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Which inhibit the uptake of iodine by the thyroid, are present in many commonly consumed plants. They are estimated to contribute approximately 4% to the worldwide incidence of goiters in humans (Liener 1986). Cabbage, cauliflower, Brussels sprouts, broccoli, kale, kohlrabi, turnips, radish, mustard, rutabaga, and oil seed meals from rape and turnip all possess some goitrogenic activity (Coon 1975). Effects of thyroid inhibition are not counteracted by the consumption of dietary iodine. The nature and extent of toxicity of glucosinolates are still the subject of debate. Although there are few, if any, acute human illnesses caused by glucosinolates, chronic and subchronic effects remain a possibility (Heaney and Fenwick 1987).
Additional foods with the potential for antithyroid activity include plants in the genus Allium(onion group); other vegetables such as chard, spinach, lettuce, celery, green pepper, beets, carrots, and radishes; legumes such as soybeans, peas, lentils, beans, and peanuts; nuts such as filberts and walnuts; fruits such as pears, peaches, apricots, strawberries, and raisins; and animal products such as milk, clams, oysters, and liver (Coon 1975). However, it has not been proven that a diet of these foods would be goitrogenic unless they comprised an excessively high proportion of the diet, a substantial amount of them were eaten raw, or they were not well cooked. Although goitrogens in foods are largely destroyed by thorough cooking, it must be acknowledged that many of the foods listed above are eaten uncooked (Coon 1975).
The most potent natural toxins responsible for human health risks are the mycotoxins. These are not strictly plant compounds but toxic metabolites produced by fungi infesting foodstuffs, especially cereals and nuts, which have been stored under conditions of elevated temperature and high humidity (NAS 1989). Among the ailments caused by these mycotoxins, the most notable historically is ergotism, or "St. Anthony's Fire," which afflicted people centuries ago. This was caused by ergot alkaloids produced by Claviceps purpureagrowing on cereal grains (NAS 1973). Although some mycotoxins have been identified as potent liver carcinogens in experimental animals, their role as human carcinogens has not been established.
The extent of the risks to human health associated with ingesting naturally occurring toxins remains a scientifically contentious matter (Watson 1987). Debate on this subject has been clouded by the absence of a systematic approach to defining and, in particular, quantifying human hazards. Although data have been assembled on the chemical properties and biological sources of most of these compounds, their long-term risks to public health have not been established. In fact, the National Research Council has concluded that the current data on human dietary exposure is insufficient and has underscored the need for new studies with larger sample sizes and refined testing methods (NAS 1996). Above all, it is important to emphasize that there is presently no firm evidence to demonstrate a link between long-term ingestion of natural toxins in commonly eaten foods and any type of chronic human illness (NAS 1989; NAS 1996).
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