PDF Summary:The Body, by Bill Bryson

Are spicy foods bad for you? Can sleep deprivation be fatal? In The Body, bestselling author Bill Bryson takes the reader on a whirlwind tour of the human body’s various systems. He examines how they work, how they can fail, and what can be done to fix them when that happens.

In this guide, we’ll focus on the practical takeaways from Bryson’s discussion and compare his advice to perspectives from other bestsellers, such as The Omnivore’s Dilemma, The 4-Hour Body, Why We Sleep, and How Not to Die. Along the way, you’ll learn why your sense of smell is probably as good as your dog’s, how hot peppers increase your life expectancy, and why you should let your teenager sleep in.

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The Capsaicin Question

While most studies corroborate Bryson’s assertion that people who eat spicy foods have lower blood pressure and lower mortality rates, no one has yet determined exactly why this is the case.

One popular hypothesis is that frequently ingesting capsaicin changes the relative populations of microbes in your digestive system, which in turn affects how you digest food. Some scientists think capsaicin affects certain microbes much like it affects your nerve cells, tricking them into perceiving it as heat, which different species of microbes may find beneficial or detrimental.

However, it is also possible that the apparent benefits of eating spicy food could be due to other factors entirely. Many of the studies that found a statistically significant reduction in mortality for people who eat peppers also found statistically significant differences in age, gender, and socioeconomic status between the populations of people who eat peppers and people who don’t. It is difficult to compensate for these other differences with complete accuracy.

Hygiene

In addition to what you feed your body, the microbes that you expose your body to can have a significant impact on how well your body works. Bryson explains that your body is inhabited by trillions of microorganisms from thousands of different species.

(Shortform note: As Bryson explains, your body is full of microbes. According to some estimates, there are actually more bacterial cells in your body than human cells.)

We can infer two main applications from Bryson’s explanation of viruses, bacteria, and how these pathogens can be transmitted from one person to another.

Wash Your Hands

First of all, Bryson’s discussion highlights the importance of washing your hands to avoid transferring pathogens from one person to another—especially if you work in the medical industry.

Bryson points out that most of the microbes that call your body home live inside you and don’t survive outside the body well enough for scientists to even study them effectively. However, a few hundred species of bacteria make their home on your skin, where they readily spread from one person to another by physical contact. Moreover, the particular species are different from person to person, and some species are harmless to one person but infectious to another.

(Shortform note: We can infer from other sources that the reason some microbes will harm one person but not another has to do with your immune system’s ability to recognize them. As your immune system develops, it accepts your unique population of microbes as part of your body. But if your immune system encounters foreign microbes, it attacks them. Since everyone has a slightly different set of microbes, any of the species that differ could potentially trigger an immune response. And sometimes the immune response manifests in symptoms such as fever or swelling.)

Similarly, Bryson points out that even when you are completely healthy, there are thousands of types of viruses inside your body, as well as in the surrounding environment. Less than 0.3% of all known viruses are disease-causing, but like bacteria, viruses often affect different people differently. For example, studies estimate that the Covid-19 virus is relatively harmless to at least 45% of the people who contract it, but it causes severe or even fatal illness in others.

(Shortform note: One reason that viruses affect different people differently is that viruses work by splicing themselves into a cell’s genetic code, and everyone has slightly different genes. Environmental factors and immune-system familiarity (as discussed above) also likely play a role, and scientists acknowledge that they don’t yet know all the factors that contribute to the differences.)

Furthermore, Bryson explains that you have to scrub your hands with soap and water for at least a whole minute to wash off viruses and bacteria. He estimates that, even in modern-day American hospitals, inadequate hand-washing leads to more than a million serious infections per year.

How Much Hygiene Is Right for You?

Some researchers assert that removing the native bacteria cultures from your hands (or other parts of your body) actually poses certain health risks. Specifically, scientists believe that certain types of microbes play an important role in your immune system’s development, such that systematically removing them increases your risk of autoimmune diseases.

Bryson’s discussion of handwashing focuses mostly on medical personnel, where frequent, thorough handwashing is undeniably important: Any slight increase in your risk of getting an autoimmune disease is vastly outweighed by the risk of transferring an infectious disease from one patient to another.

But what about the rest of us? If you don’t work in medicine, food service, or other venues with special sanitation requirements, perhaps removing the bacteria from your hands too frequently is actually counterproductive.

Rethink Social Greetings

The second thing we can infer from Bryson’s discussion of disease transmission is that hand-shaking spreads microbes more effectively than most other types of greetings. Specifically, he says studies have shown that you’re actually more likely to pick up pathogens from someone (or transfer pathogens to them) if you shake their hand than if you kiss them.

In the West, shaking hands is a common greeting ritual in both business and personal relationships, while other cultures have historically used various customs such as kissing on the cheek or bowing to one another in this way. It might not be socially appropriate to go around kissing your coworkers, but if Bryson’s studies are correct, it may be time to rethink the methods you use to greet them.

(Shortform note: Since the start of the Covid-19 pandemic, many psychologists have taken an active interest in studying and even developing physical greetings that are less likely to spread viruses. One study concluded that the best alternative greeting to promote consisted of clasping your own hands together in front of you for the other person to see. They reached this conclusion after considering and subsequently dismissing alternatives such as bowing, elbow-rubbing, hand-waving, and foot-bumping.)

Fitness

Just as what you feed your body and the microbes you expose it to affect how well it works and how long it lasts, so does how you use your body. In particular, Bryson presents a number of reasons why you should exercise.

He notes that all your joints are lined with cartilage, which, unlike the rest of your bodily tissues, doesn’t have a blood supply. The only way your body can maintain the cartilage in your joints is by circulating the synovial fluid in your joints, and the only way you can make your synovial fluid circulate is by moving your joints. If the cartilage in your joints deteriorates too much, movement becomes painful and difficult, a condition known as osteoarthritis.

(Shortform note: Bryson’s observation of how exercise benefits your body in unexpected ways extends to other systems as well. Just as the cartilage in your joints needs movement to circulate synovial fluid, so does your lymph system. Lymph cells are part of your immune system. They identify and eliminate unwelcome cells, such as harmful bacteria and cells that have become cancerous. Instead of circulating through your body in your bloodstream, lymph cells are carried by lymph fluid through a separate circulatory system that relies on the movement of your body to move fluid through the system.)

Furthermore, Bryson says that exercise not only strengthens your body, but also bolsters your immune system, improves your mental health, and reduces your risk of heart attacks, diabetes, and cancer.

(Shortform note: In The Joy of Movement, Kelly McGonigal corroborates Bryson’s list of benefits and elaborates on how exercise improves your mental health. She explains that when you exercise, your muscles produce hormones called myokines, which circulate through your bloodstream and stimulate your brain (as well as other parts of your body). She asserts that myokines increase your cognitive performance and alleviate both physical pain and emotional depression.)

How Much Is Enough?

Despite extolling the benefits of exercise, Bryson points out that there’s no consensus on how much exercise you should get. He says the idea that you should take at least 10,000 steps a day is a baseless myth, and the CDC’s recommendation of 150 minutes of exercise per week is based on what experts think the average person can realistically work into their schedule, not on how much exercise anyone thinks is actually ideal.

(Shortform note: In The 4-Hour Body, Tim Ferriss presents his philosophy of using the minimum effective dose to get the outcome you’re looking for at a minimum cost. In the case of exercise, this means keeping your workout to the bare minimum that will allow you to achieve a certain goal, such as losing a certain amount of weight. Ferriss himself was able to achieve his fitness goals by investing a total of only four hours a month in exercise and related activity. Based on his experience, he recommends doing a short workout consisting of three specific exercises twice a week.)

Bryson goes on to discuss a study that found people who spent six or more hours a day sitting down were twice as likely to suffer from heart failure or diabetes and had a higher overall mortality rate than people who spent less than six hours a day seated, regardless of how much exercise they got the rest of the day.

(Shortform note: While many other studies support the idea that sitting for more than six hours a day carries significant health risks, very few support the conclusion that it can’t be alleviated by exercise. Currently, most sources suggest that you can reduce the detrimental effects of prolonged sitting by taking short breaks for walking or other light physical activity every 30 minutes.)

Sleep

As we’ve discussed, being active plays an important role in keeping your body functioning properly, but so does resting. In particular, Bryson’s discussion emphasizes the danger of not getting enough sleep. He discusses a study that found the US economy loses about $60 billion per year to sleep deprivation. This is because sleep-deprived workers are less productive, and Americans are now sleeping an average of two fewer hours per night than we did 50 years ago.

(Shortform note: While Bryson considered sleep deprivation from a macroeconomic perspective by citing its estimated cost to the economy as a whole, entrepreneurs David Fried and Jason Hansson address the microeconomic side of the issue in Rework. One of their key principles for building and maintaining the momentum of your company is to get more sleep. This is because getting enough sleep enhances your productivity and creativity, boosts your morale, and improves your ability to make good decisions on the job.)

Furthermore, Bryson asserts that sleep deprivation can be fatal. He provides two pieces of evidence for this. First, he cites a study in which lab rats were prevented from sleeping: All the rats died within a month. Autopsies of the rats showed no particular cause of death—after a certain amount of time without sleep, their bodies just shut down.

Second, he discusses a rare genetic disorder that can cause people to completely lose the ability to sleep. There is currently no effective treatment for the disorder, and every documented case has proved fatal: Without sleep, the victim’s organs eventually stop functioning.

(Shortform note: Some scientists question whether sleep actually serves a vital function. They observe that almost all animals sleep, but certain species of fruit flies sleep very little and can be prevented from sleeping without harming them (unlike humans or lab rats). One possible explanation is that most animals only perform certain vital functions (such as purging toxins from cells in certain organs) while they sleep, but that some are able to perform the same functions while they are awake. If this hypothesis is correct, then sleep accompanies vital functions in many animals, but is not a vital function itself.)

How Much Is Enough?

According to Bryson, how much sleep you need depends on your age, and the ideal number varies from one person to another even within a particular age group:

• Infants need about 19 hours of sleep per day.
• Preschool children need about 14 hours of sleep per day.
• School children need about 12 hours of sleep per day.
• Adolescents need about 10 hours of sleep per day.
• Adults need about 8 hours of sleep per day, plus or minus an hour or so, depending on the individual.

(Shortform note: Contrary to Bryson’s recommendations, in The 4-Hour Body, Tim Ferriss asserts that an adult can be healthy and productive with as little as two hours of sleep per day. According to Ferriss, a “polyphasic sleep schedule” makes this possible: Instead of sleeping all night and being awake all day, you take a 20-minute nap every four hours, around the clock. However, Ferriss’s assertions remain controversial because polyphasic sleep has not been studied much by scientists, and existing studies tend to contradict the idea that you can get by on just two hours of sleep.)

Let Teenagers Sleep In

In addition to pointing out that adolescents need about two more hours of sleep per day than adults, Bryson observes that our biological clocks tend to shift by about two hours during adolescence. As such, he argues that it’s biologically normal for adolescents to stay up later and sleep in later than adults.

Thus, based on the numbers Bryson presents, allowing adolescents to start their day four hours later than adults would be merely equitable. Let’s say adults get up at 6:00 AM. If teenagers go to bed two hours later because of the differences in their biological clocks, and sleep two hours longer because their bodies require more sleep, then they should get up at 10:00 AM.

(Shortform note: Psychologist and neuroscientist Matthew Walker explores this issue in more detail in Why We Sleep. He suggests that the shift in teens’ sleep schedules plays a role in their transition from childhood to adulthood. Specifically, if the adolescents in a community go to bed later than their parents, that affords them time to socialize without adult supervision. This, in turn, allows them to develop the social skills that they will need to function as independent adults.)

Medication

We’ve discussed several things you can do to keep your body functioning well, but now let’s turn our attention to what happens when you do get sick or injured. As he considers the body’s various ailments, Bryson discusses many historical and modern medical practices, covering the whole spectrum from ingenious to horrifying.

Two problems that he notes are still prevalent in the US today are overtreatment and failure to take gender into consideration when designing or prescribing medications. We’ll now consider these problems and what you can do about them.

Beware of Overtreatment

Bryson asserts that overtreatment (medical treatment beyond what would actually be beneficial) is a common problem in the developed world, especially in the United States. As such, whenever a doctor proposes treatment, take a moment to critically consider whether the treatment would actually help. Bryson discusses three particular situations that lend themselves to overtreatment:

Treatment at the End of Life

When an elderly person is diagnosed with early-stage cancer, doctors usually prescribe aggressive treatment to eradicate the cancer before it spreads. However, if the person is likely to die of other causes before the cancer becomes noticeable, then there’s no point treating the cancer. Bryson points out that early-stage cancers often take years to develop. And all current cancer treatments are hard on the body. So, depending on the person’s age and overall health, the side effects of chemotherapy or other aggressive cancer treatments might actually shorten her life expectancy more than leaving the cancer alone would.

(Shortform note: Michael Greger’s discussion of how medical treatments can result in premature death provides additional support for Bryson’s point. In How Not to Die, Greger notes that each year, over 100,000 people die from side effects of medications, and another 12,000 die from complications of unnecessary surgeries. These numbers help to illustrate the danger involved in receiving medical treatment. As Bryson points out, if the risk of dying from the treatment is greater than the risk of dying from the condition being treated, then the treatment probably isn’t worth it.)

Treatment Due to False Positives

Many tests for cancers, diseases, and other conditions are prone to “false positives,” that is, the test indicates a problem that doesn’t actually exist. For example, Bryson says that if a mammogram comes back positive, that indicates only a 10% chance that the woman who was tested actually has cancer. Overtreatment can happen when a person is treated for a condition she doesn’t have due to a false-positive test.

(Shortform note: Bryson cites Strange Glow by Timothy Jorgensen as his source of information that 90% of positive mammogram tests are false positives. However, different sources provide different numbers. One study reported that for every 10,000 mammograms, there are an average of two cases of cancer and 170 false positives, implying a false-positive rate of almost 99%. But another study asserts that the rate of false positives for mammograms is not more than 14.4%, and varies from one region to another, with many countries averaging a false positive rate around 5%.)

Treating Symptoms Instead of the Source

Treatments that address symptoms instead of underlying causes can be a form of overtreatment. For example, Bryson says that people with high blood pressure often take medication to reduce it because high blood pressure increases the risk of heart disease. He says that taking blood pressure medications reduces your blood pressure but actually doesn’t reduce your risk of heart disease, because high blood pressure is just a symptom that accompanies heart disease, not the actual cause.

Subconscious Assumptions Drive Doctors to Treat Symptoms

Bryson draws his blood pressure medication example from an article that investigated several medical procedures that are commonly prescribed, despite data showing they don’t improve patient outcomes. The original article provides additional insight on why doctors continue to employ treatments that they know are unlikely to help.

The issue seems to be that even trained physicians subconsciously oversimplify the human body: In this case, we know that people with high blood pressure are at greater risk of heart disease, so we assume a simple cause-and-effect relationship between high blood pressure and heart failure.

After subconsciously making this assumption, it seems obvious that reducing a person’s blood pressure would reduce his risk of heart disease. Even when we see data to the contrary, we have trouble accepting it, until we recognize the error in our assumption: High blood pressure corresponds to an increased risk of heart failure, but it doesn’t directly cause heart failure. And it’s particularly difficult to consciously realize this error if we made the assumption subconsciously in the first place.

Consider Gender When Planning Treatment

From Bryson’s discussion, we infer that you should ask your doctor about gender-specific side effects whenever your doctor prescribes a new medication, especially if you’re a woman.

Bryson warns that many treatments affect men and women differently. In some cases, a drug will cause severe side effects (such as brain hemorrhaging) in one gender but not the other. This is because male and female bodies metabolize chemicals differently, and it is particularly problematic because drugs are not always tested on both genders.

According to Bryson, as of 2007, 80% of all available drugs had been tested only on men during their respective clinical trials. Pharmaceutical companies used male-only test populations because they were afraid that women's menstrual cycles would introduce an additional variable, making the test results less clear.

(Shortform note: This seems to be improving. Clinical trials for new drugs now include equal representation of both sexes, and in 2019, 72% of all trial participants were women because of increased development of drugs specifically for women. However, it’s difficult to say exactly how much it still needs to improve, because gender-specific data still isn’t available for all medications on the market. A study that analyzed 137 common medications in 2018 found that women were adequately represented in clinical trials for all the drugs that they had adequate data to assess, but they were only able to assess 28% of the medications because gender data wasn’t available for the others.)

Bryson also warns that women are more often misdiagnosed because the symptoms of many medical conditions manifest differently in men and women. Heart attacks, in particular, are often mistaken for other conditions in women because a woman having a heart attack often experiences abdominal pain and nausea, whereas doctors more readily associate chest pain with heart conditions (which is the classic symptom of heart attacks for men).

(Shortform note: Experts at the Mayo Clinic describe additional symptoms of heart disease in women. Besides abdominal pain and nausea, symptoms may include pain in the neck, jaw, arms, or shoulders, as well as fatigue, dizziness, or heartburn.)

Where to Find Information

How do you determine if a treatment or medication is safe for you, given your gender? The obvious answer is to discuss it with your doctor—or perhaps with another doctor, if the physician who prescribed it isn’t well versed in how it might affect men and women differently.

One resource you can also use is the National Library of Medicine. You can search their database for a drug by name and get a comprehensive overview of its uses, directions for use, known side effects, and any special precautions you should take when using it. The information isn’t perfect, because, as Bryson points out, not all medications have been studied with respect to how they affect different genders. But it’s a starting point that can bring any known issues to your attention.

Another resource is the FDA’s database of post-market drug safety information. If you look up a given drug on this website, it will provide information on any issues with a given drug that surfaced after it was released onto the market, or references to aftermarket studies on the drug.