Toxins and Contaminants

Fruits and vegetables contain large numbers of non-nutrients, some of which are toxic, but they are rarely harmful under ordinary conditions. Anti-nutritional substances in plant foods include antienzymes that interfere with digestion, antivitamins, and substances such as oxalates, phytates, tannins, and dietary fiber that can interfere with the absorption of some minerals. Glucosinolates, which are responsible for the characteristic flavor of vegetables of the families Cruciferae and Brassicaceae, are goi-trogenic but appear to be an insignificant hazard to human health in the amounts usually eaten. In addition some legumes contain lectins in amounts sufficient to have been the cause of occasional cases of food poisoning when incompletely destroyed by cooking.

Most of these substances are destroyed by heat and have been found harmful to animals when included in feed in the raw state. Some are present in salad vegetables that are eaten raw, but in amounts too small to be harmful.

Cassava, especially the bitter variety, contains cyanide. This is usually removed in traditional food processing but not infrequently is a source of harm when the food is not properly treated.

Plantains eaten as a staple provide sufficient 5-hydroxytryptamine to affect central and peripheral nervous systems. Unripe ackee fruit (Blighia sapida) contains a toxin (hypoglycin) which causes vomiting sickness and hypoglycaemia. Rhubarb contains oxa-late, and although the amounts in stems are harmless, poisoning has resulted from eating the leaves which contain a much higher concentration.

Potatoes contain small, usually harmless, amounts of solanine, but this is increased to toxic levels by exposure to light and subsequent 'greening.'

Some plant foods have been the cause of occasional outbreaks of poisoning in special circumstances. For example Jimson weed, Datura stramonium, contains alkaloids including scopolamine which produce hallucination; and the hemp plant, Cannabis indica, and the peyote, containing mescaline, have been consumed deliberately for their psychic effects.

Contamination

Some vegetables accumulate environmental toxins such as lead and radioactive fallout but the main cause for concern is from residues of agricultural chemicals, pesticides, and weedkillers. The danger of these chemicals is mainly to those handling them in production and manufacture, but there is concern over the small amounts remaining in the crops since these may be consumed over a long period, and the toxins may possibly be cumulative. Among these are the organo-chlorine insecticides including DDT and dieldrin. Both the substances and their degradation products persist in crops and so find their way into the human food chain. The fact that they are fat soluble and accumulate in the adipose tissue has given rise to concern but there is no evidence that these quantities merit alarm.

Nevertheless, they are restricted in use and are under continuous observation.

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