The man had been a home-brewer and was regularly exposed to the brewer’s yeast, the fungus used to ferment beer. He likely had an antibiotics treatment that had killed off his original intestinal flora and replaced it with brewer’s yeast. 

From then on, carb-rich foods such as bread or bagels would ferment after he ate them, making an alcoholic brew in his gut within the next 24 hours.

He took daily anti-fungal medications for six weeks to kill off the yeast and followed a very strict no sugar/carb/alcohol diet, until eventually he was back to normal.