Five men have been poisoned in Japan after allegedly asking a restaurant to serve them banned parts of the world's most toxic fish. The men were having dinner on Friday night in the western city of Wakayama when they ate puffer fish liver, which prompted vomiting and breathing difficulties early the next morning. Puffer fish - also called blowfish or fugu in Japanese - are the world's most toxic group of fish and their livers, ovaries and skin contain tetrodotoxin, 100 times more lethal than cyanide. Dicing with death: Five Japanese men were poisoned after eating banned parts of the puffer fish (file photo) Yet eating the fish's flesh is a tradition in Japan, where wealthy diners play the Russian roulette of the restaurant world by dining on sashimi slivers so thin they are almost transparent. According to a city health official, the men asked to eat a banned, toxic part of the fish - which the most foolhardy fugu aficionados are said to enjoy for the tingling it produces on the lips. Popular: The tradition of eating thin slices of 'fugu' persists despite dozens of poisonings a year . The tingling is only the first symptom of tetrodotoxin poisoning, and is seen as part of the thrill for lovers of the fish. But if the toxin is served in any significant quantity it then paralyses the muscles, suffocating victims as it reaches their chests and diaphragms while they are still conscious. The health official said the men were in their 40s and 50s and the restaurant was shut down for five days from yesterday amid an investigation. 'They consumed a dinner that included liver, which is regarded as toxic material,' she told AFP, reading from a statement. Calls to the restaurant went unanswered. Chefs who want to serve puffer fish must train for two years before taking a rigorous test which a third of applicants fail. The most prized species is the tiger puffer, takifugu rubripes, whose flesh is safe to eat but whose liver, ovaries and intestine can all kill. It is not clear which species of puffer fish the men ate. Puffer fish account for the majority of Japan's food poisoning deaths but the delicacy shows no sign of dying out, although it is heavily regulated. Government figures show 30 to 50 people are hospitalised every year, with a few of those cases leading to death.
Men in their 40s and 50s ate at restaurant in Wakayama, Japan, on Friday . They fell ill the next morning with breathing difficulties and vomiting . Health official claimed they asked to eat highly poisonous fish's liver . Restaurant has been shut down for five days as investigation continues . Puffer fish toxins kill - but aficionados eat just enough for tingling on lips .