Figure 2 Structures of thiamin and thiochrome.

quantitatively to thiochrome (Figure 2). In addition, anti-thiamin factors in food can accelerate thiamin losses. Paralysis in foxes fed raw carp led to the discovery of the thiaminase enzymes. Two thiami-nases are found in food. Thiaminase I is found in fish, shellfish, ferns, and some bacteria and catalyzes a base exchange reaction between thiazole and another base. Thiaminase II is a hydrolytic enzyme that cleaves the vitamin at the methylene bridge and is found mainly in bacteria. The thiaminases are heat labile, so only food that is eaten raw or fermented may loose thiamin during its preparation or in the gastrointestinal tract. There are also heat-stable anti-thiamin factors that are found in ferns, tea, betel nuts, large numbers of plants and vegetables, and some animal tissues. Anti-thiamin factors bind with varying degrees of attachment to thiamin and may or may not interfere with the bioavailability of thiamin. Diphenols, especially those with the hydroxyl groups in the ortho position, tend to react to give products that are both thiochrome negative and microbiologi-cally inactive (i.e., thiamin is deactivated). Thus, in areas of northern and northeastern Thailand where tea drinking, chewing fermented tea leaves, chewing betel nuts, and consuming raw/fermented fish are common practices, thiamin deficiency still occurs despite thiamin intakes of 0.44-0.50 mg/4.2MJ.

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