PDF Summary:Gut, by Giulia Enders

According to Giulia Enders, the gut is a system of organs as complex and intricate as the brain. As you go about your day, your gut is hard at work breaking down food, delivering nutrients to your body, and even training your immune system to fight infection.

In Gut, Enders explains how your digestive system works and why it’s important to keep it healthy. In this guide, we’ll cover Enders's ideas on topics like where the food you eat goes, what your appendix actually does, and how tiny organisms in your intestines influence your immune system (and possibly your mood). We’ll fact-check Enders’s claims about the gut and elaborate further on the organs in this system. We’ll also discuss studies that have updated our scientific knowledge of the gut since the book’s publication.

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When the stomach is done with your food, it no longer resembles what you put in your mouth. Instead, it’s a pulp of nutritious carbohydrates, fats, and proteins called chyme.

The small intestine’s job is to absorb the food particles found in chyme into the bloodstream. From there, as we’ve already noted, the particles travel throughout the body, providing fuel for our cells and building important structures.

There are three important parts to the small intestine’s job: breakdown, absorption, and cleanup. Let’s look at each in turn.

Step 1: Enzymes and Digestive Juices Break Down Chyme

Enders explains that as the chyme enters the small intestine, the liver and pancreas release enzymes and digestive juices through a small hole called the duodenal papilla. These juices break down any food that wasn’t fully broken down in the stomach.

Step 2: Villi and Microvilli Absorb Nutrients

Once digestive juices and enzymes have broken down the chyme even further, the small intestine begins to absorb macronutrient molecules.

To do so, Enders explains, the small intestine has a massive surface area, full of twists, turns, and folds. Tiny, fingerlike protrusions called villi pepper its surface, joined by their smaller relatives, microvilli. Enders claims that if all of these folds, villi, and microvilli were ironed out into a straight, smooth line, it would measure four and a half miles in length.

(Shortform note: If four and a half miles seems unbelievable, it may be because you're familiar with the more common estimate of the length of the small intestine: around 22 feet. Both this statistic and Enders's are technically correct—Enders’s estimate includes the villi and microvilli, while the 22 feet estimate doesn’t.)

Synchronized by bioelectric pulses, the villi and microvilli move the chyme through the small intestine. They also absorb carbohydrate and amino acid molecules into the bloodstream. These molecules then travel to the liver, which filters out bad particles such as toxins.

Whereas carbohydrates and amino acids directly enter the bloodstream, fat must take a different route. Enders explains that our villi and microvilli can’t absorb fat because it’s not soluble in water. Instead, our lymphatic system brings fat from our gut directly to our heart, which then pumps it to the liver for screening.

(Shortform note: Fat going “directly to the heart” sounds scary given the connection between the overconsumption of unhealthy fats and heart disease. However, the structure of our lymphatic system is not the reason that fat can cause heart disease. Instead, saturated fats contain a harmful type of cholesterol, which builds up in the arteries and blocks blood flow. Further, a recent study suggests that food high in saturated fat makes us more susceptible to harmful bacteria. These bacteria can damage our small intestine and make it harder to absorb saturated fat, increasing the risk of heart disease.)

Step 3: A Wave of Leftovers Cleans the Small Intestine

About an hour after digestion, the stomach sends whatever leftovers remain into the small intestine. Enders describes this as a wave that cleans the gut, creating the gurgling sound that we often associate with hunger. (Shortform note: The technical term for this gurgling sound is “borborygmi,” and it can be louder or more frequent depending on the amount of gas in the gut.)

The Role of the Liver and the Pancreas

Enders briefly mentions the liver and the pancreas several times in this section of Gut. But she focuses less on these organs and more on the organs that food travels through (including the stomach, small intestine, and large intestine).

Still, the liver and the pancreas are crucial parts of the digestive system. Here are several of their most important functions.

The liver regulates your blood levels, making sure that you have the right balance of nutrients. It also produces cholesterol, stores nutrients such as iron, and breaks down medicine into a usable form.

Furthermore, the liver filters harmful substances from your blood. Enders mentions this in passing, but she doesn’t specify how the process works. Research shows that the liver filters blood by passing it through channels lined with immune cells. These immune cells attack toxins and harmful substances, causing the liver to excrete them in a substance called bile. Bile then goes through the rest of the gut and ends up in your poop.

The pancreas produces three crucial enzymes: lipase, protease, and amylase, which break down fats, proteins, and carbs respectively. It also produces hormones such as insulin, which, as we mentioned earlier, regulates your blood sugar levels.

The Large Intestine and Appendix

Enders notes that the small intestine is remarkably efficient at absorbing food particles. However, it can’t process everything. This is where the large intestine comes in: Helped by a huge population of bacteria, its job is to digest nutrients that the small intestine can’t absorb.

Attached to the large intestine is the appendix. Despite its reputation as a useless organ, Enders argues that the appendix helps digestion by maintaining the gut’s population of healthy bacteria.

Let’s take a closer look at the role of each organ.

The Large Intestine

According to Enders, the large intestine is in charge of breaking down and absorbing nutrients and water left over by the small intestine. It slowly absorbs water and electrolytes from the chyme, ensuring that we don’t lose too many fluids when we poop.

Enders explains that the large intestine houses the vast majority of the bacteria in your gut. As we’ll explain in more detail in Part 2, these bacteria use enzymes to squeeze the last nutrients out of the chyme. The walls of the large intestine then absorb the nutrients into the bloodstream.

(Shortform note: The large intestine absorbs water through osmosis. Osmosis is a process in which water moves across a semi-porous membrane—a barrier that only allows certain particles through. In the large intestine, water moves from inside the gut, through the gut wall membrane, and into the bloodstream. Once osmosis is complete and the chyme is dehydrated, the large intestine produces mucus to help keep it moving and bind it together into poop.)

The Appendix

The appendix attaches to the large intestine on the lower right side of your abdomen. Enders maintains that, contrary to popular belief, the appendix does have a use. According to Enders, the appendix consists of the same tissue that makes up your tonsils. Researchers theorize that, like the tonsils, the appendix captures and kills bad bacteria.

Researchers also suggest that the appendix repopulates our large intestine with helpful bacteria. When we have diarrhea, we poop out a lot of our gut bacteria. To compensate, Enders explains, the appendix releases helpful bacteria into the large intestine. This restores a healthy balance and prevents harmful invaders from settling in the gut.

(Shortform note: Recent research has supported Enders’s claims and extended scientists’ understanding of the appendix. A 2017 study analyzed over 500 mammal species and found that species with appendices had more immune tissue than those without them. This supports the theory that the appendix captures and kills bad bacteria. Furthermore, a 2018 study suggested that the appendix both creates an immune response and serves as a repository for good bacteria, backing up Enders's claims.)

The Final Stage: Pooping

Even after the large intestine’s job is done, there are some substances that your body can’t or won’t absorb. These substances appear in your poop. We generally think of pooping as a simple, mundane act. However, Enders maintains that it’s a fascinating and complex process and devotes almost two chapters to the subject. Here, we’ll provide a brief explanation of this process.

Enders explains that pooping works through a negotiation between our internal and external sphincters: muscles that prevent the contents of our guts from leaking. We consciously control the external sphincter, but not the internal one.

When we need to poop (or fart), Enders continues, the internal sphincter lets out a little bit of liquid (or gas). The external sphincter then sends a message to the brain, telling it that we’re ready to release our bowels. The brain then decides whether to open the external sphincter and let out the poop (or gas) or keep the sphincter closed until later.

The Highs and Lows of Pooping

Despite being a somewhat taboo topic, as Enders notes, pooping consistently is a crucial part of being healthy. What’s more, scientists have found that pooping can biologically lift your mood. It does so by stimulating the vagus nerve, which controls blood pressure and heart rate. This can lead to a feeling of relaxation while you’re on the toilet.

However, problems can also occur that make pooping more difficult. One such problem is fecal incontinence, in which we can’t control our bowel movements. This happens when there’s an issue with our sphincters: Either the sphincters aren’t able to close properly or communication between the rectum and the brain breaks down. In the latter case, we’re no longer able to recognize when we need to poop.

Part 2: Microbiomes and Bacteria

In the previous section, we discussed how the gut’s organs work together to digest food and keep us healthy—but our organs are only one part of the picture. As we mentioned in Part 1, we have huge populations of bacteria in our guts. These bacteria play a vital role in digestion, and emerging science suggests that they also have other important effects. These include strengthening our immune system and possibly impacting our mental health.

In this section, we’ll further explore the role of these bacteria populations, how we develop them, and how we can maintain a healthy gut.

The Role of Bacteria in the Gut

Our bodies are filled with and covered in microbes—tiny organisms that can only be seen through a microscope. Taken together, these microscopic organisms form what scientists call your microbiome, an ecosystem of particles operating inside your body.

Almost all of the bacteria in your microbiome live in your gut. While scientists are still discovering everything they do, it’s clear that these bacteria serve several important functions, including keeping us healthy and providing us with nutrients.

Let’s take a closer look at these functions of our gut bacteria.

Function #1: Training the Immune System

According to Enders, the bacteria in your gut train your immune system, helping you to fight off infections, viruses, and bad bacteria. She explains that your gut holds microbes in the mucus membrane, an area that prevents them from being absorbed through the gut wall. This ensures that bad bacteria won’t impact other parts of the body and cause sickness.

Enders describes the mucus membrane as a testing ground for the immune system. When your immune cells in the mucus membrane come in contact with the microbes held there, they learn to distinguish the good bacteria from the bad. In the future, this training will help your immune cells kill the bad bacteria while leaving the helpful ones alone in other parts of the body.

We Need Further Research on Bacteria and the Immune System

Enders's discussion of the gut’s impact on the immune system is somewhat vague. This is likely because our understanding of this topic is still incomplete.

It’s generally accepted that bacteria impact our immune response and train our immune system. That said, most of the studies that suggest a link between the microbiome and the immune system have been conducted on animals, not humans, including a study that found that mice without helpful gut bacteria were far more likely to develop diseases.

Even several years after the publication of Gut, scientists are uncertain about the mechanism behind the gut-immune system connection. One 2020 study highlights areas for further research, including how we develop immunity as infants, the connection between the microbiome and specific diseases, and the exact function of specific bacteria in the gut.

Function #2: Digesting Food and Producing Nutrients

In addition to training the immune system, bacteria also help in the digestive process. As we mentioned in Part 1, they help you extract nutrients from your food.

Your bacteria make your gut more efficient by breaking down foods that your body can’t process alone. Enders explains that sometimes your gut lacks the enzymes to digest a particularly complex substance, such as lactose or plant starches. Your gut bacteria can produce these enzymes to help you break down and absorb the food. In this way, according to Enders, your microbes provide you with about 10% of your nutrition.

The Many Uses of Enzymes

Enzymes are proteins that serve as catalysts in our bodies. This means that they initiate and control the speed of chemical reactions. In the digestive system, they speed up the chemical reactions that allow us to break down certain foods.

In recent years, bioengineers have become increasingly interested in using digestive enzymes outside of our bodies. For example, scientists have recently engineered enzymes from gut bacteria to produce biofuel (fuel made from plants such as corn). These enzymes can break down cellulose, which is an important and often costly step in manufacturing biofuel.

Can Bacteria Influence Mental Health?

While your gut bacteria help you stay healthy and get the nutrients you need, recent evidence suggests that they can have negative effects as well. Enders explains that one of the emerging fields of research in gastroenterology (the study of the gut) is the connection between the gut and the brain. Some scientists believe that the composition of our gut bacteria can impact our susceptibility to stress, anxiety, and even depression.

While she acknowledges that more research needs to be done on the brain-gut connection, Enders cites several studies that offer support to this theory. Let’s explore these studies and their conclusions in detail.

Laboratory Studies on Animals

According to Enders, studies conducted on mice and rats have shown a strong connection between the gut and the brain. One study, conducted in 2011, found that mice with healthier guts showed fewer depressive tendencies, performed better on memory tests, and produced fewer stress hormones.

Another study found that gut health affected mice’s personalities. Researchers used two species of mice: one that was more gregarious and curious, and one that was more shy. They switched the species’ gut bacteria and found that these personality traits reversed. A further study, conducted in 2016, took bacteria from humans with depression and put them into rats. They found that the rats exhibited symptoms of depression.

These studies demonstrate a connection between the gut and the brain in mice and rats. However, Enders is careful to note that we can’t automatically translate these results to humans.

Studies on Humans

While most of our knowledge of the gut-brain connection comes from research on animals, Enders does cite several studies with human participants. One study found that the presence of the bacteria Bifidobacterium bifidum slightly reduced anxiety before a test, but only in subjects who did not sleep well. Another study found that participants who took probiotics—a supplement of helpful bacteria—for three to four weeks saw a slight increase in mood. However, a third study, conducted in 2017, found that sustained probiotic use had no effect on depressive symptoms. Overall, Enders maintains that these individual studies are inconclusive and that more research is required.

Updates on the Gut-Brain Connection

Since Enders published the updated edition of Gut in 2018, further research has confirmed and extended her claims. Here’s a brief update on scientists’ understanding of the gut-brain connection.

For studies conducted on animals, the sources that Enders cites suggest a connection between gut health and depression, memory, stress, and personality. Further animal studies have largely confirmed these findings. For example, a 2020 study found that a probiotic regimen in rats reduced depressive symptoms and elevated the activity of neurotransmitters in the brain. Other recent studies have found that a healthy gut can help reduce anxiety and chronic stress in mice.

Meanwhile, studies with human participants have continued to show a connection between the gut and mental health, although the extent of that connection is still unclear. There’s more evidence that probiotics can help in the treatment of depression: A 2019 study found that the probiotic Lactobacillus plantarum 299v improved cognitive functions such as reasoning, memory, and learning in patients with depression. The authors recommended probiotic use as a supplement to depression therapy.

Furthermore, in a 2020 study, researchers at the University of Oxford found a correlation between the composition of gut bacteria and personality traits in humans. According to the study, participants with high populations of the bacteria Lactococcus, Akkermansia, and Oscillospira tended to be more extroverted and communicative. On the other hand, less sociable people tended to have higher populations of the bacteria Desulfovibrio and Sutterella. The researchers’ results are the first to show a connection between personality and gut bacteria composition in humans.

Developing and Maintaining Your Microbiome

As we discussed in earlier sections, our microbiomes are crucial to our health in multiple ways. But how do we get these helpful bacteria in our gut? In this section, we’ll explain how we develop our microbiomes. Then, we’ll review Enders's advice on how to maintain a healthy gut.

Microbiome Development

According to Enders, we begin to develop our microbiomes within seconds of being born. Enders claims that under normal circumstances, the womb is germ-free. This means that we first come into contact with bacteria when we enter the birth canal. These bacteria begin to train our immune system and help us digest food.

(Shortform note: Recent studies challenge Enders’s claim that the womb is usually free from bacteria. Some scientists theorize that the mother passes bacteria to the fetus through the bloodstream, citing studies that have found bacteria in the placenta. However, other scientists question the methods of these studies and maintain that more research must be done.)

As we continue to grow, our microbiomes change based on our surroundings. Enders notes that babies put many items that they find in their environments in their mouths, which exposes them to all sorts of new bacteria. While this increases the risk of illness, it also helps develop immunity and create a healthy microbiome.

While our environment influences our microbiomes, Enders also suggests that family members can pass gut bacteria down from one generation to the next. (Shortform note: Because she claims that the womb is germ-free, Enders presumably means that family members pass down bacteria through contact, not through genetics.)

Microbiome Development: Environment vs. Genetics

Other scientists agree that, as Enders notes, a child’s surroundings play a major role in microbiome composition. One study found a high correlation between the microbiomes of adopted children and their foster parents. It’s also clear that this microbiome composition is beneficial: Other studies suggest that exposure to more bacteria as a child, including exposure to a pet, can decrease the risk of Type-1 diabetes, asthma, and allergies.

But some scientists suggest that genes also play an important role in microbiome development. One study found that having certain genes can lead to an abundance of certain bacteria in the microbiome—for example, people who are genetically more likely to experience lactose intolerance often have more Bifidobacterium in their guts, a strain of bacteria that can break down lactose. Another study found that twins have statistically significant similarities in microbiome composition due to their shared genetics. However, the connection between genetics and the composition of our gut bacteria is still a relatively new field of study, and more research is required to understand the true nature of this connection.

How to Maintain a Healthy Microbiome

By the time you’re three years old, you have a basic composition of gut bacteria that will remain consistent throughout the rest of your life. Although this basic composition doesn’t change, you can still influence the health of your microbiome as you get older. Enders provides three tips on how to do this.

Tip #1: Take Prebiotics and Probiotics

Enders's first piece of advice is to nourish your gut bacteria by taking probiotics and prebiotics. While people often group them together, probiotics and prebiotics serve different functions in the gut.

Probiotics flood your gut with good bacteria. These bacteria have all of the positive effects on your gut that we discussed earlier: They help train your immune system to fight infection and provide you with nutrients. Many food and drink products contain probiotics, including pickles, kombucha, kefir, and tempeh. Pharmacies also sell probiotics in supplement form.

On the other hand, prebiotics feed the good bacteria that are already in your gut. Like probiotics, prebiotics are present in both foods and supplements. Foods that contain prebiotics are rich in dietary fiber and include beans, peas, apples, and cocoa.

The Benefits and Drawbacks of Prebiotics and Probiotics

As Enders notes, taking prebiotics and probiotics can have positive effects on health. Furthermore, studies show that taking the two together can make them more effective. This is because prebiotics rich in dietary fiber serve as food for probiotic bacteria, helping them grow, reproduce, and make your gut healthier.

However, there can be drawbacks to taking probiotics and prebiotics. The FDA doesn’t regulate supplements as strictly as food, so there are lower standards for prebiotics and probiotics bought at the pharmacy. In some cases, probiotics and prebiotics can also cause sickness. Furthermore, they can cause side effects such as gas and bloating, especially when taking both in the same sitting. To minimize side effects, try taking prebiotics and probiotics at different times during the day.

Tip #2: Avoid Bad Bacteria

While most bacteria in our gut are helpful, some bacteria can make us sick. Enders tells us to avoid bacteria such as salmonella, which is commonly found in poultry and livestock and can cause vomiting and diarrhea. She recommends thoroughly washing cutting boards, knives, and surfaces that touch eggs and raw meat, and washing your hands when cooking these foods.

(Shortform note: Even if you follow the strategies Enders suggests, you might still contract salmonella. If this happens, it’s likely to pass in four to seven days. Clinicians recommend drinking lots of water to compensate for the fluids lost to vomiting and diarrhea. If symptoms persist beyond a week, go to the hospital: You may need treatment for dehydration.)

Tip #3: Take Antibiotics Only When Necessary

Scientists designed antibiotics to cure bacterial diseases such as pneumonia by killing off bad bacteria. Enders acknowledges that antibiotics work and supports taking them when they are medically necessary. However, she explains that antibiotics also kill good bacteria. Needlessly taking antibiotics (for example, to treat a common cold) depletes the good bacteria in our guts and can even make bacteria immune to antibiotics in the future.

The Potential Risks of Antibiotics for Young Children

Some scientists maintain that overusing antibiotics is especially dangerous for babies and toddlers. These scientists theorize that there is a “critical period” in which small changes to our microbiota—such as the death of bacteria due to antibiotics—have large consequences due to the lack of diversity in infants’ microbiomes.

Studies on mice support this hypothesis, suggesting a link between antibiotic usage in early life and the development of asthma. However, other scientists maintain that more research needs to be done to establish and understand these connections in humans. As Enders recommends, take your doctor’s suggestions when deciding whether to give antibiotics to young children.