Among the many probing questions she asks in her new book, Mary Roach brings up an apparent paradox of the stomach: “Humans eat and digest animal stomachs all the time — how come we don’t digest our own stomachs?”
It turns out that we do digest our stomach lining, but we also continually rebuild it, producing an entirely new one every few days. The book is replete such awesome facts, presented in Roach’s clever and idiosyncratic way. She’s a funny lady, this one.
“Gulp: Adventures on the Alimentary Canal” explores some of the taboos and controversies related to our inner world of guts and mucus, such as academic disagreements on what the ideal standards for excrement frequency, consistency, and shape ought to be. However, Roach also brings up the science behind the topics she discusses. For instance, research shows that people tend to prefer cheaper wines (about $10 per bottle) to more expensive wines (about $50), suggesting that high price does not equal strong preference. Kurt Lewin says, “We don’t eat what we like; we like what we eat,” meaning we develop our food preferences to some degree based on the availability of the food source. In a study using female college students, only 14 percent claimed to like evaporated milk, but, after 16 exposures repeated over a month, 51 percent said they liked it. The brain can recognize the flavor of a familiar food via sensory receptors in the tongue and can call up past memories of relative digestive ease and feeling sated. The signals from the taste cells (smell is a big part of this too) travel through the cranial nerves feeding the olfactory and gustatory processing sites in the brain. These centers have a stored representation of the food memory distributed across
the multimodal association cortex.
People who’ve experienced many different odors and tastes presumably have taste and smell centers as elaborate as London taxi drivers’ hippocampi relative to us normal folk (navigating the intricate streets of a city leads to hippocampal growth). Trying to cultivate her own sensory areas, Roach unsuccessfully attempts to develop skills as an olive-oil taster but gives up distinguishing between flavors after feeling ill from ingesting too many samples. The brain can also retrieve memories associated with foods that have made us sick, also known as taste aversions. These are powerful memories that can be learned in a single trial, compared to the many exposures required to get a measly 37-percent bump in liking evaporated milk. The ability to learn about safe and dangerous foods is an advantageous tool — if an animal repeatedly eats foods that cause gastric distress and general malaise, that animal will not survive.
However, as Roach points out, the brain cannot do this alone; it requires communication with the stomach brain known as the enteric nervous system. This is accomplished by a cranial nerve that projects deep into the abdomen called the vagus nerve — no relation to people who get so drunk in Vegas, they get sick. But what happens in vagus doesn’t stay in vagus. It is not only a sensory nerve — it can also send signals to visceral structures from the brain. Signals of gastric distress travel along the vagus nerve to the nucleus of the solitary tract and into the taste centers, where they are integrated with what the tongue is telling the brain. Together, the signals are saying something like “This tastes like x and makes me feel like $#!@.”
Interestingly, there are cells in the viscera that can detect toxins and initiate vomiting. These are our first line of defense in the event that we eat something we haven’t learned about but that our stomach brain doesn’t feel so good about. Just another of the fun facts in “Gulp,” which is an accessible collection of things we don’t normally think about. In addition to the enthralling discussion of brain-gut interactions, Roach also explores differences in cultural norms related to diet and ingestion (such as whether or not excessive chewing can lower the national debt), making “Gulp” a fun and light yet thoughtful read.