In much of the world, monosodium glutamate or MSG is a beloved ingredient. It’s in stock cubes and potato chips, and sprinkled on everything from soups to salads, adding a savory flavor sometimes referred to as umami or the “fifth taste”.

It was invented in commercial form in Japan by Kikunae Ikeda, who founded the firm Ajinomoto to sell the product at home and abroad.

At Ajinomoto’s factory outside of Tokyo, a steady stream of visitors join tours where they sample miso soup with and without MSG, and snap selfies with the firm’s mascot — the red and white AjiPanda.

But elsewhere the substance is regarded as less benign, with articles dubbing it a “killer condiment” and people reporting side-effects including headaches, sweats and flushing.

The unsavory reputation dates back decades, to a 1968 letter in the New England Journal of Medicine by Chinese-American doctor Robert Ho Man Kwok.

He described symptoms he experienced while eating at Chinese restaurants in the US, including “numbness at the back of the neck… general weakness and palpitation”.

His Chinese friends — “all well educated” — experienced similar sensations, he wrote.

He proposed several potential reasons, including soy sauce, cooking wine, MSG, or high sodium content, and suggested “friends in the medical field” research this “peculiar syndrome”.

The letter was picked up by the media and made its way into the public imagination, creating a lasting association between MSG and various, poorly defined health effects.