Sex Sleep Eat Drink Dream (9 page)

Of course, more pleasant associations may also dictate your choice of lunch foods. Most of us lean toward the familiar. The food that sustained my family was a mix of Jewish, 1950s American, and German cuisines: matzo brei, meat loaf, bratwurst (those stout pork sausages that release down the chin a stream of hot, spicy juice), and, with the arrival of my adopted sister from Seoul, a touch of the exotic: Korean beef and kimchi, a dish that elicits the intense oral burn that some find so pleasurable.

Roasted chicken is my comfort food, soothing in part because of its intimate link with my grandmother. "Eat," she would say, "eat," as she thrust another slab of tender white meat on my plate in her tiny Upper West Side apartment. When I pleaded a full stomach, she would wrap in brown paper the leftover pullet, complete with its pan juices, and stuff it in my briefcase for my plane trip home. This fragrant, dripping package I would tuck into a corner of the overhead baggage compartment, whence wafted aromatic vapors of rosemary and garlic to torture my fellow travelers. I did this culinary shipping trick not out of a sense of obligation or duty but because I relished the sweet flesh, so tender it seemed to evaporate in my mouth. Grandma's magic touch with poultry she passed on to her son, my father, who would dish out to his flu-stricken daughters loving bowls of hot chicken soup—Jewish ampicillin—for me, the flavor of familial love.

There may be more to comfort food than simple familiarity. Some kinds of fare contain substances that appear to boost mood. Foods such as cold-water sardines, tuna, salmon, and walnuts, loaded with omega-3 fatty acids, may have an important impact on how we feel. In 2005, William Carlezon and a team at Harvard discovered that, in rats anyway, these compounds work at least as well as prescription antidepressant drugs at lifting mood. A likely explanation for this effect is the positive impact the compounds have on the brain's mitochondria (the energy-producing power plants of all body cells), which may ultimately enhance communication among neurons in key areas of the brain. However, Carlezon emphasizes that it took a month of feeding rats an omega-3-enriched diet to see these effects. "Shorter treatment periods were not effective," he says. "So an occasional piece of fish won't do it—you need a sustained change in diet." Carlezon's finding bolsters earlier research showing a correlation between fish consumption and lower prevalence of major depression. "This work provides more evidence that our behavior—including the selection of the foods we use to fuel our body—can have a tremendous influence on how we feel and act," he says.

Another study suggests that some fare may not only boost psychological comfort but relieve physical discomfort. Researchers have found that foods rich in butter, oil, and other kinds of fat can reduce the perception of pain. Subjects fed a meal of pancakes loaded with cream and melted butter ninety minutes before having their forearm immersed in freezing water reported feeling less pain than those who had eaten pancakes of equivalent calories, but made with skim milk and water. The greatest pain relief occurred about an hour and a half after the meal. Because a liquid meal failed to offer the same relief, the scientists suspect that the effect may depend on so-called orosen-sory stimulation—smelling, tasting, and feeling those rich fatty pancakes—which may trigger the body's natural painkilling opioids.

Chocolate, known for its uplifting effects, may work its magic by the same method, sparking such a chemical kick in the brain that feels good. One study suggested that eating chocolate may create a positive mood not just in the one who's indulging, but also—if she's pregnant—in her baby. When researchers at the University of Helsinki looked for a link between the amount of chocolate eaten by pregnant women (especially those feeling stressed) and the behavior of their babies, they found that babies born to women who ate chocolate daily during pregnancy were rated more active, more likely to smile and laugh, and less fearful than the babies of those mothers who didn't indulge.

For flavor or familiarity, comfort or craving, you've selected your meal, perhaps egg salad with greens and a thick wedge of chocolate pie.

Take a bite of that pie. The mouth is full of food sensors, and not just those devoted to taste. As you tuck into creamy chocolate and buttery crust, the highly sensitive receptors in and around your teeth help to modulate the secretion of saliva, a fluid made of 99 percent water and 1 percent magic—magic in the form of sodium ions, enzymes, and a host of other organic substances, among them bacteria-fighting mucins, without which our teeth would decay. Special mechanoreceptors on your tongue sort the pieces of your mouthful by size so as to place large, tough ones between the teeth for chewing. Inside the tooth and in its sockets are still other sensors—thousands of nerve endings, the highest density in the body—which are there not for the purpose of enhancing toothache or the pain of the drill, says Peter Lucas, an anthropologist at George Washington University, but to offer fine-scale detection of forces. This helps with the up-front decisions we make about the taste, texture, and quality of food, and whether or not to swallow.

Take a look at your teeth in the mirror. The shiny white enamel crowning each tooth is the strongest tissue in your body, and necessarily so. According to Lucas, our jaws put as much as 128 pounds of pressure on our teeth when we chew, creating tensile forces to crush, grind, slice, and break apart food into particles. All of this pressure, or mechanical loading, is important not just for grinding food but for maintaining bone in our jaws; without it, the bone would slowly shrink over time. Remove a tooth, and hence reduce the pressure of chewing, and the jawbone in that area will decrease by 25 percent.

Glance again at those pearly ones. Chances are they do not shine as stellar examples of ideal dentition. By animal standards, human teeth are extraordinarily disordered and the only part of the body that requires regular surgery. We may have both evolution and diet to thank for this. Because tool use and cooking have reduced our food to small particles or mush, like egg salad and mashed potatoes and chocolate pie, we don't chew nearly as much as our ancestors did. On average, we spend only an hour a day chewing (a sixth of the time a chimp spends for the same calorie intake). And even during that solo hour, with our diet of soft and processed foods, we don't generate much force. Compared with a raw potato, a cooked one reduces stress to molars by more than 80 percent.

Chewing, or the lack thereof, can quickly transform the anatomy of our jaws, says Dan Lieberman, a biological anthropologist at Harvard. When Lieberman fed a soft diet of cooked food to small furry animals called hyraxes, or rock rabbits, he found that their snouts developed thinner, shorter bones than hyraxes fed a diet of raw food. In Lieberman's view, something similar has been happening to our tribe. "Since the Paleolithic, our faces have been reduced in size by about 12 percent," says Lieberman, "and most of this shrinkage has occurred in the mouth and jaw." Our teeth, on the other hand, have largely maintained their number and size despite this facial diminishment, causing crowding and other dental ills.

Even with the help of saliva and plenty of chewing, swallowing is no easy feat. I grasped this for the first time while watching what transpired inside the pink gullet of a medical student at the University of Virginia School of Medicine. An otolaryngologist had numbed the young woman's throat and inserted through her nose a fiber-optic tube with a camera attached, which projected the image on a giant movie screen.

"You're looking at Lisa's pharynx," said the anatomist in charge, Dr. Barry Hinton. This is the cavity where the hollows of the mouth and nose join in the back of the throat, familiar to all who have experienced postnasal drip. On the big screen, the pharynx looked for all the world like a pulsating pink cave. Dr. Hinton asked Lisa to breathe normally as he pointed out the details of her larynx, also known as the voice box, the organ that plays so crucial a role in breathing and speaking: its opening, or glottis, and its little fold of vocal cords, which widened and narrowed with beautiful proficiency as she inhaled and exhaled. Here the channels for air and food diverge, leading, respectively, to the trachea, passage to the lungs, and to the esophagus, tunnel to the stomach.

"Speak a little, if you can," Dr. Hinton said. Lisa achieved her mission with some difficulty, gagging at first, then mastering the task, the window in her glottis narrowing and opening wide with the voicing of the
p
in
please
and the
t
in
take,
as in "Please take out this tube."

"One final task," said Hinton. "Swallow." Lisa grimaced. Then the coral circles of muscle in her throat contracted in a quick spasm, raising the larynx and swinging the epiglottis into place over its opening to shut off the respiratory tract so that she might swallow without choking. It seemed nothing less than wondrous.

You're slowing down on your pie now, nibbling here and there as your appetite tails off. The human stomach expands to receive a meal of as much as two and a half pints (that's about half the capacity of a dog's stomach, and just a hundredth that of a cow). This it holds for a few hours, depending on the amount of food, before delivering it, by degrees, through waves of contraction, to the small intestine.

Stretch receptors in the stomach help to signal fullness. But the matter is not so straightforward: At least half a dozen messages from the stomach and intestines reinforce the "stop eating" message. Two hormones, CCK and PYY, made by intestinal cells and secreted in response to the presence of food in the gut, play a key role in delivering the satiety signal to the brain. Give people an infusion of these hormones, and they'll cut back on food intake and end their meal earlier. In one recent study, people injected with PYY and then offered a free-choice, all-you-can-eat buffet two hours later ate a third fewer calories than people who were injected with a saline solution; these appetite-suppressing effects lasted twelve hours.

How quickly you feel satisfied also depends on what you eat. Foods are not equally effective at suppressing hunger signals. Those rich in fiber, which move more slowly down the gut, may trigger more PYY than do fast foods made of refined carbohydrates, which are quickly dissolved in the stomach. David Cummings and his team have shown that protein and sugar both suppress ghrelin, triggering a quick 70 percent decrease in the hunger hormone, while fat makes ghrelin levels fall more slowly and only by about 50 percent. The researchers suggest that this weak suppression of ghrelin by high-fat foods could be one of the mechanisms underlying the weight gain that comes with high-fat diets.

Whatever the content of the cuisine, however, the message eventually gets through:
enough.

5. POST-LUNCH

T
HE SUN IS HIGH,
the breeze light, the meal heavy in your stomach. Best to walk the mile or so back to the office. As you stride along the sidewalk, threading through the crowd, you're setting in motion more than fifty bones in the ankle and foot—a quarter of the bones in your body—as well as multiple muscles and ligaments, all interacting dynamically with the ground.

"If I could not walk far and fast, I think I should just explode and perish," wrote Charles Dickens. Goethe composed poems while walking. So did Robert Frost and Dante. Some observers have even credited walking, legs and arms swinging in pendulum time, with imparting rhythms for famous poems and prose, including Dante's
Purgatorio,
with its measures that mimic the human gait.

Whether or not we need walking for sane mind or sound meter, we do seem built for it. To fathom what's going on in the body during such a seemingly simple act of human locomotion, scientists analyze the movement of limbs and expenditure of energy of subjects walking or running on a treadmill. As a volunteer subject in one such experiment in 2005, I was wired up and put through the paces at Dan Lieberman's laboratory at Harvard University. On my feet were pressure sensors to show my heel and toe strikes. Electromyography sensors revealed the firing of my muscles, and accelerometers and rate gyros on my head detected its pitch, roll, and yaw. Small silver foam balls attached to my joints—ankle, knee, hip, elbow, shoulder—acted as infrared reflectors for three video cameras mapping in three-dimensional space the location of my limb segments. Later I would wear a mask connected to equipment that gathered information on how much oxygen I consumed while walking and running, a measure of my energy expenditure.

Ail this gear was about as comfy as a hair shirt, especially the head part, improvised from elastic, foam, and wires. But learning the lore of locomotion was worth the discomfort.

Walking feels easy because it easily converts the body's potential energy to kinetic energy, Lieberman explained. A walking human body is not unlike an inverted pendulum. The body pivots over a relatively rigid or stiff leg, with little need for energy input—the potential energy gained in the rise roughly equals the kinetic energy expended in the descent. By this trick the body stores and recovers so much of the energy used with each stride that it reduces its own workload by as much as 65 to 70 percent.

Watching on a computer screen the tabulated results from the experiment, I had to marvel at the ingenuity of the moving body, the clockwork firing of the muscles, the regular pumping action of arms and shoulders, the consistency of our long-legged stride. Walking is a highly efficient form of locomotion for our species—at least at optimal speed. Around 4.2 feet per second, or a little more than 3 miles an hour, is most economical, says R. McNeill Alexander, a biologist at the University of Leeds, in part because muscles work best at the stride length and frequency characteristic of this pace. As pace increases or decreases from this optimum, the cost to the body rises rapidly. But somehow the body knows how to minimize its costs, even when it's forced to move in awkward strides. In one study, Canadian scientists asked athletes to tread with weird gaits—little mincing steps or strange plodding strides. They found that the athletes automatically compensated for the odd gaits and minimized their energy expenditure by adjusting their pace and stride frequency. When we walk, say the researchers, the relationship between our speed and our stride length and frequency is not an accident of mechanics. The body is monitoring gait all the time and making necessary adjustments, all without a conscious thought from us.