A second group of antivitamins is the antithiamine factors. They interact with vitamin Bv also known as thiamine. Antithiamine factors can be distinguished as thiaminases, tannins, and catechols. The interaction with vitamin B1 can lead to serious neurotoxic effects as a result of vitamin B1 deficiency. Normally, antithiamine factors pose no appreciable risk to humans. They only cause thiamine deficiency in people whose diet is already low in thiamine.
Thiaminases are found in many fish species, freshwater as well as saltwater species, and in certain species of crab and clam. These antithiamine factors are enzymes that split thiamine at the methylene linkage (Figure 3.3).
ch2oh ch3 n nh2
ch3 n ch2ch2oh
Thiaminases contain a nonprotein coenzyme, structurally related to hemin, the red pigment component of hemoglobin. The coenzyme is the actual antithiamine factor. Cooking destroys thiaminases in fish and other sources.
Antithiamine factors can also be of plant origin. Tannins, occurring in a variety of plants, including tea, are believed to be responsible for inhibition of growth in animals, and for inhibition of digestive enzymes. A study in volunteers on the effects of tannins in tea leaves, tea infusions and betel nuts on thiamine, has shown that the tannins were responsible for thiamine destruction. Tannins are a complex of esters and ethers of various carbohydrates. A component of tannins is gallic acid.
Gallic acid is obtained by hydrolyzing tannins. The interaction of these substances with thiamine is oxygen-, temperature-, and pH-dependent. It appears to proceed in two phases: a rapid initial phase, which is reversible on addition of reducing agents, such as ascorbic acid, and a slower subsequent phase, which is irreversible.
A variety of antithiamine factors are the ortho-catechol derivatives. A well-known example is present in bracken. So-called fern-poisoning in cattle is attributed to this factor. Possibly, there are two types of heat-stable antithiamine factors in this fern, one of which has been identified as caffeic acid (3,4-dihydroxycinnamic acid).
CH = CHCOO
CH = CHCOO
Caffeic acid can also be formed on hydrolysis of chlorogenic acid by intestinal bacteria. Chlorogenic acid is found in green coffee beans and green apples. Other ortho-catechols, such as methylsinapate occurring in mustard or rapeseed, also have antithiamine activity.
Was this article helpful?
WHAT IT IS A three-phase plan that has been likened to the low-carbohydrate Atkins program because during the first two weeks, South Beach eliminates most carbs, including bread, pasta, potatoes, fruit and most dairy products. In PHASE 2, healthy carbs, including most fruits, whole grains and dairy products are gradually reintroduced, but processed carbs such as bagels, cookies, cornflakes, regular pasta and rice cakes remain on the list of foods to avoid or eat rarely.