Out of 300,000 plant species, around 2000 species are considered as poisonous/toxic. This toxicity in plants is due to the presence of Naturally Occurring Toxins or NOTs, which are considered as one of the major chemical hazards responsible for causing serious health consequences when ingested in sufficient large quantities.
Chemical Composition of Plant
Plant-derived foods are assumed as mixtures of chemicals, which are grouped into two broad categories: intrinsic components which are inherent constituents of the plants and extrinsic components which are chemicals of both natural or industrial origin, reaching the food either by direct addition (food additives), by contamination (e.g. pollutants, mycotoxins, packaging migrants) or indirectly as a result of agricultural practices (e.g. pesticide residues).
Intrinsic components include a wide range of chemicals such as:
- Macro and Micronutrients – These include proteins, fats lipids, sugar, vitamins, minerals etc. which determines the nutritional value of the plant food.
- Anti-nutrients: These are the class of components that reduces the nutritional value of the plant food (e.g. protease inhibitors blocking protein digestion, phytate inhibiting absorption of minerals such as iron).
- Inherent plant toxicants: These are non-nutrient secondary plant metabolites which causes toxicity in humans. E.g. Glycoalkaloids, cyanogenic glucosides etc.
Naturally Occurring Toxins or NOT’s are defined as low molecular weight endogenous toxins or secondary metabolites secreted naturally by plant, which might give rise to adverse effects in humans when the plant or plant products are ingested. These are natural compounds secreted by plant because of its defense mechanism against predators, insects or microorganisms or extreme climatic condition. These chemical compounds have diverse structures and differ in biological function and toxicity. There are more than 500 varieties of NOT’s identified in food till date.
Some Major Plant Based Naturally Occurring Toxins
These are majorly present in sprouts and sun greened portion of potato. Exposure of potatoes to light in the field or marketplace can lead to glycoalkaloid concentrations that are unsafe for human consumption The FDA considers the maximum acceptable glycoalkaloid content to be 20–25 mg/100 g fresh. Cooking cannot destroy these toxins. Therefore, it is advisable to avoid eating green parts of potato and to reduce its production, it is important to store potatoes in a dark, cool, and dry place.
These are considered as stress toxins which act as natural pesticides and aid plants in defense against viruses, bacteria, fungi, insects, and animals. Its Concentrations may also increase after exposure to UV radiation, changes in temperature, prolonged storage, or treatment with hypochlorite or copper sulfate. They are phototoxic and photo mutagenic in nature. They are mainly found in plants belonging to Rutaceae (e.g., citrus fruits) and Umbelliferae (e.g., parsnip, parsley, celery, carrots) families. The FDA considers the maximum acceptable limit to be 1.31 mg/day.
- Hydrogen cyanide
They are amino-acid-derived constituents of plants which are secreted as secondary metabolites and is found in raw cassava and bamboo shoots, consumption of which can lead to poisoning. Hydrogen cyanide is also found in several types of fruits having pits. Toxicity of cyanogenic glycosides-containing plant is due to the cyanide produced by the action of hydrolytic enzyme (beta-glycosidase). The acute lethal dose of hydrogen cyanide for humans is reported to be 0.5-3.5 mg/kg body weight.
Lectins are also known as hemagglutinins which are a group of glycoproteins that has ability to agglutinate RBCs and are present in beans. Severe stomach ache, vomiting, and diarrhea can occur after consuming raw beans. Lectins are destroyed by soaking the beans in water for 5 hours, followed by boiling in fresh water for 10 minutes. Lectins are present in high levels in black beans, soybeans, lima beans, kidney beans and lentils and grain products.
- Oxalic acid
Oxalic acid is present in rhubarb. Oxalic acid is concentrated in the leaves and these should not be eaten. Muscle twitching, cramps, decreased breathing and heart rate, vomiting, pain, headache, and convulsions are some common symptoms of oxalic acid poisoning. In severe cases, even coma can occur.
These occur in zucchini (courgetti) or summer squash. These toxins give zucchini a bitter taste. Eating bitter zucchinis can cause vomiting, stomach cramps, diarrhea, and collapse.
As outlined above, foods often contain natural toxins which do not cause any harmful effect, when present below critical limit. The food plants can be safely ingested when appropriate measures are taken like careful selection, sufficient processing plus cooking and limitation of intake. The consumption of fresh vegetables and fruits reduces the potential risk of dangerous health issues such as vomiting, nausea, diarrhea etc. .People are advised to be cautious regarding the amount of intake and are recommended to observe the effects after ingestion, particularly for elderly people and children.