Sex Sleep Eat Drink Dream (11 page)

Once while traveling in Guatemala, I succumbed to a craving for salad at a hotel restaurant and nibbled a bite or two of fresh lettuce and tomato. Not long thereafter, I lay in a feverish sweat in my hotel room, every few minutes stumbling to the bathroom, a victim of turista. (After twenty-four hours of agony, just as a candlelight Christmas procession passed by the window, I sat up in bed fully recovered—which my Catholic husband took for a miracle and I chalked up to a well-tuned immune system.)

Most of us have suffered in similar fashion. We endure a misbehaving bowel until—miracle or not—the immune system learns the nature of the new bacteria.

A much more serious disruption can result from the use and misuse of antibiotics. Such environmental meddling may create an imbalance in the normal bacterial consortia, wiping out some of the gut's residents and allowing a single strain—often a pathogen such as
Clostridium difficile—
to multiply. Even worse, in the dense, gene-swapping microbial communities of the gut, it may also encourage the evolution of microbial pathogens resistant to antibiotics.

But many of our abundant microscopic denizens are neither potential troublemakers nor passive bystanders, says Jeffrey Gordon of Washington University: "They're companions essential to our digestive well-being, symbionts that have coevolved with us and benefit from the association, just as we gain from our alliance with them." For years we've known that friendly microbes, or commensals, help us make vitamins and establish tight-knit communities that keep out potential pathogens. They also metabolize nutrients so that we can absorb them more readily (especially such otherwise indigestible components as plant cell walls). But how they accomplish their good works has been poorly understood. Most of these microorganisms are fiendishly difficult to study. It's hard to keep them alive outside the gut. And even if scientists could sustain them in the lonely isolation of a petri dish, bacteria in culture probably wouldn't behave the same way they do in their normal ecosystem in the intestines.

Gordon realized that the only way to get real insight into these beneficial bacteria is to study them in their natural setting. So he and his colleagues have developed an ingenious approach. In germ-free plastic bubbles they raise germ-free mice, which have none of the trillions of microbes that would normally reside in them. Then they introduce common gut bugs one at a time and study their effects.

What they are learning is revolutionizing our view of ourselves and how we process the food we eat. Without our resident bacteria, Gordon has discovered, our intestines would not grow properly. One way the gut protects itself from natural toxins and its own powerful acid secretions is by shedding its own lining every week or two. As the replacement cells mature, they travel from the base to the tips of those little finger-like villi lining the intestines. They do so, Gordon has found, only with the help of bacterial signals, which ensure their healthy development. Without these microbial messages, our intestines and their all-important villi would fail to grow normally.

Gut bacteria also protect this intestinal lining. Scientists at Yale discovered that the bugs help to activate the body's machinery that repairs injured cells. In killing off our friendly bacteria, antibiotics can inhibit the processes necessary for this protection and healing. Moreover, certain bugs help us tolerate harmless food proteins and other innocuous foreign matter floating inside the alimentary tract. If our immune cells react to these, triggering inflammation, it's bad news for us. One prominent microbe with the cumbersome name
Bacteroides thetaiotaomicron
ensures that our immune systems leave alone these innocent interlopers.

But here's the real shocker: Our
B. theta
and other bacteria may also help determine our girth by influencing how many of our calories are transformed into fat. Gordon and his team have found that germ-free mice can eat 29 percent more food than mice with normal microflora and still maintain a svelte figure, with 42 percent less body fat. Adding a community of gut bacteria to the intestines of the germ-free mice caused them to increase their body fat content by 60 percent in two weeks, even though they didn't eat any additional food. "That's because these bacteria improve the efficiency of calorie harvest from the diet and help the body deposit the extracted calories in fat cells," Gordon explains. When he and his colleagues probed the genome for
B. theta,
they found that many of the microbe's genes are dedicated to processing carbohydrates that we don't have the genes to digest. Without bacteria such as
B. theta,
the carbs would simply pass through our system without caloric gain.

Recently, Gordon and his lab mates took their experiments a step further. In comparing the gut bacteria of fat and lean mice, they found that fat mice had a larger proportion of a type of bacteria called Firmicutes and a smaller proportion of Bacteroidetes. When they transplanted the Firmicutes-rich microbial mix from fat mice into germ-free mice, the recipients put on more body fat than those receiving a mix of microbes from lean mice. In human studies, the team found that similar Firmicutes/Bacteroidetes proportions held true for obese and lean people. And as the obese people in their study lost weight over the course of a year under the scientists' supervision, their gut populations became more like those in the lean people.

"The message from these experiments," says Gordon, "is that the amount of calories available in the foods we consume may not be a fixed value but rather influenced by the nature of our gut microbes." The compositional differences in our resident microbes may affect the caloric density of the foods we eat, and ultimately our predisposition to obesity. Take-home lesson: Consume those nutritional labels with a grain of salt. Depending on your gut bacteria, that doughnut might have more calories for you—possibly as much as 30 percent more—than for your neighbor.

I've come to respect and admire the swarming pool of diverse creatures inhabiting my body. I like to think of them skiddling about in my gut after lunch, freely offering their genetic inventions, sidling up to my villi to whisper words of encouragement to young cells, harvesting nutrients and calories, or just idly spinning circles in the swamp water of my lumen, keeping turista at bay.

How long it takes for bowel, bugs, and brain to digest a meal depends on what we eat and when we eat it. Fat-rich meals take longer to digest than meals rich in protein or carbohydrates. About 50 percent more time is required to empty dinner from the stomach than breakfast—in part because the nighttime velocity of the so-called housekeeping waves responsible for gastric emptying is half what it is during the day.

Other aspects of the gastrointestinal tract also show daily rhythms: the activity of enzymes in the small bowel, the secretion of gastric acid, and the rates at which substances are absorbed in the intestines. Franz Halberg of the University of Minnesota determined that the body processes calories in different ways at different times of day. Eat a single daily meal of two thousand calories for breakfast each day, and you may well lose weight. Eat the same meal at dinnertime, and you'll likely gain pounds—perhaps because the body burns off carbohydrates more rapidly in the morning than in the evening.

If daily rhythms influence how we handle food, the reverse is true, too: Our meal schedule affects the pattern of our circadian rhythms. Scientists have discovered that some of those peripheral clocks in our body depend on feeding time to set their schedules. A pattern of regular meals, three times a day, is the dominant
zeitgeber
for the clocks residing in the cells of our liver, kidneys, and pancreas. This makes good sense from a physiological point of view. The body's major organs have to anticipate the handling of food and water, preparing for the required tasks ahead of time, so they're ready to absorb food, secrete digestive enzymes, and control urine production.

Toy with this regular eating pattern—as some shift workers and jet setters necessarily do—and you may screw up those peripheral clocks, wreaking havoc with your intestinal tract. (One recent study showed that daytime feeding of normally nocturnal rodents completely inverts the schedule of clocks in their peripheral tissues.) This may help to explain why shift workers and jet-lagged travelers, who eat at off hours, frequently suffer digestive upset until they adapt to the dictates of their new schedule.

So, under normal conditions, how long does it take for your egg salad and pie to pass through your inner highways and byways? Studies on so-called whole-gut transit time are few and far between, say scientists, because it's not practical to measure this in large groups in the field. But not long ago, gastroenterologists hurdled the obstacles. In one sampling of 677 men and 884 women in East Bristol, England, participants were persuaded to record the details of their diet and defecations, including careful notations on stool form (using the Bristol scale, from 1, "small hard lumps, like nuts," to 6, "fluffy pieces with ragged edges"). From these records, as well as systematic questioning about bowel habits, the researchers estimated the transit time of meals from food to feces to be fifty-five hours for men and seventy-two hours for women. This may seem like an awfully long time, and it's tempting to doubt the universality of these figures given the usual British diet. But other studies confirm that the average rate is between two and two and a half days.

There is, however, a great deal of variability from person to person and from meal to meal. "A meal is typically a mixture of chemically and physically diverse materials," explains the physiologist Richard Bowen. "Some substances show accelerated transit while others are retarded in the flow downstream." Alcohol consumption tends to quicken transit in both sexes, as does intake of dietary fiber; in women, oral contraceptives slow it. Transit time is generally shorter in older women than younger ones; the change occurs around the age of fifty, which suggests that female sex hormones may have some effect.

Want to speed things up? The safest and most natural way, say the experts, is to eat more dietary fiber.

Food spends only a few hours in the stomach and a few more in the small intestine. After our intestinal cells have done their work, what's left passes on in liquid form to the colon. The remaining dozens of hours are spent here, where water is absorbed—on the order of more than two gallons a day—and wastes prepared for elimination. Because of their bacterial content, the latter must be handled by the body "with circumspection," observes Michael Gershon, confined but also propelled through the colon's sole portal, usually once a day.

***

We often think about food. We seldom think about what it becomes. Feces, from the Latin for dregs, are made mostly of water, mucus, bile pigments (which lend stools their brown color), some fats, dead cells, gases, plenty of roughage (primarily undigested cellulose, or fiber, from plant-based foods), quite a lot of bacteria that have lost their grip on the colon, and some 1,200 different types of viruses. Roughage provides most of the bulk. Some kinds of fiber go right through our alimentary tract pretty much intact, offering no calories but a sensation of fullness, as well as exercise for our colon, giving it something to squeeze.

A diet with little roughage will produce about four ounces of excrement a day; one rich in fruits, vegetables, and grains, about thirteen. A diet of meat makes for the strongest smell; of milk, the mildest. The stink in feces arises from skatole (also present in bad breath), a byproduct of the breakdown of the amino acid tryptophan. The human nose is highly sensitive to skatole but does not always find it revolting. In fact, the compound is said to be used in small quantities as a flavoring in vanilla ice cream.

Smell rarely escapes from feces stored in the colon except in the event of flatulence. Breaking wind—farting is the more common term, used at least since the time of Chaucer, who wrote, "This Nicholas anon let flee a fart"—is the release of a bubble of intestinal gases, carbon dioxide, hydrogen, nitrogen, and methane, produced in part by the swallowing of nitrogen and in part by the action of intestinal microbes on food. Most of us fart about once an hour, depending on what we've eaten and whether we're under stress.

Preventing the occurrence is exceedingly difficult. Scientists investigated the phenomenon in a case report on a thirty-two-year-old male computer programmer who experienced extreme flatulence. "Conscious efforts to stifle air swallowing seldom are effective," say the researchers, "and the only 'treatment' alleged to inhibit air swallowing is the prevention of jaw closure by holding an object between the teeth. Our patient ... put this maneuver to the test. Unfortunately, this treatment was ineffective, as evidenced by 66 gas passages over a 13.5-h period during which he clenched an object between his teeth."

So what about modifying our bacterial flora through the use of antibiotics or by eliminating the fibrous substrate on which they thrive? "We have found that ingestion of a diet in which all carbohydrate is supplied in the form of white rice reduces flatus output," say the scientists. (A rather draconian solution if you consider the limited nutritional value of white rice.) Antibiotics fail to appreciably reduce the problem. Consuming so-called probiotics, live bacteria cultures, to induce a flora that efficiently consumes hydrogen might be useful in theory, they say; however, such "floral modification" has not yet been achieved.

So much for what goes to waste. What's happening to the energetic fruits of your meal encapsulated in the egg salad and pie? A raft of new discoveries has exposed some of the secrets of how your body uses the calories it consumes, shedding light on such mysteries as why your slender colleague Esme can eat anything she pleases and never gain an ounce while plump Phoebe consumes many fewer calories, is constantly dieting, and yet stubbornly retains her excess pounds. If you're more like Phoebe than Esme, there may be a thing or two you can do to shift the balance.