Sex Sleep Eat Drink Dream (10 page)

But now you're running a little late, so you pick up your pace. As you cast aside energy efficiency in favor of urgent speed, your breathing grows more labored. At rest, you breathe in and out about sixteen times a minute, inhaling some eight quarts of air. But switch into high gear to hustle back to the office or sprint across a busy intersection and your need for air will jump fifteen- or twentyfold. How does the body know when it's low on oxygen and needs to breathe harder?

For more than a century, scientists have been searching for the elusive oxygen sensor. Not long ago, biochemists at the University of Virginia discovered a likely candidate in a type of nitric oxide known as SNO. Nitric oxide is the gas generated during a lightning storm and best known for reacting with ozone to cause smog. The body, it turns out, makes nitric oxide in its own cells for a range of purposes, from controlling the muscles of the gastrointestinal tract to dilating blood vessels. Now it's believed that the SNO form of nitric oxide is also the messenger that allows the blood to communicate with brain regions that control respiration.

I love this idea that a gas born of lightning also sparks the heavy breathing necessary to carry us back to the office on fleet feet.

A little breathless but invigorated by your walk, you duck into the bathroom to freshen your breath with a quick tooth brushing. Here's a little-known fact to enliven your dental scrub: Brushing is not a simple matter of scouring the scum from your teeth; rather, it's an experiment in social evolution, suggests Kevin Foster, a biologist at Harvard. Like it or not, your mouth is home to a teeming society of bacteria that occupy distinct niches on tongue, teeth, and gums. "Brushing may mix bacteria that were previously surrounded by their clone mates with unrelated bacteria from another part of your mouth," says Foster. This mixing affects the evolution of their communities, which in turn determines whether they cause troubles such as tooth decay and bad breath.

That the maw is neighborhood to a secret microscopic life was first discovered by the seventeenth-century Dutch draper and naturalist Anton van Leeuwenhoek. One day, in a characteristic moment of curiosity, Leeuwenhoek scraped a little plaque from his teeth and put it beneath his microscope. He saw "with great wonder ... very many little living animalcules, very prettily amoving [which] hovered so together, that you might imagine them to be a big swarm of gnats or flies."

Only lately have we learned that the mouth hosts truly cosmic microbial communities, easily exceeding in number the six billion or so people on Earth. (Consider this: In one slow kiss, partners swap more than five million bacteria.) The six hundred or so different species of oral occupants are not uniformly distributed or haplessly floating about, but flourishing in organized communities that adhere together in "biofilms" and settle into specialized niches. These biofilms protect the bacteria and encourage their growth in family groups. The so-called Red Complex, for example, is an alliance of three species that appears to contribute to gum disease. Brushing disrupts these social relationships, says Foster, inhibiting their ability to grow, thrive, and rot your teeth, irritate your gums, or promote halitosis.

Research suggests that bad breath is mainly the result of these tiny mouth microbes satisfying their taste for proteins. In digesting the proteins, they produce what one microbiologist, Mel Rosenberg of Tel Aviv University, calls a bouquet of "truly fetid substances": hydrogen sulfide (that rotten-egg odor), methyl mercaptan and skatole (which produce the odor in feces), cadaverine (the smell of rotting corpses), putrescine (the odor of decaying meat), and isovaleric acid (the stench of sweaty feet).

Perhaps the world's expert on breath odor research and a self-described smell-ologist, Rosenberg developed a clinical test for bad breath called a Halimeter and a user-friendly litmus test called the OK-2-Kiss test, which measures the presence of troublesome bacteria and malodor. Rosenberg lists twenty-two species of bacteria known to cause bad breath. Normally, saliva washes away both bacteria and their stinky metabolic products, he says, but sometimes saliva doesn't reach the back of the tongue, where bacteria can hide and "putrefy" postnasal drip. A mouth dry from a long night of mouth breathing or a morning of fasting can worsen the situation. So can too much talking. (It's a particular scourge for politicians.) However, Rosenberg does not advise trying to do away with oral bacteria. Some species play an important protective role, he says: When their populations are reduced, say by the chronic use of antibiotics, the tongue becomes prey to colonization by Candida, a yeast-like organism that causes disease.

So how to avoid the dreaded halitosis? According to Rosenberg, Italians chew parsley. Iraqis gnaw on cloves; Brazilians, cinnamon; and Indians, fennel seeds. Thais munch on guava peels, and the Chinese drink rice wine with crushed eggshells or eat persimmon or grapefruit or red dates. If you're without access to such spices, herbs, or fruit, Rosenberg recommends keeping your mouth moist and brushing and flossing after meals, especially after eating foods rich in protein.

***

Back at your desk with a full stomach and relatively fresh breath, you're ready to tackle that stack of papers, organize the afternoon, supervise staff. You've forgotten all about your egg salad. Fortunately, your body hasn't. It's just starting the business of digestion, overseeing the millions—no, billions—of obscure little laborers managing the hard toil of eggs, salad, and pie, quietly, invisibly, so you are able to think of other things.

The clandestine events of digestion were long ago described by William Beaumont, who gained an excellent view of the subject thanks to the strange misfortune of a nineteen-year-old Canadian trapper named Alexis St. Martin. Beaumont, a U.S. Army surgeon, was called one June morning in 1822 to treat St. Martin for a large wound in the abdomen. The poor trapper had found himself at the wrong end of a shotgun. The gun had fired accidentally and struck him at a distance of only three feet, "literally blowing off integuments and muscles of the size of a man's hand," wrote Beaumont. The gaping wound was such that the trapper's death seemed certain. Despite great loss of blood and days of high fever, St. Martin survived. But the injury left a permanent hole in his stomach, a kind of valve the size of a forefinger that had to be plugged so food wouldn't ooze out during meals. The hole allowed Beaumont to see inside St. Martin's stomach to a depth of five or six inches and to conduct more than a hundred groundbreaking experiments on the workings of the stomach, its secretions, and the process of digestion.

"Pure gastric juice ... is a clear, transparent fluid; inodorous; a little saltish; and very perceptibly acid," wrote Beaumont. "It is the most general solvent in nature ... even the hardest bone cannot withstand its action." It's true. The gastric juice sloshing about in your core is one powerful brew, made of pepsin, an enzyme that breaks down the proteins in food, and hydrochloric acid—a substance so caustic it can demolish bacteria and dissolve iron—which provides the acidic environment that pepsin requires to do its work. Smelling or tasting food, or just thinking about it, stimulates cells in the lining of the stomach to secrete hydrochloric acid. Among the stomach's more famous feats is its ability to digest, say, boiled beef, with the help of this acid without burning up its own tissue or digesting itself—a talent it owes to its inner walls, which possess a layer of mucus and bicarbonate that shields it from its own corrosive contents. When gastric juice leaves the protected environment of the stomach and backs up into the esophagus, the result is the painful sensation of heartburn. If occasional, this backup is only bothersome, but if frequent, it's dangerous, as gastric juices can erode or destroy the lining of the esophagus. The production of these juices is lowest in the morning and peaks from about 10
P.M.
to 2
A.M.,
which explains why peptic ulcers act up and heartburn flares during these hours.

Despite its special equipment, your stomach is dispensable. An effective storage facility and preparer of food for digestion, kneading it into small particles, pulverizing and sterilizing it, the stomach otherwise plays just a small part in the actual process of digestion and virtually no part in absorption (except of certain drugs, such as alcohol and aspirin). The work of absorbing takes place through finger-like projections in the intestines called villi.

These days, detailed study of digestion no longer requires a bullet hole; with special scopes and chemical tools we can observe in the murky fundus or dim niches of the duodenum, even in the tiny villi, events occurring at the level of individual cells and molecules. We can track these activities over time, listen to signals flying to and from the gut, and gawp at its unexpected "intelligence."

That we digest our meals without taxing the brain is largely due to an independent, self-sufficient "brain within the belly," according to Michael Gershon of Columbia University. The brain in the head controls what goes on at the top and bottom of the digestive system, but what happens in between is managed primarily by what Gershon calls the "brain gone south."

Inside the thirty-two-foot tube of your intestinal tract lies an intricate web of millions of nerve cells that runs things, controlling both the movement and the chemistry of digestion. Only in the past few years have scientists begun to unravel the secrets of this intelligent network, known as the enteric nervous system. Gershon was among the first to suggest that the system was driven by the very same chemicals that transmit instructions in the brain. He and others have found at least thirty brain chemicals of different types operating as messengers in the bowel. These chemical messages allow the enteric nervous system to perform a plethora of tasks without the help of the brain—from sensing nutrients and measuring acids to triggering the waves of motion that propel food along the digestive tract and coordinating with the immune system to defend the gut.

According to Gershon, a steady stream of signals flies back and forth between the two "brains." Think of those butterflies in your stomach before you delivered your presentation. "We all experience situations in which our brains cause our bowels to go into overdrive," says Gershon. But as it turns out, the message traffic is heavier going north, from midriff to mind, by an order of about nine to one. "Satiety, nausea, the urge to vomit, abdominal pain, all are the gut's way of warning the brain of danger from ingested food or infectious pathogens," Gershon explains.

Your intestinal tract is surprisingly smart, versatile, brain-like. But its ultimate achievements are not yours alone: Your resident bacteria play a far larger role in digestion than ever imagined.

You may have been a sterile, singular being in the womb, but once you entered the birth canal and then the world of nipples and hands and bed sheets, you picked up an ark of microbial handmaidens. Soon the little buggers were everywhere, like words filling a page, in folds of skin, in orifices of nose and ears, and especially in the warm, cozy tunnels of your digestive tract, from mouth to anus. By the age of two, "the human body is grossly contaminated with microbes," notes David Relman, a microbiologist at Stanford University. In fact, "of all the cells that make up the healthy human body," he says, "more than 99 percent are actually microorganisms living on the skin, in the gut, and elsewhere." The small intestine is densely settled, with 100 million bacterial cells per milliliter (.06 cubic inch); the large intestine, or colon, 100
billion
per milliliter. The total weight of all these bugs has been estimated to be more than two pounds.

In 2005, scientists for the first time attempted to enumerate the different microbial species inhabiting the gut. Microbiologists used genomic sequencing to take a census of the gut flora of three healthy adults and discovered close to four hundred species, more than half of which were entirely new to science. The researchers suspect that this is the tip of the iceberg, that the number of species of gut microbes may be closer to six or seven thousand. Hundreds of these species bring with them genes that endow us with traits and functions that are useful to us—and that we have not had to develop on our own. In this way, they expand our own genome and act as master physiological chemists in our bodies. In fact, say the scientists, your body can best be thought of as a kind of genetic superorganism, a rich amalgam of human and microbial genes.

My bugs are probably different from yours. Studies of twins and their marital partners suggest that our genetic makeup helps determine the types of bacteria that are attracted to our alimentary tracts and set up shop there. But a host of environmental factors also play into the picture: where we live, what we eat and drink, our hormones and our hygiene. The bacteria we meet as infants help shape the populations we'll carry throughout life. Babies born by cesarean section may have different species than those born through the vagina. (Baby mice, at least, swallow the bacterial bits and pieces floating around in the birth canal as they make their way down.) Breast-fed babies tend to be colonized by bifidobacteria, and generally have fewer gut troubles than formula-fed infants, who are inhabited by more Clostridia, bacteroides, and streptococci. Early use of antibiotics, too, can deeply affect the populations.

As long as our microbial communities remain stable, we coexist in peace. The load of bugs is potentially dangerous, but the dense competition among them usually keeps any one player from dominating. Also, the possibly destructive ones are held at bay by the body's immune cells, which come to know the resident bacteria, learn to neutralize the toxins they make, and mount an effective attack if the interlopers venture beyond the walls of the digestive tube. However, if something changes the composition of this intestinal community—say, a bacteria-laden piece of fresh fruit or vegetable, eaten in a place where the local microbes differ from those at home—there may be unpleasant consequences.