Toxic substances in food

Introduction

All substances are potentially toxic. Some are toxic when ingested in abnormally large quantities, for example, common salt and water. Others such as caffeine and certain other alkaloids are toxic in moderate amount.

A toxic substance ingested might be present in food in its natural state or be produced by micro-organisms contaminating the food. Other contaminants may be residues from agricultural chemicals such as pesticides, insecticides, weedicides and antibiotics. Substances added intentionally as food preservatives (melamine) may also have deleterious effects.

Natural toxins in food could affect metabolism in a variety of ways. Some affect protein utilization (protease inhibitors). Others affect utilization of minerals (oxalates, phytates, dietary fibers, goitrogens). Some act as antivitamins (ascorbic acid oxidase in plants, thiaminase in fish, avidin in egg-white, dicoumarol, anti-pyridoxine, anti-vitamin E). Certain foods have non-nutrient substances with pharmacological activity (vasoactive amines in tomatoes, cheese, avocado and pineapple). Other non-nutrients may produce adverse reactions that are immunologically mediated, resulting in a phenomenon referred as ‘food allergy”.

Despite the widespread occurrence and consumption of numerous toxins, very few cases of injury have been reported. Humans are able to metabolize small amounts of toxins even when they are consumed regularly. There are, however, more acute problems such as favism, lathyrism and poisoning by certain sea foods which are consumed, by choice or by necessity, even though the dangers of their consumption are known

Residues of chemicals used to protect crops during growth and storage is a major, continuing problem. In many countries there is strict control over the use of such chemicals and maximum tolerances have been established. Some of these chemicals might not only be on the surface of the vegetables or fruit at harvest, but might also be absorbed by the plant and be present in the cells of leaves and fruit and yam eaten, making them potentially more dangerous.

Common toxins

1. Trypsin inhibitors
2. Phytohemagglutinins
3. Goitrogens
4. Cyanogenic glucosides
5. Anti-vitamins
6. caffeine
7. organ specific toxins
8. toxins in flavouring agents
9. toxins in sea food
10. Toxicity due to micro-organisms

Trypsin/protease inhibitors

Substances capable of inhibiting proteolytic activity of digestive enzymes are found throughout the plant kingdom and particularly in legumes.

Phytohemagglutinins/phytohaemagglutinins

Haemagglutinins are proteins capable of agglutinating red blood cells. Such substances present in plants are called phytohaemagglutinins. They are present in all legumes.

Goitrogens

These are the substances that can give rise to goiters. Goitrogens are found in cabbage, cauliflower, radish, turnips and rape seeds.

Antivitamins

Common foods that contain antivitamins are kidney beans and soya beans. Several plants such as ferns and bracken contain antimetabolites of thiamine which, if eaten in quantity, can lead to thiamine deficiency

Caffeine

Caffeine stimulates central nervous system, exert a diuretic effect on kidneys and stimulate heart muscles. The food items which contain caffeine are tea and coffe.

Organ specific toxins

Neurotoxins and nephrotoxins are the commonest organ specific toxins. Most of the legumes are known to contain these toxins.

Toxins in flavoring agents

Toxins found in flavoring agents can precipitate allergicreaction. Some of them are carcinogenic.

Toxins in sea food

A large number of species of fish may be poisonous. The gonads and liver of puffer fish. Porcupine fish and box fish are poisonous. These toxins may cause neurological or hematological manifestations.

Toxicity due to micro-organisms

The number of types of micro-organisms important in food toxicity is very large. Salmonella, cholera and botulism are some toxic organisms found in food. The can be myotoxic, neurotoxic or hemotoxic.