Atypical subjects rarely faze Mary Roach, a science writer who has penned books about everything from cadavers to the feasibility of sending humans to Mars. For the latest line in her taboo-oriented books, “Gulp: Adventures on the Alimentary Canal,” Roach leads a journey through the miraculous inner workings of the human body akin to a fact-filled version of “Fantastic Voyage.”

“Gulp,” available in bookstores Tuesday, reads as stomach-churning at times and thoroughly enlightening at others, an expedition devoted to the overlooked milieu of a topic considered foul.

“It’s interesting to look at why things are taboo because they tend to be things that are a part of everyone’s life: death, sex, eating, excreting,” she said. “These are all things we all do, and yet we don’t want to think about them.”

Her previous book about National Aeronautics and Space Administration’s research into the hazards of human space flight provided plenty of coarse material. As she pored over scientific literature for 2010’s “Packing for Mars,” she found a choice article to help guide her exploration of all things related to guts.

“It was the weirdest study I’d ever seen,” she said. “A bunch of students were put into a metabolic chamber and were served a slurry of dead bacteria — they were looking into bacteria as space food for a Mars mission.”

The paper’s title sums up the study’s excruciating backfire for the six young men who gave up several days to perilous research in 1968: “Human Intolerance to Bacteria as Food.” Even if the concept of “bioregenerative” foods farmed from astronauts’ waste never took off after the article’s publication, the study gives Roach a comic and viscera-covered launching point for “Gulp,” a no-holds-barred approach to entertaining and informative science reportage.

“It just got me thinking of eating not as a sensual pleasure, but a physiological, biological process,” she said. “We tend not to think about it that way.”

Roach relishes the absurdity of her subject while grounding the bizarre history and current research with informational factoids. As she explores the complex ecosystem of the mouth, “Gulp” enlightens with facts about the immense antiviral properties of saliva, which is not just a “bacterial cesspool,” she said.

Working her way to the anus, she makes a stop along the tour to discuss the rectum in all its glory as a storage facility. She doesn’t just take cues from scientists — Roach contacted the California Department of Corrections and Rehabilitation to get an inside peek at prisoners who brazenly use their rectums as “handy pockets,” she said.

“It was sort of a once-in-a-lifetime scenario to sit down and talk about rectal smuggling with an inmate who was a convicted murderer,” she said.

The prisoner informs the anus-centered chapter of “Gulp” as well as the historical figures and researchers also backing the book. Roach also discovered the backdoor cavity’s immense potential: Prisoners regularly smuggle cellphones, rolling papers, tobacco and other drugs.

Roach plunges into her subjects as an ignorant and sarcastic outsider, wholly interested in her subjects without the technical expertise to bog down a casual read. “Gulp” may not represent a thoroughly methodical account, but that’s hardly the point.

“I don’t necessarily always want to go to a lab or a hospital — that’s not always the best way into the subject,” she said. “This isn’t a book about digestive health. It’s about the human food processor, a marvelous and very strange machine.”