Gut: The Inside Story of Our Body’s Most Underrated Organ Book by Giulia Enders

Human digestion is a sticky, complex subject. Even in previous reading on the gut microbiome, I didn’t know how digestion actually worked. Reading Gut: The Inside Story of Our Body’s Most Underrated Organ by Giulia Enders helped fill in some blanks. Enders presents the facts in plain language. Gut explains the digestive system detail by detail, from common bodily functions and misfunctions to microbial digestive contributions.

An Anatomy Lesson

Food enters via the mouth.

Literally, everyone knows food enters via the mouth. Lesser known is how quickly digestion starts. While our teeth break food down, increasing its surface area, saliva begins the digestive process.

  • Per Enders: “Saliva is basically filtered blood” excreted from papillae in our cheeks. “The salivary glands sieve the blood, keeping back the red blood cells…” This is why we can test hormones, including stress hormones, with saliva samples.
  • Saliva also contains a painkiller, opiorphin, “that is stronger than morphine.” This is why sore throats feel better after a meal.
  • If you follow your tongue into your mouth far enough, you’ll encounter the root of your tongue. Its nodules, filled with immune cells, investigate everything we swallow. This is part of a ring of immune tissue encircling our throat.

The gastric bubble in the stomach.

The esophagus, a 1-inch wide tube, connects the mouth to the stomach. However, the esophagus connects not to the top of the stomach, but the side. This prevents gastric pressure from forcing its way up and out. As a result, there’s a “gastric bubble” of air at the top of the stomach.

  • Enders offers two examples of the gastric bubble in action. 1) Swallowing air helps air trapped in your stomach make its escape (aka the burp). 2) Flipping to your left side when lying down will often relieve gastric pressure.
  • The shape of the stomach encourages liquids to move quickly along the short edge toward the small intestine while forcing solids to linger longer.

A close-up of the small intestine, its villi and microvilli.

The small intestine is 10-20 feet of twisty tube between the stomach and the large intestine. While that sounds long, its many folds increase surface area to make it relatively compact.

  • As partially-digested food enters the small intestine, our body squirts digestive enzymes and fat solvents onto it.
  • Each square inch of its surface contains about 20,000 villi, “tiny fingerlike projections … [protruding] into the mush of partly digested food…” Villi are covered by microvilli, which, “are, in turn, covered with a velvety meshwork made of countless sugar-based structures that look a little bit like antlers.”
  • Our body absorbs most of the nutrients from our food in the small intestine. “Each individual villus contains a tiny blood vessel—a capillary—that [takes in] the absorbed molecules.”
  • Our “rumbling stomach,” mistakenly attributed to hunger, is actually a cleanup process in our small intestines. “Our bellies don’t rumble when we’re hungry, but when there is a long enough break between meals to finally get some cleaning done!”

We find the appendix near the junction where small and large intestines meet. Mostly made of immune tissue, it’s home to a large population of bacteria.

  • Conventional wisdom states the appendix is useless. Not so! Enders clarifies: “[The appendix] acts as a storehouse of all the best, most helpful bacteria.”
  • Diarrhea can flush away gut microbes, leaving the terrain open for new microbes to settle. This is a risk! The wrong microbes can cause trouble. To help, “the appendix team steps in and spreads out protectively throughout the entire large intestine.”

The large intestine, digestion’s penultimate stop, extracts the last bits of usefulness from the small intestine’s leftovers.

  • The large intestine goes to work on the leftovers for about sixteen hours. Without an extended time, we’d miss out on many useful substances from our food. 
  • The majority of our gut bacteria live in the large intestine. There, they break down the last nutritious substances that our human cells weren’t able to liberate. 
  • Our symbiosis with microbes yields, “energy-rich fatty acids, vitamin K, vitamin B12, thiamine (vitamin B1), and riboflavin (vitamin B2).”

The last bit of the large intestine is called the rectum. There, two sphincters, one internal and one external create a barrier between the large intestine and the outside world. They collaborate to ensure a balance between waste disposal and social standing.

  • Proper #2 position is squatting, as validated by experimental analysis. “Hemorrhoids, digestive diseases like diverticulitis, and even constipation are common only in countries where people generally sit on some kind of chair to pass their stool.”

The Microbes

The most adorable microbes.

Microbes and digestion go together like horse and carriage. After explaining the various digestive organs, Enders discusses microbes’ roles. Most of us know that bacteria impact our nutrition. They take what’s left of our food and extract nutrients we previously missed.

  • We get the good stuff. “Different bacteria manufacture different substances: acids, gasses, fats,” Enders writes. These useful substances are ones we can’t extract on our own. About 90% of our nutrition comes from our food, while the rest is a byproduct of our bacteria. And another fun fact: “Bacteria produce nutrients that are so tiny we can absorb them directly into the cells of our gut.”
  • The recipe for yogurt. Bacteria sometimes pre-chew our food like mother birds. “Yogurt is nothing other than milk that has been predigested by bacteria. Much of the sugar in the milk (lactose) has already been broken down and transformed into lactic acid (lactate) and smaller sugar molecules. That is why yogurt is both sweeter and sourer than milk.”

An empty seat for microbes on top of the mucus layer.

We should thank the bacteria for their service! But where do we find them?

  • Location, location location. Some bacteria reside in the upper gut, but the vast majority live in the large intestine and rectum. Some bacteria prefer the small intestine, while others live exclusively in the colon. “More than half the bacteria that grow in our digestive tract are just too well adapted to living there to…survive outside the gut.”
  • Home sweet home. “If we say a microorganism is particularly suited to our gut, we mean it appreciates the architecture of our gut cells, copes well with the climate, and likes the food on the menu.”
  • The “seat’s taken” defense. A big way microbes keep us safer is by occupying space. Bacteria can only latch at specific locations. Friendly or benign microbes keep harmful ones from moving in.
  • Have we met? New microbial research regularly surprises us. We don’t even always know where we’ll find microbes. “In 2011, a group of researchers in the United States decided to examine the flora of volunteers’ belly buttons, just for fun. One subject naval was found to contain bacteria that were previously known to live only in the seas off the coast of Japan—despite the fact that the volunteer had never been to Asia.”

Other interesting tidbits about bacteria.

  • Tylenol and the gut. “The pain-relief drug acetaminophen can be more toxic for some people than others: some gut bacteria produce a substance that influences the liver’s ability to detoxify the drug. Whether you can pop a pill to cure your headache without a second thought is decided partially in your gut.”
  • Bacteria can modulate cholesterol. In a 2011 experiment, 114 Canadians ate yogurt rich in digestive-resistant Lactobacillus reuteri bacteria. “Within six weeks, their levels of bad LDL cholesterol sank by 8.91 percent. That’s about half the improvement attained by taking a mild anti-cholesterol drug—but without the side effects.”

Everyday Cleanliness

Though I finished Gut earlier this year, April 2020 sees us amidst a viral pandemic. Popular opinion swings toward the absolute eradication of microbes with hand sanitizer and disinfecting spray. Considering some pathogens are quite hazardous, this is appropriate at times. However, it’s also useful to consider what it means to be “clean.”

“Cleaning means removing a film of fats and proteins from surfaces. Any bacteria living in it or under it will be removed along with the film. We usually use water and cleaning fluid to achieve this.” The aim of cleaning, “…should be to reduce bacteria numbers—but not to zero. …A couple thousand Salmonella bacteria in the kitchen sink are a chance for our immune system to do a little sightseeing. They become dangerous only when they turn up in greater numbers.”

Wash your produce!

Enders offers a few tidbits on cleanliness that should help us manage our bacterial exposure. 

  • Why we wash produce. “Washing [fruit and vegetables] dilutes most soil-dwelling bacteria to such a low concentration that they become harmless to humans.”
  • Avoid kitchen sponges! “[Sponges] offer the perfect home for any passing microbe—nice and warm, moist, and full of food.”
  • And dish towels. “The same is true for [damp] dishtowels or drying-up cloths. They are most useful for spreading a nice even layer of bacteria on your utensils than for drying them.”
  • Drying surface. “Bacteria cannot breed on dry surfaces. Some cannot survive [dryness] at all. A freshly mopped floor is at its cleanest after it has dried.”

Reading Gut felt like diving into the deep end of a pool, not quite knowing the water’s depth. Swimming through the digestive system, though, proved exhilarating. In addition to learning about digestion and microbes, I even gained some useful, timely tidbits about cleanliness. Even as long as I’ve rambled, this summary is a kiddie pool compared to Enders’ book. It’s highly recommended reading for anyone who’s enjoyed splashing around here!

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