Soybeans Soybeans Soybeans
Alfalfa, common beans (Phaseolus vulgaris), peas (Pisum sativum)
All legumes contain at least small amounts of these factors; however, certain varieties of lima beans (Phaseolus lunatus) may contain much larger amounts Fava beans (Vicia faba)
Many species of mature dry legume seeds, but not peanuts; the immature (green) seeds contain much lower amounts Peanuts and soybeans
All legumes contain trypsin inhibitors; these inhibitors are destroyed by heat Lathyrus pea (L. sativus), which is grown mainly in India Common vetch (Vicia sativa) may also be lathyrogenic Soybeans, Peas (Pisum sativum) Occurs in all legumes to some extent
Source: Ensminger et al., Food and Nutrition Encyclopedia, 2nd ed., CRC Press, Boca Raton, FL, 1994.
A variety of plants contain a third group of type B antinutritives, the glucosinolates, also known as thioglucosides. Many glucosinolates are goitrogenic. They have a general structure, and yield on hydrolysis the active or actual goitrogens, such as thiocyanates, isothiocyanates, cyclic sulfur compounds, and nitriles. Three types of goiter can be identified: (1) cabbage goiter, (2) brassica seed goiter, and (3) legume goiter. Cabbage goiter, also known as struma, is induced by excessive consumption of cabbage. It seems that cabbage goitrogens inhibit iodine uptake by directly affecting the thyroid gland. Cabbage goiter can be treated by iodine supplementation. Brassica seed goiter can result from the consumption of the seeds of brassica plants (e.g., rutabaga, turnip, cabbage, and rape), which contain goitrogens that prevent thyroxine synthesis. This type of goiter can only be treated by administration of the thyroid hormone. Legume goiter is induced by goitrogens in legumes such as soybeans and peanuts. It differs from cabbage goiter in that the thyroid gland does not lose its activity for iodine. Inhibition of the intestinal absorption of iodine or the reabsorption of thyroxine has been shown in this case. Legume goiter can be treated by iodine therapy. Glucosinolates, which have been shown to induce goiter, at least in experimental animals, are found in several foods and feedstuffs: broccoli (buds), brussels sprouts (head), cabbage (head), cauliflower (buds), garden cress (leaves), horseradish (roots), kale (leaves), kohlrabi (head), black and white mustard (seed), radish (root), rape (seed), rutabaga (root), and turnips (root and seed). One of the most potent glucosinolates is progoitrin from the seeds of brassica plants and the roots of rutabaga. Hydrolysis of this compound yields 1-cyano-2-hydroxy-3-butene, 1-cyano-2-hydroxy-3, 4-butylepisulfide, 2-hydroxy-3, 4-butenylisothiocyanate, and (S)-5-vinyl-oxazolidone-2-thione, also known as goitrin. The latter product interferes, together with its ^-enantiomer, in the iodination of thyroxine precursors, and so the resulting goiter cannot be treated by iodine therapy.
Type C antinutritives are naturally occurring substances that can inactivate vitamins, form unabsorbable complexes with them, or interfere with their digestive or metabolic utilization. They are also known as antivitamins. The most important type C antinutritives are ascorbic acid oxidase, antithiamine factors, and antipyridoxine factors.
Ascorbic acid oxidase is a copper-containing enzyme that catalyzes the oxidation of free ascorbic acid to diketogluconic acid, oxalic acid, and other oxidation products. It has been reported to occur in many fruits (e.g., peaches and bananas) and vegetables (e.g., cucumbers, pumpkins, lettuce, cress, cauliflowers, spinach, green beans, green peas, carrots, potatoes, tomatoes, beets, and kohlrabi). The enzyme is active between pH 4 and 7 (optimum pH 5.6 to 6.0); its optimum temperature is 38°C. The enzyme is released when plant cells are broken. Therefore, if fruits and vegetables are cut, the vitamin C content decreases gradually. Ascorbic acid oxidase can be inhibited effectively at pH 2 or by blanching at around 100°C. Ascorbic acid can also be protected against ascorbic acid oxidase by substances of plant origin. Flavonoids, such as the flavonols quercetin and kemp-ferol, present in fruits and vegetables, strongly inhibit the enzyme.
A second group of type C antinutritives are the antithiamine factors, which interact with thiamine, also known as vitamin B1. Antithiamine factors can be grouped as thiaminases, catechols, and tannins. Thiaminases, which are enzymes that split thiamine at the methylene linkage, are found in many freshwater and saltwater fish species and in certain species of crab and clam. They contain a nonprotein coenzyme structurally related to hemin. This coenzyme is the actual antithiamine factor. Thiaminases in fish and other sources can be destroyed by cooking. Antithiamine factors of plant origin include catechols and tannins. The most well-known ortho-catechol is found in bracken fern. In fact, there are two types of heat-stable antithiamine factors in this fern, one of which has been identified as caffeic acid, which can also be hydrolyzed from chlorogenic acid (found in green coffee beans) by intestinal bacteria. Other ortho-catechols, such as methylsinapate occurring in mustard seed and rapeseed, also have antithiamine activity. The mechanism of thiamine inactivation by these compounds requires oxygen and is dependent on temperature and pH. The reaction appears to proceed in two phases: a rapid initial phase, which is reversible by addition of reducing agents (e.g., ascorbic acid), and a slower subsequent phase, which is irreversible. Tannins, occurring in a variety of plants, including tea, similarly possess antithiamine activity. Thiamine is one of the vitamins likely to be deficient in the diet. Thus, persistent consumption of antithiamine factors and the possible presence of thiaminase-producing bacteria in the gastrointestinal tract may compromise the already marginal thiamine intake.
A variety of plants and mushrooms contain pyridoxine antagonists. These compounds interfere with the use of vitamin B6 and are called antipyridoxine factors. They are hydrazine derivatives. Linseed contains the water-soluble and heat-labile antipyridoxine factor linatine (g-glutamyl-1-amino-D-proline). Hydrolysis of linatine yields the actual antipyridoxine factor 1-amino-proline. Antipyridoxine factors have also been found in wild mushrooms, the common commercial edible mushroom, and the Japanese mushroom shiitake. Commercial and shiitake mushrooms contain agaritine. Hydrolysis of agaritine by g-glu-tamyl transferase, which is endogenous to the mushroom, yields the active agent 4-hydroxymethylphenylhydrazine. Disruption of the cells of the mushroom can accelerate hydrolysis; careful handling of the mushrooms and immediate blanching after cleaning and cutting can prevent hydrolysis. The mechanism underlying the antipyridoxine activity is believed to be condensation of the hydrazines with the carbonyl compounds pyridoxal and pyridoxal phosphate (the active form of the vitamin), resulting in the formation of inactive hydrazones.
In addition to these antinutritives, foods can contain a variety of toxic substances as shown in Table 1.12 (see Reference 4, pp. 24-36, 1284-1285, 1776-1785, 1790-1803, 2082-2087). Some of these toxic substances are added inadvertently by the food processing methods, but some occur naturally. If consumed in minute quantities, some of these toxic materials are without significant effect, yet other compounds (e.g., arsenic), even in minute amounts, could accumulate and become lethal.
Table 1.13 contains information about plants commonly thought of as weeds.6 Some of these plants may have toxic components that affect certain consumers. There can be considerable variability among humans with respect to plants that can be tolerated by them. Plants can differ from variety to variety and indeed from one growing condition to another in the content of certain of their herbal or nutritive ingredients. Lastly, Table 1.14 provides a list of toxic plants that should not be consumed under any circumstances (see Reference 4, pp. 24-36, 1284-1285, 1776-1785, 1790-1803, 2082-2087).
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