BANGKOK — Thanat Tantinam, a colorectal surgeon in Thailand, knows a lot of things from his medical training, like the ins and outs of the human digestive system, or bowel cancer rates. But also: Never eat Kentucky Fried Chicken at work.

Since he was a medical student, about a decade ago, the U.S. fast food franchise has been the subject of an odd superstition among Thai doctors, many of whom believe that eating its chicken at work will lead to intolerably busy shifts.

“It’s a serious superstition in some hospitals,” said Thanat, 35. “Your colleagues might be mad if you bring KFC.”

He recently tested this theory.…