The Body: A Guide for Occupants

In a single day, we breathe about 20,000 times, enough to inhale at least one oxygen molecule from the breath of every human who ever existed. Some of the air fills pockets in the skull near the nose, the sinuses, whose function no one really understands. The diaphragm, a muscle at the bottom of the chest, pulls downward, and the lungs expand to take in air. Air that exits the nose on a wintry day meets the cold outside air, and moisture from the breath condenses in the nostrils, sometimes causing a runny nose. 

The respiratory system is very good at keeping clean, with mucus that lines the passage from nose to lungs and traps foreign particles. Millions of tiny paddles called cilia, waving 16 times a second, push the particles up to the throat, where they are swallowed and dissolved. Larger objects get sneezed out; the moisture in a sneeze can travel 25 feet and float in the air for 10 minutes. 

Asthma affects 15% of children and 5% of adults, more often in cities than in the country, and more commonly in boys and young women. In recent decades, asthma rates have climbed in the developed world. Half of asthma cases begin with allergens, but the other half have no known cause. Many researchers believe it has something to do with the lifestyle in developed nations. 

It took a long time to figure out that smoking causes lung cancer. Most men smoked in the 1940s, yet only some got cancer, while some non-smokers also contracted the disease. In 1950, research began to suggest a link between cigarettes and cancer. The tobacco industry downplayed these announcements but introduced filters in 1953. The filters made little difference except to flavor, so producers used stronger tobacco; as a result, filtered cigarettes introduced more tar into people’s lungs. 

In 1964, the U.S. Surgeon General issued a report unequivocally linking cigarettes and lung cancer, but the medical community did not finally accept the dangers of tobacco until the late 1970s. Today, only 18% of Americans smoke, though that figure jumps to one in three below the poverty line, and lung cancer is still responsible for one-fifth of all deaths. 

One lung problem that seems trivial yet causes misery is hiccups. It’s caused by a spasm of the diaphragm, but beyond that, little is known about it. Chronic sufferers—one hiccupped continuously for 68 years—are pretty much on their own. 

Food contains energy, measured in calories, that the body uses to fuel its processes. An early proponent of the calorie as an important measuring tool was chemistry professor Wilbur Olin Atwater. In the late 1800s, Atwater put volunteers into a cabinet-sized box for up to five days, measured the foods they ate, the gases they exhaled, their urine and excrement, and produced a book, The Chemical Composition of American Food Materials, that listed the energy content, in calories, of 4,000 foods. 

The book made Atwater famous and helped get nutrition science on its feet. Atwater believed, erroneously, that the only value of food was its energy content—no one yet knew about vitamins, minerals, and other nutritional values—and he therefore recommended eating lots of meat and avoiding fruits and vegetables. 

The calorie as a measuring tool has flaws: Many calories are empty of any other food value, and not all the caloric energy of a given food is completely absorbed by the body. One way to increase energy absorption is to cook food, which “kills toxins, improves taste, makes tough substances chewable, greatly broadens the range of what we can eat, and above all vastly boosts the amount of calories humans can derive from what they eat” (229). A cooked potato, for example, is 20 times more digestible than a raw one. Pre-humans used fire to cook food as early as 1.8 million years ago. 

Humans need four macronutrients: water, proteins, carbohydrates, and fats. Beginning in the early 1900s, scientists discovered a number of micronutrients—minerals and organic chemicals, including vitamins—that are required for health, most of which can only be obtained from foods. Many vitamins and minerals are needed only in a certain range; for instance, too little or too much Vitamin A or iron can cause illness. Many people suffer from micronutrient deficiencies, while others get too much, often from overuse of nutritional supplements. 

Twenty percent of body weight is protein. Proteins are large molecules made from 20 amino acids that collect into peptides and polypeptides to construct the complex protein chemicals that do the work of, and form the structure of, the main tissues and organ systems of the body. Twelve amino acids can be made internally, but eight must be consumed whole from food, either from meat or from specific combinations of plant food, such as rice and soybeans or corn and black beans. 

Carbohydrates, a chief energy source from food, are made up of chains of carbon, hydrogen, and oxygen atoms. The smallest chains, called monosaccharides, are simple sugars like glucose, fructose, and galactose. In nature, two sugars, for example glucose and fructose, combine to form a disaccharide, in this case sucrose, or table sugar. Short carbohydrates taste sweet, but large forms, called polysaccharides, make up the starches in foods like potatoes and pasta. All carbohydrates are broken down into monosaccharides by the body for use as energy. 

Fats and carbohydrates both are made merely of carbon, oxygen, and hydrogen atoms. Carbohydrates exist in plants as energy storage or for structure; fats store the same food energy in a form efficient for storage in animals. When called on for use as energy, fats are paired with certain proteins and cholesterol for transport; these are called lipoproteins. Though highly useful in the body, too much of one kind of cholesterol circulating in the blood stream can lead to clogged arteries. A diet high in fiber reduces the amount of “bad” cholesterol absorbed from food. 

Fats from animal and certain plant foods are saturated while most plant fats are unsaturated. Hydrogenated, or “trans” fats, once thought to be a healthy alternative, are now considered bad for the liver and for cholesterol levels, “essentially a form of slow-acting poison” (237). Since 2018, trans fats have been banned in the U.S.. 

People consume about two and a half quarts of water a day, much of it from food. The idea that people should drink eight glasses of water a day stems from confusing the total amount absorbed with the total amount that should be drunk from a glass. Meanwhile, caffeinated drinks may have some diuretic action, but they do provide more water than is lost. Drinking too much water can be lethal; people have died from consuming three or more gallons in a sitting. 

In the past, people ate what they could; today, food is so plentiful that many people eat what they like, much of it junk. Worldwide, more humans now are fat than starving. A seven-nation study in the 1950s by nutritionist Ancel Keys—developer of the K ration food pack for World War II soldiers —suggested that too much fat was bad for the heart but that a “Mediterranean” diet with less fat was better for health. Two much larger 2010 studies suggest that dietary fat is not connected to heart disease. 

A more sinister culprit may be sugar: Americans consume 22 teaspoons of sugar a day, whereas the World Health Organization (WHO) recommends no more than five teaspoons. Today’s fruits contain much more sugar and are poorer in vitamins and minerals than were fruits of the 1950s. The WHO also suggests no more than 2,000 milligrams of salt per day, but Americans average 3,400 milligrams. Studies suggest that high salt intake harms people with existing heart problems. 

Diets and other health habits are complex and intertwined; teasing out the true causes of health problems is difficult, and the issue remains unresolved. Nutritionists often divide up into pro- and anti- camps with regard to certain nutrients, often ignoring the other side’s findings, a form of confirmation bias. The best advice for now is to consume a diet that’s moderate and balanced. 

Food travels through the alimentary canal at various rates, depending on a person’s activity levels, what they’ve eaten, and how much they’ve consumed. On average, food takes more than two days in men to go from mouth to anus; three days for women. 

The stomach bathes food in hydrochloric acid, which helps break it down and also kills off microbes that might cause illness. In the U.S., 3,000 people die each year from food poisoning. The most serious attacks come from E. coli and Salmonella bacteria; as many as 1 million or more Americans get salmonella poisoning every year. The biggest source of infection, at one-fifth of all cases, is leafy green vegetables. 

The stomach was a complete mystery until a Canadian fur trapper, Alexis St. Martin, was shot by accident in 1822. The hole into his stomach never completely healed, and U.S. Army surgeon William Beaumont took the trapper into his home and observed, through the hole, St. Martin’s stomach and how it affected the foods he ate. Beaumont proved that hydrochloric acid was the chief digesting chemical used by the stomach. His report in book form became the only source of knowledge about the stomach for nearly a century. St. Martin lived another 60 years. 

Below the stomach lies the small intestine, 25 feet of tubing that does most of the digesting. The small intestine, along with the rest of the alimentary canal, avoids self-digestion because it is lined with epithelial cells that protect themselves with a thick mucus and replace themselves every few days. 

The large intestine—the bowel, or colon—contains billions of bacteria that digest the fiber left over from food and that extract vitamins that are absorbed into the body along with a large amount of water. People produce roughly half a pound of fecal material per day, or seven tons in a lifetime. At the base of the large intestine is the appendix, a reservoir of bacteria that becomes dangerously inflamed in about 6% of people in developed countries.

Farts are generated in the colon and consist roughly of half carbon dioxide and half hydrogen and nitrogen. Three parts per million are hydrogen sulfide, enough to give farts their distinct smell.