Secondary plant metabolites
Natural toxins found in plant foods. These toxins are the defences used by plants to defend themselves against predators: bacteria and viruses, fungi, insects, humans and animals. Some of the toxic effects of secondary plant metabolites include interference with hormonal function, or with the activity of digestive enzymes or the absorption of nutrients, damage to blood vessels and interference with blood clotting and enzyme activity. Aminopropionitrile in chick peas can cause a type of poisoning called lathyrism, which affects the bones and blood vessels. Dioscorine in wild yams can cause paralysis of the central nervous system. Plants belonging to the deadly nightshade family (potatoes, tomatoes, aubergines, tobacco and peppers) contain toxic alkaloids. Lectins in pulses are highly toxic but are broken down by fast boiling for at least ten minutes.
Sometimes the production of plant toxins is stimulated by damage or stress inflicted on the plant by insects, fungi or frost, for example. A number of human deaths have occurred from solanine in potatoes, produced as a result of exposing the potatoes to light. Psoralens produced by celery which has been attacked by moulds or viruses can cause skin rashes. Both natural and artificial toxins originating in poisonous plants or from pesticides consumed by an animal can be passed on to the human consumer in the meat, milk or eggs of that animal.
Under normal circumstances, natural toxins in common plant foods can be easily dealt with by the human liver, which has developed enzymes capable of detoxifying these substances. However the overloading of enzymes in the liver’s detoxification system, which may occur in individuals who have been subjected to severe toxic assault, or the overconsumption of certain foods (which may occur in starvation conditions where a varied selection of foods is not available) may lead to the accumulation of toxic levels of one or more plant toxins.