Back in 2007, after a series of mostly ineffective treatments prescribed by doctors, Ms. Enders, then 17, decided to take matters into her own hands. Convinced that the illness was somehow associated with her intestines, she pored over gastroenterological research, consumed probiotic bacterial cultures meant to aid digestion and tried out mineral supplements.

The experiments worked (although she is not sure which one did the trick), leaving her with healthy skin and a newfound interest in her intestines. “I experienced with my own body that knowledge is power,” she writes of the episode in “Gut: The Inside Story of Our Body’s Most Underrated Organ,” which was published in North America last month after its surprising success in Germany, where it has sold almost 1.5 million copies since its release in March 2014.