rashes, and can be toxic when ingested. ... Processing raw cashews can be a laborious and nightmarish ordeal, and people who work in cashew processing plants tend to exhibit greater allergies to cashew shells over time. There is a high incidence of skin rashes among people who either harvest or process raw cashews. Greater sensitivity to urushiol can lead to extreme allergic reaction when raw cashews are ingested, and anyone allergic to poison ivy could potentially have a fatal reaction to eating true raw cashews. ... At some time, pre-dating written history, the people of Brazil figured out that the fruit surrounding the cashew “nut” could be eaten, but the shell could not. Also, early Brazilians were able to understand that the nut could be used when steamed or cooked.

Buckwheat sprouts

The basic problem with buckwheat greens is that they contain fagopyrin, a naturally occurring substance in the buckwheat plant. When ingested in sufficient quantity, fagopyrin is known to cause the skin of animals and people to become phototoxic, which is to say hypersensitive to sunlight.

In addition to the burning sensation, people suffering from fagopyrism often complain of feeling a numb, fuzzy, buzzing impression when they scratch or inadvertently hit their hands on something. Sufferers also report that their hands feel painfully cold when placed in cold water or when in contact with a cold object, even if only for a few seconds. Some have had numbing, itching and tickling on the face, nose and ears after sun exposure causing a desire to scratch the skin, thus worsening the condition. The eyes may also become hypersensitive to light. If a large enough portion of the skin is exposed, one may experience dizziness. If a person is forced to remain in the sun for a long period of time, the skin may swell up and remain abnormal for up to several days.

Fagopyrin is not activated by ultraviolet light but reacts to a different portion of the sunlight spectrum. Therefore, the normal application of sunscreen offers no protection at all. Glass, which filters out ultraviolet rays, does not however filter out the reactive range relative to fagopyrin. This means that sunlight coming through home or car windows also causes a phototoxic reaction.

Grains

Loren Cordain, an anthropologist at Colorado State University:

For the vast majority of mankind’s presence on this planet, he rarely if ever consumed cereal grains. With the exception of the last 10,000 years following the agricultural ‘revolution’, humans have existed as non-cereal-eating hunter-gatherers since the emergence of Homo erectus 1.7 million years ago… the inability of humans to physiologically overcome cereal grain anti-nutrients such as phytates, alkylresorcinols, protease inhibitors, and lectins is indicative of the evolutionary novelty of this food for our species.

Lectins are toxic protein compounds found in heavy amounts in many seeds, grains and legumes. Large amounts of lectins can damage the heart, kidneys and liver, lower blood clotting ability, destroy the lining of the intestines, and inhibit cell division.

The antinutrients in grains anti-amylases and phytates are affected by heating, but phytates require  fermentation for further neutralization, which is only partial. Soaking and sprouting reduces phytates.

Soaking on its own was not found to be a good method for improving mineral bioavailability but the results showed that, in combination with other treatments, or with optimized soaking conditions, it could nevertheless prove useful.

Wheat

Wheat germ agglutinin (WGA) perturbes the integrity of the gastrointestinal epithelium and increased the permeability of the tissue in a dose- and time-dependent manner. WGA did not induce cell death but increased the permeability of individual cells to 7-AAD (a fluorescent chemical compound) which is normally not uptaken by viable cells. These data allows to define a toxicity threshold for WGA on epithelial cells.

WGA induced extensive hyperplastic and hypertrophic growth of the small bowel by increasing its content of proteins, RNA and DNA. An appreciable portion of the endocytosed WGA was transported across the gut wall into the systemic circulation, where it was deposited in the walls of the blood and lymphatic vessels. WGA also induced the hypertrophic growth of the pancreas and caused thymus atrophy.

WGA lectin is an exceptionally tough adversary as it is formed by the same disulfide bonds that make vulcanized rubber and human hair so strong, flexible and durable. Like man-made pesticides, lectins are extremely small, resistant to break-down by living systems, and tend to accumulate and become incorporated into tissues where they interfere with normal biological processes. Indeed, WGA lectin is so powerful as an insecticide that biotech firms have used recombinant DNA technology to create genetically modified WGA-enhanced plants.

WGA has been overshadowed by the other proteins in wheat. ... These pharmacologically active, opiate-like proteins in gluten are known as gluten exorphins (A5, B4, B5, C) and gliadorphins. They may effectively anesthetize us, in the short term, to the long term, adverse effects of WGA. Gluten also contains exceptionally high levels of the excitotoxic l-aspartic and l-glutamic amino acids, which can also be highly addictive, not unlike their synthetic shadow molecules aspartame and monosodium glutamate.

WGA is most concentrated in the seed of the wheat plant, likely due to the fact that the seeds are the “babies” of these plants and are invested with the entire hope for continuance of their species. Protecting the seed against predation is necessarily a first priority. WGA is an exceedingly small glycoprotein (36 kilodaltons) and is concentrated deep within the embryo of the wheat berry...

According to the U.S. Centers for Disease Control it takes only 500 micrograms (about half a grain of sand) of ricin (a lectin extracted from castor bean casings) to kill a human. A single, one ounce slice of wheat bread contains approximately 500 micrograms of WGA, which if it were refined to its pure form and injected directly into the blood, could, in theory, have platelet aggregating and erythrocyte agglutinizing effects strong enough to create an obstructive clot such as occurs in myocardial infarction and stroke.

Dr. Paul L. McNeil, cell biologist at the Medical College of Georgia:

It's possible that this bioactive property of lectin that binds to our cells could have long-term consequences taken even in small amounts. ... Maybe the bloating and gas is telling us something about lectin when it's just a minor irritation.

Cooking destroys most but not all lectin. Lectin is among the top-10 causes of food poisoning.

Good info !  Just goes to show you can "overdose" on almost anything and the toxins will get you.  I once ate almost nothing but great wild mangoes in Africa for days.  Tasted great.  Felt great. - Then got a weird skin outbreak that I'd never had before or since... (Quit the large quantities of mangoes and the skin thing went away.) 

Yes, transformation of quantity into quality  (dialectics :), seems to be one of the key considerations in choosing food.

Do you remember the sort of the mangoes? How did they look?

I was experimenting with mono-eating papaya for days few times (big Caribbean papaya) and I did not felt well on the 3-4th day. Even my body got papaya aroma. I quit and did not repeat the regime again. I found later some data on light toxins in it and other types of fruit - I need to search for it again. I did not happen to me on apple- grapes- and watermelon-dieting (monotrophicly). OK, a beginning of another chapter:

Legumes

Enzyme inhibitors

"It has been shown that certain foods have high amounts of enzyme inhibitor, notably soybeans and other legumes. When on a diet of high soy, animals would develop larger pancreases, and their enzyme secretion would increase."

Kidney beans

Raw red kidney beans at a level as low as 1% of diet cause death rats in two weeks.

Not sure, it was many years ago, a typical sort of mango, nice and ripe, good quality, right off the tree.  Just too much of a good thing I guess...

Also I found that tolerance for certain types of foods changes with age (or internal microflora ?).  For example, I used to love avocados, now they give me stomach aches (maybe I should try really small quantities and work up).  And btw. the same happened to me (stomach ache) with a papaya recently (not sure what type of papaya).  It's no big deal.  I'll just try them again some other time. 

Wow, what an eye-opening resource you have compiled for us here Lena! It's amazing how seriously brainwashed we have been into eating so many of these foods—especially potatoes and tomatoes!

I assume ripe tomatoes are ok?

Completely ripe tomatoes are perfectly OK :)

Green Tomatoes

Green tomatoes contain the highest concentration of the alkoloid solanine, called tomatine. Up to 500 mg per 1 kg of fruit weight can be found in an immature, green tomato. In many regions of the United States, especially in the south, green tomatoes are used in culinary creations, usually in relishes or fried. The green tomatoes used in these dishes are in the early stages of ripening, though still green to green yellow color, and contain lesser amounts of solanine than the harder, immature green tomato. Additionally, frying the green tomatoes in hot oil diminishes the solanine content, rendering the green tomato less toxic.

Red Tomatoes

The solanine, or tomatine, level dramatically decreases in fully ripe tomatoes, to levels less than 5 mg per 1 kg of fruit weight. These low levels pose no health problems to humans, but even in the ripened form the solanine remaining in a tomato is harmful to cats. Vine ripened tomatoes, fresh from the garden are the safest for low solanine content. Hothouse grown tomatoes that are often sold in grocery stores will contain higher levels of solanine. This is because the hothouse and commercial tomatoes have been harvested while the fruit is still green, just beginning to turn, and still have a firm, hard feel. The softer and sweeter the ripe tomato is, the less solanine it contains.

• California Polytechnic State University; NightShade Alkaloid Toxins; Lisa Branstad
• Dr. Abdul Aljuboori: Beware Eating Green (Unripe) Potatoes, Tomatoes and Eggplant
• Journal of Agricultural and Food Chemistry; Tomato Glycoalkaloids: Role in the Plant and in the Diet; Mendel Friedman; 2002

• Juicing for Health: Tomato-Nutrition Information And Facts
• University of Missouri; Growing Home Garden Tomatoes; David H. Trinklein; April, 2010

But not overripe till being spoiled though:

Among 27 mold isolates from decaying tomatoes, culture filtrates or ethyl acetate extracts of 8 isolates grown in yeast extract-sucrose medium were markedly toxic (mortality, greater than 50%) to brine shrimp larvae. The toxicity of six of these isolates could be attributed to the presence of citrinin, tenuazonic acid, or T-2 toxin. Ethyl acetate extracts of five Alternaria isolates and one Fusarium isolate were mutagenic for Salmonella typhimurium strains. In ripe tomatoes inoculated with toxin-producing isolates and incubated at 25 degrees C, one Alternaria alternata isolate produced tenuazonic acid in seven of seven tomatoes at levels of up to 106 micrograms/g and alternariol methyl ether in one of the seven tomatoes at 0.8 microgram/g. Another A. alternata isolate produced tenuazonic acid or alternariol methyl ether at much lower levels in only three of seven tomatoes. Patulin and citrinin were produced by a Penicillium expansum isolate at levels of up to 8.4 and 0.76 microgram/g, respectively. In tomatoes incubated at 15 degrees C, a Fusarium sulphureum isolate produced T-2 toxin, HT-2 toxin, and neosolaniol at levels of up to 37.5, 37.8 and 5.6 micrograms/g, respectively. If these mycotoxins are thermostable, they may occur at detectable levels in tomato products whenever partially moldy tomatoes are used as raw material.