Gulp: Adventures on the Alimentary Canal

Gulp: Adventures on the Alimentary Canal (2013) by Mary Roach is a work of contemporary popular science writing that explores the topic of human digestion. Roach, a bestselling science writer, is known for her ability to make complex scientific subjects accessible through humor and meticulous research. Building on the success of her previous works—like Stiff (2003), which examines human cadavers from a scientific perspective, and Packing for Mars (2010), which explores the psychological and physiological discomforts of space travel—Gulp likewise tackles an unusual subject with both wit and respect for scientific inquiry. The book follows the progression of food through the digestive tract, focusing on the medical, historical, and cultural implications of food and bodily processes.

This study guide refers to the 2025 W. W. Norton & Company eBook edition.

Content Warning: The source material and guide feature depictions of physical abuse, emotional abuse, antigay bias, illness, and death.

Summary

Roach opens Gulp by describing a 1968 NASA experiment at UC Berkeley where volunteers consumed meals made from bacteria for two days while confined in a metabolic chamber. This failed attempt to create sustainable food sources for Mars missions from astronauts’ biological waste sets the tone for Roach’s exploration of the taboo and uncomfortable aspects of human digestion. She argues that the digestive system deserves the same attention as gourmet cuisine, positioning her book as an investigation into the biological realities that society either romanticizes or ignores.

Roach reveals that what humans experience as “taste” is actually 80-90% smell. Humans can only detect five basic tastes (sweet, bitter, salty, sour, and umami), but they can perceive an almost infinite number of smells through orthonasal and retronasal olfaction. Through encounters with professional sensory analyst Sue Langstaff and failed attempts at olive oil tasting, Roach demonstrates that flavor identification is a learned skill requiring extensive practice. Her experiences show how visual cues can override actual sensory information, as demonstrated by wine students who described the same white wine differently when artificially colored red.

The book explores how cultural conditioning shapes food preferences through examination of traditional Inuit eating practices, where organ consumption provides essential nutrients in environments with limited vegetation. Roach contrasts this with mainstream America’s resistance to organ meats, even during World War II when the government hired anthropologist Margaret Mead’s team to promote “variety meats.” The most effective method involved public pledges capitalizing on wartime patriotism, though these efforts largely failed due to associations with poverty and lower social status.

Roach’s investigation of the pet food industry at AFB International reveals the fundamental challenge of balancing animal nutritional needs with human consumer preferences. She discovers that dry pet food became popular during World War II tin rationing, leading to the development of palatants—flavor coatings that make nutritionally complete but bland kibble appealing to animals.

A central narrative focuses on the complex 30-year relationship between Army surgeon William Beaumont and Alexis St. Martin, whose 1822 gunshot wound created a permanent opening into his stomach. This unprecedented research opportunity allowed Beaumont direct access to observe human digestion. However, Beaumont treated St. Martin like a servant, and he performed household duties while serving as Beaumont’s experimental subject. Roach suggests that the uncomfortable intimacy of their scientific partnership illustrates both the dedication necessary for scientific advancement and the problematic power dynamics of 19th-century medical research.

Roach then explores the sophisticated science of mastication through research at Food Valley in the Netherlands. There, she learned that people possess unique chewing patterns that are as distinctive as fingerprints. She also learned that jaw muscles are the body’s strongest muscles in terms of pressure and that they also possess an intricate protective system: The jaw automatically reduces its force when hard food gives way, preventing teeth from colliding destructively through a reflex called the “silent period.”

The book examines the bizarre historical phenomenon of Fletcherism, promoted by Horace Fletcher in the early 1900s, which advocated for extremely thorough mastication—sometimes over 700 chews per bite. Despite lacking medical training, Fletcher gained remarkable influence, attracting followers including Henry James and Franz Kafka, and he briefly influenced government policy during World War I.

Roach reveals saliva’s critical protective functions through research with Erika Silletti, discovering that humans produce two distinct types: stimulated saliva from eating, and unstimulated background flow. Saliva neutralizes acids that would otherwise dissolve tooth enamel, and it contains digestive enzymes similar to those found in commercial detergents. Roach also addresses cultural boundaries surrounding saliva, noting how this bodily fluid becomes repulsive once it leaves the mouth despite its essential antimicrobial and wound-healing properties.

The book examines whether humans could survive being swallowed by whales, using the biblical story of Jonah as a starting point. While baleen whales have narrow throats making human consumption impossible, sperm whales possess gullets that are wide enough to accommodate a person. However, Roach explains that sperm whale stomachs function like powerful grinding mechanisms, exerting crushing forces sufficient to crush glass and metal.

Roach also explores the combustible nature of digestive gases through agricultural and medical contexts. Bacteria in oxygen-free environments like the human colon produce hydrogen and methane, creating potential explosion hazards during medical procedures. A 1977 colonoscopy incident in France, where electrical equipment ignited hydrogen gas from a patient who had consumed mannitol, led to changes in medical practices. NASA’s concerns about astronaut flatulence in sealed spacecraft also illustrate practical applications of this knowledge. The book traces flatulence research from crude 1940s methods using rectal tubes to sophisticated breath-hydrogen testing. Gastroenterologist Michael Levitt’s groundbreaking research identified three sulfur compounds responsible for gas odors and developed synthetic mixtures to test odor-reducing products, most of which proved ineffective.

Roach chronicles dangerous constipation cases through historical medical specimens and the controversial theory surrounding Elvis Presley’s death. She examines patients like J.W., who suffered from Hirschsprung’s disease and worked as “Balloon Man” in a Philadelphia freak show before dying while straining in a bathroom. The book explores how Elvis Presley’s physician theorized that the singer died from complications related to severe constipation rather than drug overdose, likely involving the dangerous Valsalva maneuver that can trigger fatal cardiac arrhythmia.

The book then explores rectal smuggling in prisons, where inmates conceal contraband by overriding natural defecation reflexes. Roach visited Avenal State Prison to examine confiscated items worth thousands on the prison black market. She interviewed an inmate who explained how humans can suppress bodily urges through conscious effort. She contrasts prison smuggling with international drug trafficking and addresses cultural taboos surrounding rectal use.

Roach concludes her book by discussing fecal microbiota transplantation (FMT). She recounts attending a colon-themed party and observing actual transplant procedures. She explains that gut bacteria play crucial roles in digestion, drug efficacy, and disease prevention. Despite FMT’s proven effectiveness, bureaucratic obstacles and resistance from the medical establishment severely limit patient access to this life-saving therapy, partly due to pharmaceutical companies’ financial interests in ongoing treatments rather than cures.

Throughout Gulp, Roach challenges cultural squeamishness about bodily functions while celebrating scientists who pursue uncomfortable questions others avoid. She demonstrates how societal taboos surrounding digestion may prevent important medical discussions and proper treatment of potentially life-threatening conditions. The book ultimately argues that humans should better understand their internal organs’ remarkable complexity and beauty rather than dismissing natural processes.