Plant food is heavily promoted as a healthy and important part of human diet. However, many plants contain various natural toxins. These toxic compounds in plants, including tannins, saponins, glucosinolates, cyanogenic glycosides, furocoumarins, lectins, and oxalates, are part of plants’ defense mechanisms against pests, fungi, and predators, including humans. They can impact our health in various ways, causing nutritional deficiencies and toxic effects when consumed in high amounts. For instance, grains and certain vegetables and herbs are rich in saponins and tannins. Nightshade vegetables are known to contain toxic alkaloids.

Adding to the complexity, genetically modified (GM) foods, such as GM corn and soy, carry their own potential health risks. GM plants are created in labs where genetic material from one species is inserted into the DNA of an unrelated plant. Some GM crops are designed to survive high doses of toxic weed killers or to produce their own insecticides. These modifications lead to the presence of these toxins in our food, contributing to health disorders like cancer, birth defects, and hormone disruption.

Evolutionary Fitness

Evolutionary fitness and the production of toxic compounds in plants are deeply intertwined. As immobile organisms, plants have developed elaborate metabolic systems to produce a range of specialized compounds as a means to survive in their environment. These compounds play a crucial role in plant survival and reproduction. Their presence and diversity are a product of adaptation to challenging environments. The biological functions conferred by these compounds include deterring herbivores, resisting microbial pathogens, and coping with environmental stressors. The development of these traits often involves adaptive mechanisms at various levels, from subcellular to interspecies levels​​.

One classical theory explaining the evolution of plant chemical diversity suggests that new defenses arise from a pairwise co-evolutionary arms race between plants and their specialized natural enemies. However, this perspective has been challenged and expanded, with new research suggesting that plant defense chemicals can evolve under multiple and potentially contrasting selective pressures.

Natural toxins

Many of these plant-produced compounds, while beneficial to the plant’s survival, can pose risks to human metabolic health. These substances, often referred to as “natural toxins,” are compounds produced by living organisms that can be toxic to other creatures when ingested. Plants produce them as a natural defense mechanism against predators, insects, microorganisms, or in response to environmental stresses like drought or extreme humidity. Toxin producing organisms like mould additionally introduce toxins into food through infestation​. These toxins can cause a variety of adverse health effects, ranging from allergic reactions to severe gastrointestinal issues, and even death in extreme cases. Long-term health consequences can include impacts on the immune, reproductive, or nervous systems, and even cancer​.

When consumed by humans, these natural toxins can cause a range of adverse health effects. Acute poisoning can result in allergic reactions, severe stomachache, diarrhea, and even death in severe cases. The World Health Organization (WHO) and the Food and Agriculture Organization of the United Nations (FAO) jointly convene an international body, JECFA, to evaluate the health risks from natural toxins in food, while the Codex Alimentarius Commission establishes international standards and codes of practice to limit exposure to these toxins​.

Cyanogenic glycosides

Plants produce cyanogenic glycosides as a defense mechanism against herbivores. When a plant’s tissue is damaged, for example by being chewed or crushed, enzymes present in the plant break down the cyanogenic glycosides, releasing hydrogen cyanide. This hydrogen cyanide is toxic and can deter or harm animals that eat the plant.

Effects

In terms of human health, the consumption of food high in cyanogenic glycosides can have the following effects:

1. Cyanide Poisoning: If a large amount of cyanogenic glycosides is consumed and metabolized, it can lead to cyanide poisoning. Symptoms can range from headache, dizziness, and confusion to seizures, loss of consciousness, and even death in severe cases. This is riskier if a person consumes a large amount of raw food high in these compounds, like certain types of cassava or improperly prepared lima beans.
2. Thyroid Issues: Chronic consumption of foods high in cyanogenic glycosides may contribute to thyroid issues. Cyanide can interfere with the body’s ability to use iodine to produce thyroid hormones.
3. Neurological Effects: Chronic exposure to lower levels of cyanide, such as might occur from a diet consistently high in foods with cyanogenic glycosides, might contribute to neurological issues. This is a particular concern in parts of the world where foods high in these compounds, like cassava, are a dietary staple.

Furocoumarins

Furocoumarins are part of a larger group of compounds called phytoalexins. Plants synthesize phytoalexins in response to stress or damage, such as during an attack by insects or fungi. These compounds are particularly known for their phototoxic properties, meaning they become toxic when exposed to UV light. This property can deter herbivores from consuming the plant and can kill or inhibit the growth of microorganisms.

Effects

When it comes to human health, furocoumarins can have the following effects:

1. Phototoxicity: The most notable effect of furocoumarins in humans is their phototoxicity. When a person consumes a food high in furocoumarins and then exposes their skin to UV light, the furocoumarins can cause a reaction that leads to inflammation and damage to the skin. This reaction, known as phytophotodermatitis, can result in redness, itching, and blistering.
2. Drug Interactions: Furocoumarins, particularly a subset called furanocoumarins, can interfere with the metabolism of certain medications. This interference leads to increased blood levels of these medications and can cause harmful side effects. Doctors often advise people taking certain medications, including some types of statins, to avoid grapefruit, due to its high level of furanocoumarins.
3. Cancer Risks: Some research has suggested a potential link between furocoumarins and skin cancer, likely due to their phototoxic properties. However, researchers must work further to fully understand this potential risk.

Lectins

Lectins are a type of protein that can bind to specific carbohydrate molecules. A wide variety of organisms produce these proteins, including plants. The main purpose of lectins in plants is to defend against insects, microorganisms, and larger animals. They can bind to the carbohydrates present on the surface of cells in the gut of these animals. This binding causes various detrimental effects that discourage further consumption of the plant.

Lectins can also play a role in plant growth and development, and in the plant’s response to its environment. For example, they can help the plant recognize and respond to harmful microorganisms.

Effects

When it comes to human health, dietary lectins can have a variety of effects, and the specifics can depend on the type of lectin and the individual person. Here are some potential effects:

1. Gut Health: Some lectins can bind to the cells lining the gut, potentially damaging these cells and leading to a condition often referred to as “leaky gut”. This allows substances that should stay in the gut to pass into the bloodstream, leading to inflammation and other issues.
2. Nutrient Absorption: By binding to the cells in the gut, some lectins interfere with the absorption of nutrients, leading to deficiencies.
3. Immune Response: Some lectins can stimulate an immune response, leading to inflammation and other immune-related issues.
4. Effects on Blood Cells: Some lectins, like those found in uncooked red kidney beans, cause agglutination, or clumping together, of red blood cells. This can be harmful if the beans are eaten without being properly cooked first.

Oxalates

Effects

Oxalates in the human diet can have a few different effects. In general, most people can consume moderate amounts of oxalates without immediately noticeable issues. However, under certain conditions, a high-oxalate diet may contribute to kidney stones or other health issues.

1. Kidney Stones: Oxalate is a major component of the most common type of kidney stone, calcium oxalate stones. High levels of oxalate in the urine can combine with calcium to form these crystals. People who are prone to kidney stones or who have had them in the past are often advised to limit their intake of high-oxalate foods.
2. Nutrient Absorption: Oxalates can bind to minerals in the gut and interfere with their absorption. This is especially true for calcium and iron. When oxalate binds to these minerals, it forms insoluble crystals that cannot be absorbed and are instead excreted. This can potentially lead to deficiencies if intake of these minerals is not adequate.
3. Oxalosis: This is a rare genetic disorder in which the body doesn’t process oxalate properly. The deficient process leads to a buildup in the blood and urine. This can cause a variety of health problems, including kidney stones, heart and eye problems, and issues with the bones and skin.
4. Gut Health: Some research suggests that oxalates might negatively affect gut health, particularly in people with pre-existing gut issues. This is still a relatively new area of research, however, and more studies are needed to fully understand the relationship.

Glucosinolates

Plants produce glucosinolates primarily as a defense mechanism. When plant cells are damaged, such as when an insect chews on the plant leaf, the enzyme myrosinase breaks down glucosinolates into various compounds, including isothiocyanates, nitriles, and thiocyanates. These compounds have pungent flavors and are generally toxic to insects and other herbivores. This tandem helps protect the plant from being eaten.

Effects

There are some potential negative effects associated with consuming high amounts of glucosinolates. These include:

1. Thyroid Dysfunction: High intake of certain glucosinolates, particularly those found in foods like cabbage, broccoli, and cauliflower, can interfere with thyroid function. These compounds can inhibit the uptake of iodine by the thyroid gland. The uptake of iodine is necessary for the production of thyroid hormones. Over time, this can lead to goiter (an enlarged thyroid gland) and hypothyroidism (low thyroid function). This is especially an issue in individuals with an iodine-deficient diet.
2. Digestive Upset: Some people may experience digestive upset, including gas, bloating, and diarrhea, after consuming large amounts of glucosinolate-rich foods. This is generally due to the high fiber content and the sulfur-containing compounds in these foods.
3. Drug Interactions: There’s some evidence that high intake of cruciferous vegetables (which are rich in glucosinolates) can interfere with the effectiveness of certain medications, including blood thinners like warfarin.
4. Iron Absorption: There’s some evidence that compounds in glucosinolate-rich foods may interfere with the absorption of iron. This could potentially increase the risk of iron deficiency, especially in individuals who rely on plant-based sources of iron.

Tannins

Tannins, a category of polyphenolic compounds, are produced by a range of plants, encompassing fruits, nuts, and certain beans. Their production serves critical roles within the plant’s life cycle and survival. Firstly, tannins provide a defense mechanism against herbivores. By possessing a bitter taste and the ability to bind to proteins and other large molecules, they reduce digestibility, thereby deterring herbivores from consuming the plant. Secondly, tannins offer protection against pathogens. Their antimicrobial properties assist the plant in warding off bacterial and fungal pathogens, adding another layer of defense for the plant.

Effects

Tannins have noticeably negative effects when consumed in large amounts. Here are some of the potential negative effects:

1. Digestive Issues: Tannins can interfere with the digestion of proteins and other nutrients by binding to them, making them less available for your body to use. This is rarely an issue in humans, but it can be a problem in herbivores that consume a lot of tannin-rich foods.
2. Iron Absorption: Tannins can bind to iron and prevent its absorption in the digestive tract. This can potentially contribute to iron deficiency, especially in individuals who rely on plant-based sources of iron.
3. Enzyme Inhibition: Tannins can inhibit the activity of various digestive enzymes, including amylase, lipase, and trypsin. This can potentially interfere with the digestion and absorption of certain nutrients.
4. Stomach Irritation: In some people, consuming large amounts of tannin-rich foods or beverages, like strong tea, can cause stomach irritation or exacerbate existing stomach issues.

Saponins

Saponins are complex compounds prevalent in numerous plants, including everyday foods like beans, peas, and certain vegetables and herbs. The production of saponins by these plants serves key defensive roles. Firstly, saponins act as a deterrent against pests, utilizing their bitter taste and their capacity to disrupt the typical functioning of the pests’ cell membranes. Secondly, saponins exhibit antifungal and antimicrobial properties, enabling the plant to protect itself against detrimental fungi and bacteria. Thus, saponins represent an integral component of a plant’s survival strategy in its environment.

Effects

Despite their potential health benefits, including antioxidant properties and possible roles in reducing the risk of certain chronic diseases, saponins can also have negative effects when consumed in large amounts. Here are some of the potential negative effects:

1. Digestive Issues: In some individuals, consuming large amounts of saponins can lead to digestive issues like bloating, gas, stomach cramps, and diarrhea.
2. Interference with Nutrient Absorption: Saponins can bind to certain minerals and interfere with their absorption in the gut. However, this effect is generally not significant enough to cause nutrient deficiencies in people unless consumed in moderate to large proportions.
3. Hemolytic Activity: In their raw form, some saponins can cause the rupture (hemolysis) of red blood cells. This is generally not a concern with dietary saponins, as cooking typically deactivates this property. However, it can potentially be a concern with certain raw foods or supplements.

Alkaloids

The primary reason why plants produce alkaloids is for defense. Alkaloids are typically bitter and can deter herbivores from consuming the plant. They can also have various effects on the metabolism of the animals that ingest them, which can further discourage consumption. Alkaloids can inhibit the growth of other plants and some microorganisms as well, providing a competitive advantage for the plant that produces them.

The production of alkaloids is not limited to any single part of the plant. They can be found in roots, stems, leaves, flowers, seeds, and fruit, although the concentration can vary widely from one part of the plant to another, and from one species to another.

Effects

Alkaloids can have a wide range of physiological effects, and many are poisonous to humans and animals. The specific effects depend on the particular alkaloid, but they can include damage to the liver and kidneys, respiratory failure, and even death in severe cases.

Solanine, an alkaloid found in potatoes and other plants in the nightshade family, can cause nausea, diarrhea, vomiting, stomach cramps, burning of the throat, cardiac dysrhythmia, nightmares, headache, and dizziness. In more severe cases, hallucinations, loss of sensation, paralysis, fever, jaundice, dilated pupils, hypothermia, and even death have been reported. Coniine, an alkaloid found in poison hemlock, can cause paralysis and death by respiratory failure.

GMOs

Two types of GM crops are common: those that are herbicide tolerant, and those that self-generate insecticides.

• For herbicide-tolerant crops, some of the toxins from the weed killers used on these crops can end up in our food.
• Self-generating insecticide crops, such as BT-toxin crops, produce their own insecticides in each cell.

In this article we will focus on the most infamous of self-generating insecticides, BT.

BT-Toxins

Bacillus thuringiensis (Bt) is a bacterium introduced into GMO crops that produces protein crystals during the sporulation phase of its growth. These crystals contain a protein that is toxic to certain types of insects, particularly larvae of moths and butterflies, beetles, and flies. When an insect consumes the Bt protein, it binds to receptors in the insect’s gut, causing the gut wall to break down and allowing gut bacteria to invade the insect’s body. As a result, the insect stops eating and soon dies.

Bt has been used as a pesticide for many years because of its purported specificity—it generally affects only the targeted pests and closely related organisms. This specificity is inferred because the desired toxic effect of Bt requires a specific pH level in the gut and specific receptors on gut cells, both of which are found in the target insects but not in mammals.

The Bt protein has been introduced into crops, such as corn and cotton, to make them resistant to certain pests. This is accomplished through genetic modification, resulting in what is known as Bt crops. This technique has reduced the need for chemical pesticide use.

Effects

In terms of human health, Bt toxins are considered safe for human consumption by the authorities regulating their use. The U.S. Environmental Protection Agency (EPA) has stated that the Bt toxin poses no significant risk to human health because humans supposedly do not have the specific gut conditions necessary for the toxin to take effect.

However, concerns have been raised about the research methodology to ascertain risk and potential health effects of consuming Bt crops. One concern is that the Bt protein could potentially cause allergic reactions in some people. Another concern is the potential impact on non-target organisms in the environment, including beneficial insects and soil microorganisms. Studies have found Bt toxin in the blood of pregnant women and their fetuses, raising questions about how the toxin is processed in the human body and whether it could have unknown health effects. These findings have stirred controversy and underscore the need for further research.

Conclusion

While this provides a broad overview of the relationship between toxic compounds in plants and implications for human diet, there is still much to explore in this complex field. Uncovering the full extent of these relationships requires further research into the specific evolutionary pressures that drive the production of these compounds, the genetic and biochemical basis for their production, and the impacts of these compounds on human health and disease.