Toxicants are produced by a variety of animals and plants and are widely distributed throughout each kingdom, from the unicellular to the multicellular. There is striking diversity of chemical structures for toxic compounds found in nature, making classification based on structure difficult.
The presence of toxicants in food may have come about because animals or plants evolved means of producing chemicals to protect themselves from predators, or insects, nematodes, microorganisms, or even humans.
The animal world has approximately 1200 species of venomous and poisonous creatures. Venomous animals refer to animals that are capable of creating a poison by a highly developed secretory gland or group of cells and delivering the poison during a bite or sting. The venom may have one or several functions, such as offense (capturing and digesting prey) or defense (protection against predators). Usually, venomous animals are not used by humans as a source of food, but when they are, care must be taken to avoid the poisonous glands containing toxicants. Poisonous animals are those whose tissues contain toxic materials. Such animals do not have a mechanism or structure for the delivery of the poison. Poisoning takes place by ingesting the animal material, particularly the tissue containing the poison. In poisonous animals, the poison may play only a small role in the animal's offensive or defensive activities. The toxin may be a by-product or a product of metabolism or a chemical that is passed along the food chain. Examples of the latter include barracuda, snapper, and other grouper fish, which can be a threat to human health because they have fed on smaller toxic fish and marine organisms such as dinoflagel-lates. The big fish eat the little fish that have consumed the toxicant and those who eat the big fish become ill.
Many plants also produce metabolic products (waste chemicals) or secondary products that can result in adverse effects if consumed in sufficient quantities. Plants did not evolve to be food for humans. There are far fewer species of plants used for food than those that are not. Many more plants are poisonous and are therefore not safe to eat. In his studies in Africa, Leakey speculated that humans were first meat eaters until they learned how to use fire for cooking plant material to remove or inactivate toxic compounds. Many plants used for food contain small amounts of toxic chemicals. For example, potato and tomato, which are from the nightshade family, contain potentially toxic chemicals (alkaloids). The compound solanine is found in the eyes and peel of potato and if sunburned (green under the skin) or blighted, solanine levels can increase sevenfold, sufficient to harm a small child. Cooked potatoes with high concentrations of solanine have a bitter taste and can cause a burning sensation in the throat. Solanine has been shown to exhibit terato-genic effects in animals. It is likely that solanine serves as a natural pesticide to the beetle and leaf hopper. Another noxious chemical, tomatine, also an alkaloid, is found in tomato and may also serve as a natural plant pesticide. Psoralen, found in parsnip, carrots, and celery, is a chemical produced by a plant under stress. Psoralen is a skin irritant, causing rash and skin problems.
Overall, healthy individuals can tolerate naturally occurring toxicants. However, there are several conditions under which natural toxicants can create problems. Inborn errors of metabolism or certain drug interactions can make individuals prone to problems caused by natural toxicants. Whereas nutrients can be beneficial to most, they can be deleterious to some, e.g., consumption of lactose by lactose-intolerant people. Other examples include individuals with celiac sprue, sucrase deficiency, fructose intolerance, galactosemia, and phenylketonuria. Individuals taking drugs that inhibit monoamine oxidase enzymes can be affected when eating cheeses or drinking wines, which are high in tyramine. Individuals with sensitivities due to allergies can be affected by foods. Hypersensitivity to a particular substance produces anaphylactic shock. Examples of foods that cause allergies include milk, wheat, nuts, citrus, strawberries, fish (shellfish), and egg. Some individuals have bizarre food habits or diets because of which they may eat certain foods in abnormal amounts. Eating large amounts of rhubarb can result in renal damage because of the excess oxalate in rhubarb. Consumption of uncrushed bay leaves can physically damage the intestinal mucosa. Some people are prone to problems caused by natural toxicants because of abnormal food habits. Eating honey produced by bees that collect nectar from plants containing poisonous alkaloids, or eating puffer fish, can also cause people harm.
For animals, in general, seafood poisons should be distinguished from marine venoms. Many seafood poisons are not limited to any single species and are likely to be affected by the environment. Some fish poisons (ichthyotoxins) are specific to a single species or genus. As illustrated in Table 12.1, seafood poisons can be categorized according to the tissues in which the toxin is found in the animal. Toxins localized in muscles, skin, liver, or intestine are known as ichthyosarcotoxins. Ich-
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