What is Gulp: Adventures on the Alimentary Canal about?

Gulp traces the alimentary canal from mouth to colon, using Mary Roach's standard method: firsthand visits to the researchers who work in each region, archival accounts of the stranger experiments in medical history, and a consistent willingness to ask the embarrassing questions that more decorous science writing leaves out. The subject is human digestion, and Roach treats it with the same combination of rigor and self-aware humor she brought to cadavers and spaceflight.

The book begins in the mouth, with food scientists who study smell, taste, and chewing, and the surprising degree to which palatability is constructed rather than intrinsic. Roach visits a pet food tasting facility (people evaluate it, not pets), explores the history of stomach research via a man named Alexis St. Martin who survived a gunshot wound leaving a permanent opening into his stomach, and sits with researchers studying gut motility, constipation, and the mechanics of rectal evacuation — a problem that turns out to be more complex than most people realize.

The gut microbiome chapter is particularly strong. Roach wrote Gulp in 2013, just as microbiome research was producing genuinely surprising findings about the relationship between gut bacteria and health, and she is appropriately careful about distinguishing what was established from what was speculative. She covers fecal transplants, which were then a fringe treatment and have since become mainstream for C. difficile infection.

The final chapters address the colon and its contents with the directness that is Roach's signature. She visits a maximum security prison to report on a case where someone allegedly weaponized human waste, which becomes a vehicle for a serious discussion of gut chemistry and the surprising range of what the human digestive system can tolerate. Gulp is the least glamorous of Roach's books and possibly the most consistently entertaining.