It smells like a mixture of overripe Gorgonzola and putrefying mushrooms. Its texture is that of slugs stuffed with mozzarella. It is natto, or rotten soya beans, and is one of the world’s most challenging foods, as loved and loathed in Japan as black puddings are in Britain or rotten herrings in Sweden.

And in the past few days it has become a cult, as natto-mania has swept Japan, emptying supermarket shelves and leaving soya bean fermenters scrambling to catch up with the unprecedented surge in demand.

Earlier this month the television programme Revealed! Encyclopaedia of Living recommended two portions of natto a day as a way of losing weight in only two weeks. By the following lunchtime, natto had sold out nationwide.

Demand has tripled as chubby Japanese have overcome their aversion to its smell and texture. So extreme is the craze that even natto manufacturers are warning consumers not to jeopardise their health in their all-consuming desire for rotten beans. They are credited with alleviating everything from thrombosis, osteoporosis and high cholesterol to dysentery, cancer and baldness, although few of these benefits are established scientifically.

Producers have been flooded with complaints from established natto aficionados and have taken out newspaper advertisements apologising to them for the natto famine.

“If this boom turns into a reminder of the virtues of traditional Japanese food, all well and good,” said Noriyuki Ogata, of the Japan fermented soya bean federated co-operative society. “But we must emphasise that natto is not a medicine. If you want to lose weight, you need a well-balanced diet and a proper amount of exercise.”

Natto is produced over the course of a week by soaking, steaming, fermenting and cooling soya beans, and combining them with salt, sugar, yeast and bacteria. At 50 yen (21p) a packet, it is one of the cheapest items in any supermarket. It is often eaten for breakfast on top of rice, but it is also used to make sushi or omelettes, or served with noodles or spaghetti. It is even sold as a flavour of ice cream.

When prodded with chopsticks, the slimy goo coating the soya beans forms web-like strings. The flavour is nutty and surprisingly mild.

Revealed! has stoked health food booms before, such as for cocoa beans or black soya beans. But none has been so huge or unlikely as natto-mania. “We had three times as many orders as usual, and we still haven’t been able to meet them,” a spokesman for the Mizkan food company said.

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