Toxicity
The vetches grown as forage are generally toxic to non-ruminants (such as humans), at least if eaten in quantity. Cattle and horses have been poisoned by V. villosa and V. benghalensis, two species that contain canavanine in their seeds. Canavanine, a toxic analogue of the amino acid arginine, has been identified in Hairy Vetch as an appetite suppressant for monogastric animals, while Narbon bean contains the quicker-acting but weaker γ-glutamyl-S-ethenylcysteine. In common vetch, γ-glutamyl-β-cyanoalanine has been found. The active part of this molecule is β-cyanoalanine. It inhibits the conversion of the sulfur amino acid methionine to cysteine.
Cystathionine, an intermediary product of this biochemical pathway is secreted in urine. This process can effectively lead to the depletion of vital protective reserves of the sulfur amino acid cysteine and thereby making Vicia sativa seed a dangerous component in mixture with other toxin sources. The Spanish pulse mix comuña contains common vetch and bitter vetch in addition to vetchling (Lathyrus cicera) seeds; it can be fed in small quantities to ruminants, but its use as a staple food will cause lathyrism even in these animals. Moreover, common vetch as well as broad bean – and probably other species of Vicia too – contain oxidants like covicine, isouramil, divicine and vicine in quantities sufficient to lower glutathione levels in G6PD-deficient persons to cause favism disease. At least broad beans also contain the lectin phytohemagglutinin and are somewhat poisonous if eaten raw. Split common vetch seeds resemble split red lentils (Lens culinaris), and has been occasionally mislabelled as such by exporters or importers to be sold for human consumption. In some countries where lentils are highly popular – e.g., Bangladesh, Egypt, India and Pakistan – import bans on suspect produce have been established to prevent these potentially harmful scams.
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