Book Summary The Body

Book Summary: The Body, Bill Bryson

By James Razko

In This book summary of The Body, Bill Bryson, you will learn what photosensitive retinal ganglion cells are, What makes poop unique, and why of all the senses, smell is the outcast. The Body is a fascinating read in its entirety, and this summary will give you a pleasant taste of its extraordinary ability to inspire awe in something we all have and often don’t appreciate.

Book Summary:

The Body: A Guide for Occupants, Bill Bryson

For reasons that nobody understands, 7 billion billion billion atoms (7,000,000,000,000,000,000,000,000,000) that aren’t very special in any way have somehow managed to assemble and make you—a living and sentient being. As far as we know, you and all the other living things are astoundingly miraculous.

Somehow— life— happened to you, and it keeps on happening without you having to do much at all. For example, every second of every moment of your life, your body composed of 37.2 trillion cells works in near-perfect harmony — all without any help. The body performs an unimaginable and unquantifiable number of operations, keeping you going without fail for decades and decades on end. And in return, all it asks for is a little water and some organic compounds to run the show — not a bad deal.

The Cell

The cell is the basic unit of life. Oddly, it is little more than a compartment of sorts, that by itself is not so alive. Somehow, when the cell fills with other nonliving things like ribosomes, DNA, RNA, Mitochondria, and other cellular stuff, this compartment manages to make the miraculous— life. It does this magic trick without a central command center. Each component of the cell, in a basic sense, randomly jostles around, responding to other parts. This randomness not only results in a smooth cellular operation but also manages to communicate and coordinate with all the cells across your entire body.


Many experts agree the most remarkable part of you is your DNA. It is your body’s instruction manual, and it serves a single purpose— to propagate more DNA. In every one of your 37.2 trillion cells, there are two copies of DNA, about a meter of the famously twisty double helix stuff per cell. And, if you were to unpack all of it and stretch it into a single strand end to end, it would reach out 10 billion miles into the cosmos, well past Pluto. However, don’t be mislead by its extraordinary length. DNA is tiny, 20 billion strands of it laid side by side would make the width of a reasonably thin human hair.

DNA to anthropologist’s delight can last for tens of thousands of years. So stable that some scientists are trying to bring back the t-rex and woolly mammoth. Besides being durable, DNA also copies with remarkable fidelity, making only one error per every billion letters copied. Most of these errors, aka mutations, don’t ever amount to much, but every so often one makes a lasting difference in the gene pool, and that is evolution.

You, unfortunately, like all other living things, will pass away and once again become inanimate particles floating around the universe. Your genes, however, will continue to go on indefinitely so long as your offspring produces descendants. Since life began some 3 billion years ago, not a single link in the chain of your familial tree has ever broken. Every one of your ancestors starting with unicellular blobs successfully passed is genetic material into the future. If your DNA could think it might consider your body little more than its vehicle through time— albeit a lovely, complicated, and rather chatty one.

Yes, you are special and unique. There is no one like you. But so is every other human. We are all paradoxically very different and practically genetically identical. All humans share 99.9 percent of their DNA.

It’s important to note that the majority of your genes come from ancestors who weren’t human at all. Some had fins, and others were tiny hairy things digging through earth. It took 3 billion years of evolutionary adjustments to come up with your genome.

Yes, DNA is remarkable. It’s also very mysterious— only 2 percent of DNA codes proteins, or does anything (as far as we can understand now) practical. For this reason, much of DNA used to be known as junk DNA. Of course, just because we don’t yet understand something doesn’t make it a pile of worthless stuff. It has since been more aptly named dark DNA.

Skin and Hair

Your skin ( the cutaneous system) is some 20 square feet in size and is the largest organ of your body, weighing about 10-15 pounds.

The skin, unlike other organs, never fails. It always holds us together. Skin, as you have discovered, gives us many kinds of pleasures and pains. And, lucky for us, it also fixes itself.

Wipe your finger anywhere in your house, and you will likely pick up a generous helping of dead skin cells. We shed about 25 thousand flakes a minute. Touch your arm, and you are touching more dead skin cells. Your outer layer of skin is dead and replaces itself monthly. This wall of lovely decay shields you from the external world.

And then there are Meissner’s corpuscles. These corpuscles don’t exactly have a sweet ring to the ear, sounding like something growing on the side of a salty rock. However, Meissner’s corpuscles are found on your fingertips, lips, tongue, and abundantly on the clitoris and penis. Without realizing, they are your favorite corpuscles of all. Corpuscles can be extremely sensitive. For example, A Pacinian corpuscle can detect an astoundingly small jostle of 0.00001 millimeters —0.00001 millimeters is almost no movement at all. That was a lot of corpuscle talk.

Fun Skin Facts

If you’ve ever unknowingly sat in a wet chair, you may have spent a bit of uncomfortable time deciding if it was damp or cold. This uncertainty is due to the fact we don’t have any receptors for wetness.

You can feel things you aren’t touching at all. For example, smack a rod into the ground, and you can quickly tell if its sand or gravel.

If you’ve ever tried to tickle yourself, you might have discovered its hard. That’s because your brain tells you both how things feel and how they should feel.

All skin colors come from a sliver of skin about a millimeter thick. Throughout history and even today, that small layer of skin designed to protect us from the sun and enable absorption of vitamin-D in different climates is what separates race. However, biologically speaking, there is no such thing as race at all. It’s an unfortunate human construct.

Dark skin is humanity’s default color. It’s thought that light skin came about due to migration and agriculture. As humans moved to northern climes and stopped eating vitamin-D rich fish and game, it became an advantage to have lighter skin, which enables the synthesis of extra Vitamin-D. Vitamin D is actually a vital hormone and not a vitamin.

Naked Ape

By some estimates, the body has as much as 5 million hairs, sprouting everywhere except your lips, nipples, genitalia, and the bottom of your feet and hands.

Humans, compared to other mammals, are considered hairless.

If you had fur, goosebumps would be a useful way to add a layer of insulating air between the skin and hair. In humans, goosebumps are thought to be an evolutionary leftover of yore that doesn’t do much besides look odd. They have been so useless that not everyone has the genes to make goosebumps.


We sweat all the time, even when we aren’t doing anything at all. If it wasn’t for our unique ability to sweat and our loss of body hair, scientists think we wouldn’t have our big brains, which is our most temperature-sensitive organ.

Sweating allows us to run long distances, even in the middle of the day. It’s something our species does uniquely well. And, our ancestors used this skill to hunt animals (who cool themselves less efficiently through panting) by literally running after them like a crazed-zombie-marathon-runner until they helplessly collapsed of heat exhaustion.

Microbial You

As you’ve read earlier, you have 37 trillion cells. You also have between 30 and 50 trillion bacteria cells, but you haven’t noticed because they are invisible to the naked eye. Cellularly, you’re half-bacteria, and half-human if there is a distinction at all. However, genetically speaking, your almost entirely bacteria. Your body houses some 20 million bacteria genes and a paltry 20 thousand human genes. If your counting, you are genetically 99 percent bacteria. It may be wise to consider who is serving who?

Besides bacteria, you also host fungi, viruses, protists, and archaea. As of now, we know of about 40,000 species of microbes, about 3 pounds altogether, that our body happily hosts. Every person has a unique colony of their own. And, scientists discover new bacteria all the time.

Like genes, parents pass down some of their microbes to their offspring.

Our colony of microbes is known as the microbiome. Unfortunately, we know very little about it. We do know that the microbiome provides us with about 10 percent of our calories by breaking down foods we otherwise could not digest, and even extracts beneficial vitamins like folic acid.

The gut and the brain constantly communicate with and influence each other through the vagus nerve. Some scientists have referred to it as our second brain.

Our microbes are so essential that nursing mothers produce 200 kinds of indigestible complex sugars, known as oligosaccharides, in their milk. These oligosaccharides are there solely to feed their newborn’s budding colony of microbes.


Antibiotics undoubtedly save lives. They also like a land mine, kill bacteria indescrimantly. And as we’ve just discussed, you could say you are ½ or 99 percent bacteria depending on how you decide to count. It may be prudent to think twice about throwing antibiotic land mines down your throat.

The average westerner will receive between 5 and 25 courses of antibiotics by the time they reach adulthood. Unfortunately, estimates have demonstrated that ¾ of the 40 million antibiotic prescriptions written every year in the United States are for conditions that can’t be cured by antibiotics— ¾. Likewise, in the United States, you are very likely consuming a lot of second-hand antibiotics; In the USA, 80% of all the antibiotics are fed to farm animals, only because it makes them fatter.

Antibiotics destroy good and bad bacteria indiscriminately. A course of unnecessary antibiotics will do little but wreak havoc on your microbiome, aka your second brain. And a weakened and unbalanced microbiome has been associated with a large number of health problems. So, be kind to your microbiota.

Antibiotic Crisis

To say bacteria replicate and evolve rapidly is an understatement. And, careless overuse of antibiotics has helped harmful bacteria evolve resistance to, and in some cases, complete immunity to current antibiotic treatments.

Experts estimate that within 30 years, 10 million people per year will die because of antimicrobial resistance to drugs. It’s aptly called the antibiotic crisis, and if nothing changes, there may be a day when antibiotics are all but useless, and a mere scratch can again kill.

The Brain

Your brain is extraordinary. And, as far as we know, it could be the most complex thing in the universe. Let that sink in for a bit.

The brain is composed of water, fat, and proteins. 75-80 percent is water. Like your microbiome, It weighs an unassuming three pounds. The brain is oddly spongy.

A spongy piece of your cortex roughly the size of a grain of sand could hold 200,000 gigabytes of information. Your entire brain is estimated to be capable of holding 200 exabytes of data or all the world’s digital content. Do you still think your smartphone is smart?

Unbelievably, the human brain only requires about 400 calories per day. A king-size Milky Way bar, about 430 calories, would be excessive. Counterintuitively, no matter how hard you think your thinking or try to think, your brain uses the same amount of energy.

The folds in your brain are as unique to you as your fingerprints or iris.

Your brain does a lot of interesting things. Consider that it takes ⅕ of a second for visual information to travel along your optic nerve. While this may not seem like a long time, in situations of life or death, it can be critical. Your brain accommodates for this delay by continually forecasting the future ⅕ of a second at a time.

Your present and your entire remembered life are constructs created inside your brain. In fact and counterintuitively, photons have no color, sound waves have no sound, and molecules have no smell. What you see, hear, and smell is not reality at all, but a fanciful interpretation of invisible and weightless subatomic particles of matter. Yes, everything is created inside your head. Your favorite color, the smell of damp roses growing next to that pine bush you can’t remember the name of— it doesn’t exist anywhere but your imagination.

And, all these before mentioned higher processes of the brain happen in a 4-millimeter thick sheath off cerebral cortex.

If your mind is not sufficiently blown yet, consider that 10 to 12 thousand years ago, the average human brain was 1,500 cubic centimeters. Today’s average brain is 1,350 cubic centimeters. Somewhere along the way, humans lost roughly the size of a tennis ball of brain matter. Some (those who see the cup half full) like to believe that the shrinking of our brains is a result of them becoming more efficient. Truthfully, no one knows for sure, and we are just as likely to have become less mentally robust. Anecdotal evidence, like the fact that all domesticated animal’s brains have shrunk by the same proportion as ours, point to a startling consideration that our ancestors may have been smarter than us.

The Head

More than 40 muscles are responsible for making anywhere between 4,100 and 10,000 facial expressions depending on which expert your consult. Humans are complex facial recognition machines tuned to the subtle moods and expressions of others. This recognizing happens both on a conscious and nonconscious level. For example, when given a set of near-identical photos of women, where only the pupils were changed, men preferred the photo with larger pupils even though they had no clue as to why they had chosen the image.

Smiles are universal, but not all smiles are real, especially many of those held menacingly for selfies. A genuine smile lasts from a fleeting ⅔ of a second to at most 4. likewise, we can’t fake authentic smiles as we have no control over the orbicularis oculi muscle that surrounds each eye and spontaneously contracts during a real smile.

No matter what emotion we try to project on the surface, all humans betray themselves and flash microexpressions, which reveal their inner feelings. These expressions last no more than a quarter of a second, and thankfully most people don’t even notice them consciously anyway. However, you can learn to see them. Of course, be careful what you wish for as you may not like what you see.

We all know we have five senses. But, by some estimates, we have 33. For example, we have a sense of balance, appetite, and of time passing— the list goes on.


We are visual creatures, and ⅓ of the cerebral cortex is devoted to sight. The human eye, with its three color receptors, can see between 2 and 7.5 million colors. That sounds like a lot, and it is, until you learn that birds, fish, reptiles, and some humans have four color receptors and might be able to see 100 million colors.

Despite our intuitions, we see a relatively small window of the world at any given time. If you look at your fingernail at arm’s length, that is about the area you can focus on at any given moment. Thanks to saccades, small darting moments of the eye, your brain stitches together an impression of a greater field of view. Every day you will have ¼ of a million saccades that go unnoticed. Likewise, you fail to notice these same movements made by others.

Speaking of things that go unnoticed, you have a large hole in your visual field that your brain fills in with imagined stuff. This hole is created by the optic nerve that is about the thickness of a pencil. Do a web search to find your blind spot, and you can experience this rather large hole in your vision first hand. You can even make your finger disappear.


While every ear is unique to its owner, ears come in two distinct forms having either free-hanging or attached ear lobes. Ears house the three smallest bones in your body known as the ossicles. Like many parts of the human body, evolution used what it could to make the ear. The ossicles used to be devoted to chewing and were part of our distant ancestor’s jawbones.

Our brains can produce a sound with the eardrum moving less than the width of an atom. If we could hear any more, the world would be noise as we could listen to the molecules of air moving. Our ears, if you’ve ever been in the front row of a loud Britney Spears concert, can also hear sounds one million million times that of the quietest sound. Some have said our ears are near perfect auditory instruments.

Unfortunately, our ears built for quieter times can quickly get damaged. Over time we lose the ability to hear high-frequency noises first. For example, some children take advantage of this by using ringtones their teachers can not hear. And, modern life, in all its noisy glory, is speeding our eardrums decline. For example, evolution never saw earbuds, capable of producing roaring symphonic noises that only need to travel a few millimeters coming.

That’s not all the ear does. It also keeps you balanced through the vestibular system. As we age, this system akin to a plane’s gyroscope degrades and is one reason the elderly aren’t know for walking tight ropes.


Most of us take the sense of smell for granted, and according to one survey, half of the people under 35 said they would rather lose their sense of smell than give up their favorite electronic device. Alas, even scientific research pushes smell aside, publishing articles on sight and hearing ten to one.

Because of our lack of interest in all things smelly, we know little about how smells work.

We do now that humans can detect at least a trillion smells. Also, smell is the only sense not mediated by the hypothalamus. Smelly information goes directly to the olfactory cortex, located near the hippocampus, where memories form. Some scientists believe this proximity is why scents are powerfully associated with memories.

Without smell, things wouldn’t taste good. With that said, smell is intensely personal. And, one person’s delicacy is just as likely to make someone else gage. This difference is not all mental. Of the 350 to 400 odor receptors, only half of them are common to everyone. We are all smelling the world differently. For example, one scientist obsessed with smell keeps a vial of androsterone in his desk that he opens to let his visitors take a whiff. One-third of the people who smell the vial sense nothing at all, another third smell the pleasant aroma of sandalwood, while the last third smell a rather revolting aroma of urine.

When smell is concerned, we don’t give ourselves enough credit. In some tests, humans outperformed dogs when trying to follow a scent like a bloodhound around a grassy field. In this challenge, Humans, not surprisingly, did particularly well with chocolate.


The tongue can identify sweet, salty, sour, bitter, and umami (savory). Some taste experts also believe we have receptors for metal, water, fat, and kokumi (full-bodied).

Smell-scientists contribute 70-90% of a food’s flavor to smell. Whenever we talk about taste, we are usually talking about flavor, which is taste plus smell.

The Japanese scientist who coined and discovered umami also co-founded the company Ajinomoto which produced ⅓ of the world’s synthetic umami, aka MSG.

MSG has gotten a bad reputation in the western world. Many people see it as a toxin that produces headaches and malaise. However, MSG is the most thoroughly studied food additive, and no one has ever been able to find a problem.

It’s All in Your Head

The sensation of taste forms inside the brain, along with everything we experience. For example, your favorite ice cream is nothing more than a bunch of molecules. It’s aroma, taste, texture, color, and so on don’t exist. The brain imagines these properties for your delight.

The Heart

If you’ve pledged allegiance to a flag while holding your hand over your heart, very likely you missed the mark, it’s actually more centrally located. Your heart beats about 100,000 times a day and up to 3.5 billion times in a lifetime. It takes about fifty-five seconds for blood to journey around your body. And like the Energizer Bunny, it keeps going on and on. This nifty piece of engineering weighs less than a pound and pumps with such force that if the aorta is severed, blood can spurt three meters high. Counterintuitively, the blood passing through the heart doesn’t supply itself with oxygen. It receives its supply of blood like all the other organs through the coronary arteries.

The heart for all its durability also seems to be vulnerable, having more named conditions than any other organ in the body. And, all of those conditions from the well-known heart attack to the obscure Takotsubo cardiomyopathy are bad news.

Heart disease is a serious issue, killing as many Americans each year as cancer, influenza, pneumonia, and accidents combined. In fact, due partly to inactivity and access to the modern food supply, you are 70 percent more likely to die from heart disease than someone living in the year 1900.


Blood does a whole lot more than carry oxygen. It also transports hormones, vital chemicals, carries off waste, and tracks down and kills pathogens. A single drop of blood can house as many as 4,000 molecules.

Platelets do more than form clots. They also have a role in immune response and even aid in regenerating tissue.

The Chemistry Department

The pituitary is your master gland, doling out or regulating the production of growth hormone, cortisol, estrogen, testosterone, oxytocin, adrenaline, endorphins, and a bunch of other important stuff.

The liver is known as the body’s laboratory and takes part in over 500 metabolic processes. The liver produces hormones, proteins, and digestive juices. It also filters toxins, disposes of old red blood cells, stores and absorbs vitamins, and converts fat and proteins to carbohydrates. And, it manages glucose, a process so important that if it goes wrong for even a little bit, your organs can shut down. Interestingly, you can hack off ⅔ of your liver, and it will mysteriously grow back to the right size in a few weeks.

Beside the liver sits the pancreas and the spleen. The pancreas is an essential gland, and as far as we can tell today, the spleen is expendable.

The kidneys are primarily the body’s filters and weigh only five ounces each. These small organs process 190 quarts of water and 3.3 pounds of salt every day. That’s enough to make a very salty bath. The kidneys work hard, and to our peril, over time, they lose function. From the age of 40 to 70, they lose 50 percent of their filtration capacity.


Diabetes comes in two forms type 1 and type 2. In type 1 diabetes (mostly inherited), the body stops producing insulin altogether, while in type 2 diabetes (usually a consequence of lifestyle), insulin is less effective (aka insulin resistance). Diabetes is on the rise, from 1980 and 2014, diabetes shot up from 100 million people to over 400 million. Ninety percent of those cases were type 2. This rise around the world has thought to been caused by adopting a western lifestyle, poor eating, and little activity.

Insulin, as you have likely heard, plays a role in diabetes. It is a hormone that lasts anywhere from 5 to 15 seconds in the body. There is a constant demand for this rather small protein. Like all hormones, insulin delivers chemical messages.

In a healthy person, the body monitors and adjusts insulin every second. People with diabetes are unfortunately only able to adjust their levels through self-administration periodically, resulting in a still unbalanced body, ultimately causing harm anyhow.

The Hunger Hormone

Leptin is a hormone that shook up and surprised endocrinology experts. Endocrinologists thought hormones were only produced inside of dedicated glands. Fat cells, surprising many, produce the hormone leptin. Now we know hormones are manufactured all over the body from the stomach, kidneys, brain, and even bones.

Leptin, as scientists found out also regulates appetite. Unfortunately, nobody has found a way to harness this hormone for weight loss. The hard part seems to be that there isn’t a signal in your body to say, “Hey, stop eating;” likely because humans evolved in a food-scarce world and didn’t require it. What leptin does is communicate with your body whether or not you have enough fat reserves to start demanding projects like getting pregnant or undergoing puberty.

There is another hormone associated with hunger known as ghrelin, produced in the stomach and other organs. Among other things like helping control insulin, ghrelin also accompanies and rises with hunger, but we aren’t sure if its causes or coincides with hunger. The fact is hormones can do a wide range of jobs. The body is a beautifully complex system and we have a long way to go in figuring it all out. For example, oxytocin, known as the hug hormone, is well known for producing feelings of attachment and affection. Still, it also plays a central role in facial recognition, contractions of the uterus in childbirth, and starting up milk production in nursing women.

The Skeleton

We have around about 206 bones. The number can vary between people. And, the count of 206 doesn’t include tiny sesamoid bones that are scattered around the body but primarily found in the hands and feet. The hands and feet have more than half the bones in the body.

Bones hold us together, protect our insides, store chemicals, transmit sound, may even help our memory, influence moods, and regulate glucose thanks to a recently discovered hormone known as osteocalcin that the bones produce.

The most abundant protein in the body is collagen. It is also the most fundamental element comprising bones. Collagen does all sorts of things from making the white of your eyes, the transparent part of your cornea, and fibers in your muscles.

Bones are living things and grow bigger with exercise like muscles.

Bone makes reinforced concrete look week. It can withstand a ton of compression, and if you break a bone and it heals, it doesn’t scar. You can even take up to 30 centimeters of bone from a leg, and with a framing system, it will grow back.

Yes, bones are extraordinary. But, we are mostly muscle, comprised of 600 altogether. For example, a slender man is 40 percent muscle.

The best “technology” on earth is inside us and other living things.

For example, cartilage is smoother than glass and has a friction coefficient five times that of ice. Unlike both of the before mentioned, it isn’t brittle.


Humans are the only primates around today that walk on two legs.

Interestingly, walking is mostly falling forward while letting your legs catch up. Observe any toddler, and this is easy to see.

For us to walk upright, evolution had to make a bunch of changes to our pre-human bodies. For example, evolution made our necks longer, backs bendy, knees enormous, and gave us angled thigh bones. The bulk of the changes happened some 6 million years ago. Still, it would take another 4 million years to pass so that humans would be good at endurance running and then another 1.5 million years until we had the brainpower to make tipped spears.


All this talk of walking brings us to the topic of exercise. And, getting your heart pumping does more than change your body composition. It strengthens bones, boosts your immune system, nurtures hormones, lessens the risk of diabetes and some cancers, improves mood, and fights of senility. It seems very likely that exercise benefits every single system in the body.

Sadly, only 20 percent of people can maintain moderate regular activity, and about 80 percent of Americans are overweight. The average American (even including walking around the house and work) walks ⅓ of a mile per day. Now consider that modern hunter-gathers walk and or trot about 19 miles a day to get food.

The current generation is likely to be the first in recorded history not to live as long as their parents, primarily because of weight-related issues. The healthcare costs alone for overweight people are estimated to be $150 billion.

Losing weight is hard. You’d have to walk 35 miles or jog 7 hours to lose one pound. And, most people overestimate how many calories they burn in a workout leading to overeating afterward.

A desk worker will burn 175,000 fewer calories per year than a worker on a factory floor.

Sitting for 6 hours or more per day increases the mortality risk for men by 20% and women by nearly 40%; even if you exercise, your chances of mortality are the same. No matter how you look at it (or how much you exercise), sitting for most of the day is bad for health.

The Immune system

The Immune system is much larger than we typically think and includes external defense systems like earwax, skin, and tears. If something manages to sneak past one of your external defense systems, some of our 300 different kinds of immune cells that come from our bone marrow, lymph nodes, spleen, thymus, and other places will attack the invader.

Like your fingerprint, iris, ear, and brain, every person’s immune system is unique.

Stress and lack of sleep weaken the immune system.

Because viruses and other bad things evolve so quickly, the immune system must be able to fight off an infinite number of things. Because it’s always on guard and fighting innumerable enemies, the immune system can sometimes get mixed up and attack your cells. About 5 percent of people suffer from an autoimmune disease.

After the immune system comes into contact with something bad, it will create antibodies. Antibodies recognize and fight off previous invaders. That’s why many diseases only make you sick once, and it is also how vaccines work.

You may be surprised to find that its not an infection that makes you feel like crap but the body’s reaction to it. When the body detects an invader, it fires attack chemicals called cytokines, which makes you feel feverish and ill.

Inflammation is the heat produced by the body as it defends itself. Blood vessels expand to bring in white blood cells, and this swelling produces tenderness by compressing nerves. The pus that grossly oozes from a wound is actually dead white blood cells, which gave their lives defending your body.

Of course, too much of a good thing can be harmful. If there is excess inflammation, neighboring tissues can be damaged. Of course, if there is too little inflammation, the infection won’t be stopped. It’s a balancing act.


Allergies are on the rise, and the more prosperous the country, the more allergies their people have. Some findings indicate that nitrogen oxides from diesel fuels, antibiotics, lack of exercise, and obesity may all be contributing factors.

In 1999 .05 percent of children had allergies. Today, that number has increased to two percent. The most common but not altogether right explanation for the climbing rates of allergies is the hygiene hypothesis. The hypothesis says, those living in more affluent areas grow up in more sterile environments and do not have the chance to develop resistance to infection as well as others living in more microbial active ecosystems. The truth is nobody knows for sure why we have allergies or why they are on the rise. The immune system is, for now, to complicated to understand.


The modern diet is—not great.

You might be surprised to find out that doctors can go through medical school without being taught nutrition. It should then come as no surprise that most Americans don’t know much about nutrition either. And complicating things even further, many nutrition experts don’t agree with each other even on the most critical topics.

Take salt, for example. It is so essential we have taste buds dedicated to it. However, the problem with salt is, too little, and you die for sure, too much, and you may also shorten your lifespan. Experts can’t agree on a daily recommended amount of salt.

After reading this book summary of The Body To take an enlightening dive into salt from author and salt expert James DiNicolantonio’s point of view by reading my book summary of The Salt Fix.

Likewise, expert disagreements extend to many other foods, vitamins, and minerals. So what can we do? On simple food-philosophy states: eat less added sugar, less refined grain, and more vegetables. When shopping in supermarkets, stick to the outside aisles as everything in the middle is usually processed junk.

The Guts

We know men and women are different. But did you know we even poop differently? For the average man, it takes about 55 hours for food to travel from mouth to anus. It takes the average women 72 hours— almost a full day longer, and we don’t know why or what effect this has.

During foods transit from mouth to anus, it first stops in the small intestine to be stripped of many nutrients. Then the food is passed on to the colon (large intestine) where billions of bacteria munch on and ferment the leftover fiber. As you may know, the more fiber you eat, the happier your gut bacteria. Well-fed gut bacteria reduces the risk of heart disease, diabetes, bowel cancer, and death of all types. So, eat your fiber.

The colon and its bacteria army help us absorb more calories and capture vitamins like B1, B2, B6, B12, and K that the body needs. The colon also helps the body reabsorb large amounts of water.

For much of our food, the end of the intestinal road is the porcelain throne. Stool, floating or sinking, consists of a lot of dead bacteria, undigested fiber, intestinal cells, and red blood cell residue. A gram of poop contains around 40 billion bacteria and 100 million archaea. Also, no one turd is alike, and even one end of a turd is unrecognizable than the other. Like much of the body, we don’t understand the digestive process yet either.

For a long time, people thought the appendix was useless, and indeed for a time, many doctors removed them without cause or care. Now we know that the appendix serves as a reservoir for gut bacteria.


Sleep is a mystery, and we helplessly give a third of our lives to it. Nobody knows why we sleep, what exactly its for, how much we need for the best health and happiness, or why some have problems with it and others do not.

What we do know is that sleep benefits every part of the body. Go without sleep long enough, and besides suffering immensely, the body will shut down and die.

Some things we think we know about sleep are it is involved with consolidating memories, restoring hormonal balance, emptying the brain of accumulating neurotoxins, and resetting the immune system. Slumber is like a tune-up for the body, and like a car, you also should not miss your regularly scheduled maintenance.

It seems that all animals sleep, even flies and bacteria. However, some animals like elephants only need two or three hours, and others require a great deal more. Why some need less or more sleep is unknown.

Impressively, some birds and marine mammals can sleep with one half of their brain at a time.

For us humans, a night’s sleep consists of a series of cycles involving four or five phases depending on which expert you ask. First comes the loss of consciousness, which takes about 5 to 15 minutes. Next, the body enters a light restorative slumber (a nap) that lasts about 20 minutes. In these first two shallow sleep states, you can be asleep and think your awake. Then deep sleep starts and lasts about an hour. Some experts divide this into two phases. And lastly, you fall into rapid eye movement (REM sleep) famous for producing our vivid hallucinations known as dreams. You then cycle between deep sleep and REM sleep for the rest of the might.

We move a lot during sleep, and we are mostly unaware of it. The average person tosses and turns around 30 – 40 times a night and wakes up unconsciously for up to 30 minutes.

You might be painfully aware that the average night sleep has shrunk from 8.5 hours fifty-years ago to 7 hours today.

You also might have noticed we don’t fall out of bed easily, even in strange places. It’s thought we owe this to our ape ancestry. In any case, it seems some part of us is keeping track of the edge during all that tossing and turning.

After this summary of The Body, discover all things sleep by reading My summary of Why We Sleep By Matthew Walker PhD.

Circadian Rhythm

Likewise, some part of us is always keeping time. You have likely experienced this, waking up right before an alarm. How our brains do this may have roots in the eye and, more specifically, in a third photoreceptor cell known as photosensitive retinal ganglion cells. Maybe the long names explain why many haven’t heard of them. These cells aren’t involved in vision at all and are there solely to detect brightness and darkness. Even though these cells don’t “see” anything, blind people can use them to determine if a light is on or off all while “seeing” nothing at all.

The photosensitive retinal ganglion cells (yup, lousy name) relay this brightness information to an area of the brain known as suprachiasmatic nuclei, which are responsible for our circadian rhythm. The suprachiasmatic nuclei is the major part of the body’s alarm clock.

Our circadian rhythm influence much more than sleep. Scientists are discovering seasonal rhythms in everything from suicide, abuse, how fast our hair grows, and even the effectiveness of some medications.


Like so many other things, we are at the very beginning of understanding our mind-body. And, the more we know, the more remarkable everything seems to be. There is so much to be discovered and already so many discoveries to admire. We are fortunate to live in a time where we can ponder the effects of photosensitive retinal ganglion cells, poop, and even the outcast sense, smell.

If you want to bathe yourself in all the current awe-inspiring knowledge of the human body, pick up Bill Bryson’s book The Body. It’s boiling over with much more exciting and elevating information than is housed in this already lengthy summary. The Body is a wonderfully fascinating, page-turning dive into ourselves.

Compliment this book summary of The Body with these other summaries:

Why We Sleep, Matthew Walker PHD
The Salt Fix, James DiNicolantonio
Digital Minimalism, Cal Newport
Deep Work, Cal Newport

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