The Science of Appetite Time Mag

Somewhere in your brain, there's a cupcake circuit. how it works is not entirely clear, and you couldn't see it even if you knew where to look. But it's there all the same—and it's a powerful thing. You didn't pop out of the womb prewired for cupcakes, but long ago, early in your babyhood, you got your first taste of one, and instantly a series of sensory, metabolic and neurochemical fireworks went off.

The mesolimbic region in the center of your brain—the area that processes pleasure—lit up. The vagus nerve flashed signals to the stomach, which began to secrete digestive acids. The pancreas began churning out insulin. The liver cranked up to refine the body's chemistry to accommodate the sugar and fat and starch that were coming in. As all those complex processes were unfolding, your midbrain filed away a simple, primal, unconscious idea: Cupcakes are good. A lifetime love affair—perhaps pleasant, perhaps tortured—began.

Human beings have always had a complicated relationship with food. Staying alive from day to day requires our bodies to keep a lot of systems running just so, but most of them—circulatory, respiratory, neurological, endocrine—operate automatically. Eating is different. Like sex, it's a voluntary thing. And like sex, it's a sine qua non to keep the species going. So nature cleverly rigs the game, making sure we pursue them both by making sure we can't resist them. In the case of food, that has lately spelled trouble. Human history has usually been characterized by too little to eat rather than too much. Nature never planned for what could happen when unchecked appetites were suddenly matched by unchecked resources. But we're seeing it now.

Postindustrial humans—as any trip to an all-you-can-eat buffet will tell you—have become a soft, sedentary, overfed lot. It's not just that 67% of the U.S. population is either overweight or obese (including about 17% of children ages 6 to 19); it's that we know that fact full well and seem helpless to control ourselves. We lose weight and routinely regain it; we vow to eat healthfully and almost always lapse. Our doctors warn us about our rising blood pressure and creeping cholesterol, and we get briefly spooked—until we're offered the next helping of cheesecake or curly fries, our appetite shouts down our reason and before we know it, we're at it again. By some estimates, Americans are collectively more than 5 billion lbs. overweight. While in a population of 300 million people that's a more or less manageable 17 lbs. apiece, the weight is not distributed equally. Those of us who are overweight are often grievously so. But all of us pay the cost. "That weight is a shared burden," says geneticist and microbiologist Ronald Evans of the Salk Institute for Biological Studies in La Jolla, Calif. "Whether it's you or not, it gets factored into the medical health bill."

Just why is our appetite so powerful a driver of our behavior, and, more important, how can we bring it to heel? If that question has long defied easy answers, it's no wonder. Understanding a single biological unit—the heart, the lungs—is hard enough. Understanding a process as complex as appetite—one that involves taste, smell, sight, texture, brain chemistry, gut chemistry, metabolism and, most confounding of all, psychology—is exponentially harder. But science is trying.

Researchers in labs and institutes around the world are peering into the brain to understand the regions where appetite is perceived and satisfied, and pinpointing the receptors on cell surfaces that keep us hungry or get us sated. They're studying the neural wiring of the stomach and intestines, as well as the operation of the genes that drive our appetite, to track how satiety signals are sent and determine why they sometimes get lost. And they're peering back into human history to understand better how we were booby-trapped for overeating from the start and how we might be able, so many eons later, to cut the trip wire at last. "The scourge of body-weight disregulation has become a leading cause of death worldwide," says Dr. David Cummings, an associate professor of medicine at the University of Washington. "Understanding it is perhaps the most compelling agenda in the field of medical research."

PRIMORDIAL GLUTTONY

If you're among the 200 million Americans who have ballooned past their target weight, you can take some consolation from the fact that your early ancestors would be very proud of you. Human beings emerged into a world in which food often was scarce, often spoiled and—when we learned the art of hunting—sometimes bit back, making the idea of eating a lot when you could both sensible and necessary. If you never knew when you were going to have dinner again, it was best to gorge when the food was there.

"We were hardwired to eat and eat—and particularly eat fatty foods because we didn't get them often," says Sharman Apt Russell, author of Hunger: An Unnatural History. We're programmed not only to overeat but also to fail to recognize immediately just when we've reached that point. Mothers tell kids not to wolf their food because it's harder to enjoy it that way and also because even after you've had enough, it can take a while for your brain to get the message. By the time it does, you're not just full; you're stuffed. "The people who didn't immediately lose their appetites, who could gorge themselves and keep going, those people would survive longer during the next famine," says Dr. Jeffrey Flier, obesity scientist and professor at Harvard Medical School.

That's not to say that your body doesn't work hard to keep itself balanced. Over the course of a year, the average adult male consumes about 900,000 calories, yet his weight may not rise or fall by more than a pound. Since a pound equals about 4,000 calories, that means his annual intake is just 0.4%—or 11 calories a day—above or below precisely what he needs to keep going. "You are within a potato chip a day of matching your intake with expenditure," says Randy Seeley, professor of psychiatry and associate director of the Obesity Research Center at the University of Cincinnati.

It takes a lot to maintain such a precisely balanced cycle of fueling and burning, and in most cases, it all starts with the clock. Like other animals, we are creatures of dietary habit. Feed us at 8 a.m., 1 p.m. and 7 p.m., and we learn to get hungry as those hours approach. Throw in a snack at 3:30 or before bedtime, and we get the itch then too. At all these moments, what's fueling the feeling is a substance called ghrelin.

THE HUNGER HORMONE

Identified in 1999, ghrelin is often called the hunger hormone because that precisely captures what it does. Ghrelin is produced in the gut in response to meal schedules—and, according to some theories, the mere sight or smell of food—and is designed to give rise to the empty feeling we recognize as wanting to eat. When ghrelin hits the brain, it heads straight for three areas: the hindbrain, which controls the body's automatic, unconscious processes; the hypothalamus, which governs metabolism; and the mesolimbic reward center in the midbrain, where feelings of pleasure and satisfaction are processed. That's a neural triple play that guarantees that when ghrelin talks, the brain will listen.

Cummings has conducted studies in which he measured the hormone in people's blood every 20 min. and found that levels reliably spike as mealtimes approach. Add or subtract a daily meal, and you soon gain or lose a surge. "Grazing animals have little spikes of ghrelin all day long—20 to 30 in the case of a rat," Cummings says. One of the reasons gastric-bypass surgery can work in severely obese people—apart from the fact that it reduces the carrying capacity of the stomach—is that it also appears to turn down the ghrelin spigot. An Italian study even looked at ghrelin in anorexics and found that levels of the hormone were chronically high—a chemical alarm that the self-starvers trained themselves to ignore. All this research confirmed ghrelin's role in driving appetite, both when we really need to eat and when we merely expect to.

If ghrelin were all there was to it, we and the rats would eat ourselves to death. But even as one system is gunning our hunger higher, another is standing by to slow things down. The first step in that appetite-taming process occurs in the stomach and upper intestine, where nerves that sense stretching and distension eventually alert the brain that we're getting full. That message is reinforced by three substances that travel northward from the gut. The first, a peptide released by the upper intestine called cholecystokinin (CCK), is the most fleeting of the three, reaching the brain and increasing the feeling of heavy satisfaction that prods you to push away from the table. But cck does not last long, certainly not long enough to prevent you from eating again well before your body needs more fuel.

Racing in after CCK are two hormones, GLP-1 and PYY, that really slam on the brakes. Produced in the lower gut, they not only tell your brain you've had enough but also tell your stomach to stop what it's doing and not move anything further along into the intestines—where the real business of digestion takes place—until what's there has been broken down some. If you've ever finished a heavy meal at 8:30 p.m. and realized that you still feel stuffed when you climb into bed at 11, that's why. What's more, GLP-1 adjusts blood chemistry, stimulating the pancreas to release more insulin, which soaks up sugars released into the blood by the inrushing food and stores them in the body's fat deposits. "These two hormones go beyond meal intake and regulate overall energy balance," says Hans-Rudolf Berthoud, head of the neurobiology and nutrition laboratory at the Pennington Biomedical Research Center in Baton Rouge, La.

If despite all those obstacles in the path of overeating you still pack in too much food—and as a result pack on too much fat—the body has one other, much bigger gun it can roll out: leptin. An appetite-suppressing hormone discovered in 1994, leptin is produced by body fat itself, usually in direct proportion to how much of the tissue you're carrying. The fatter you are, the more leptin you produce. Once in the bloodstream, the hormone travels to the hypothalamus, one of the same brain regions targeted by ghrelin, seeks out a pair of neuropeptides known to stimulate appetite and partly muffles their signals. The result is, or should be, that fatter people want to eat less. Not surprisingly, the discovery of leptin was huge news in the diet community. Maybe obese people were simply suffering from a shortage of leptin; supplement the hormone with periodic injections, and the fat would dissolve away.

As it turned out, things weren't so easy. For one thing, there are hundreds of millions of obese people in the world, but even after 13 years of study, researchers have found only a handful—on the order of 10 to 20—with a congenital deficit in leptin production or function. In fact, the leptin system in most overweight people works precisely the way it's supposed to, with hormone levels climbing more or less in lockstep with weight. The problem is, at some point the stuff simply stops working—or at least stops keeping pace with the numbers on the scale. When the few people born with a leptin deficit are given supplemental injections, they respond to the treatment. But in other obese people—whose systems have been overexposed to the hormone over the years and thus grown resistant to it—the treatments do no good at all. (Some studies show that leptin sensitivity can be improved by dieting and losing body fat, making supplements a little bit more effective.)

REJIGGERING THE SYSTEM

If we haven't yet figured out how to tame our need to eat, one reason may be that ghrelin, leptin and the handful of other gut chemicals are only the big dogs of the appetite-control system. Researchers have discovered at least two dozen other hormones and peptides that play a role too. Adjusting the levels of just the few we know best is a little like upgrading the quality of the gas in your car and thinking that is going to boost it from 20 m.p.g. to 75 m.p.g. You may notice some improvement, but if you really want a better, more efficient machine, you have to open the hood and retool things in a much more fundamental way. That kind of advanced work on the human metabolic engine is just what the new generation of appetite researchers is trying to do.

Carrie Haskell-Luevano, for example, an associate professor of medicinal chemistry at the University of Florida, is peering deeper into the brain, studying the receptor sites on individual neurons to which appetite-control chemicals bond. The receptor array is massively complex, with at least 40 sites known to play a role in carrying feelings of hunger or satiation and 30 more that probably do. Not all of them are created equal, however, and one of the most intriguing is what's known as the melanocortin-4 (MC-4) receptor.

MC-4 is the neural gateway for ghrelin, leptin and up to 20 other chemicals. Haskell-Luevano and other scientists have found at least 11 genetic mutations in some obese people that appear to cause the MC-4 receptor to malfunction. In some cases, the receptor may fold improperly on the surface of the cell; in others, chemicals may bond imprecisely to it. Either way, appetite regulation can get scrambled but good. "Basically," says Haskell-Luevano, "if you block the MC-4 pathway, hormones like leptin don't work."

Haskell-Luevano is also looking at the adjacent MC-3 receptor, which, while not as powerful as MC-4, does help the brain govern energy balance and can have mutations of its own. Still, genetic problems in these two receptors would not explain all cases of obesity or even very many. Only about 6% of adults and children considered obese have any MC-4 anomalies, though Haskell-Luevano suspects that other irregularities in the genetics of metabolism or appetite control could push the overall total of gene-based obesity closer to 25%.

Flier, of Harvard, is studying neurons in the hypothalamus that are supposed to produce a protein that responds to leptin but, for as-yet-unclear reasons, sometimes fail to. Find ways to stimulate those neurons—or even add doses of the protein they're failing to make—and you could theoretically restore appetite balance. And Evans, of the Salk Institute, is studying cellular receptors known as PPARs, which control the flow of sugar and fat throughout the body. After a meal, fatty acids enter cells, triggering the receptors to rev up the body's energy-burning motor. The more active this system is, the more fat you'll burn; what you don't use gets stored. If you're obese, there's a good chance your PPAR system is idling too low.

"We'd like to use a drug to boost metabolism so you're more efficient at burning," says Evans. Working with mice, he has conducted precisely those kinds of studies, developing drugs that stimulate PPARs and boost fat burning. Five pharmaceutical companies are conducting similar studies, at least two of which have now moved onto human subjects.

Another, entirely different avenue of research is being pursued by Michael Cowley, neuroscientist and associate professor of physiology and pharmacology at the Oregon National Primate Research Center. Cowley is interested in how excessive eating mirrors the patterns of drug addiction. Few people who compulsively consume—whether the substance of choice is food, drugs, tobacco or alcohol—are unaware that their habit could kill them. Yet few have the power to stop, at least not easily. "The phrase comfort food hits the nail on the head," Cowley says. In the case of both food and drugs, ghrelin again appears to play a role.

While the hormone affects three areas of the brain, it hits the mesolimbic reward region particularly powerfully. Studies of this part of obese people's brains reveal a level of activity remarkably similar to that in the brains of drug addicts when they are exposed to their preferred substance. What's more, in both kinds of people, there is a general deficit of activity in the mesolimbic region, which suggests that the compulsive intake of food or chemicals may simply be an attempt to compensate for this shortfall. Cowley has a financial stake in this argument, since he is the founder and chief scientist of a pharmaceutical company that has now moved to human trials of an antiobesity drug designed to calm the cravings in the reward pathway.

WHAT CAN YOU DO?

Breakthrough drugs are hard to plan for, and if you're trying to get your appetite under control today, they do you no good at all. But other studies in the appetite field could produce results sooner. At the Pennington Center in Baton Rouge, investigators run what may be the most sophisticated metabolic test kitchen in the country, hoping, in part, to determine which kinds of meals satisfy us best with the smallest penalty to pay on the scale. The kitchen looks like any other, with the exception of the laboratory instruments that allow investigators to determine the metabolic impact of eating, say, 90 grams of a pancake breakfast as opposed to 113 or 123. People battling weight problems volunteer for several-day stays to test the menus and undergo other studies in order to help both themselves and others.

At the moment, some of the research in the kitchen involves trying to find a more precise way to balance the glucose loads various foods deliver to the body. That's important, since the bigger the glucose hit, the greater the sense of satiation, but only for a little while. Afterward, hunger returns stronger than ever. "High glycemic foods like refined breads and sugars push the body to refuel," says nutrition scientist Marlene Most, head of the metabolic kitchen. "In low glycemic foods, there is a constant flow of glucose and insulin, so we don't need to refuel as much."

Pennington neuroscientist Christopher Morrison has looked at another, comparatively fast-track approach to appetite control. If leptin supplements have been such a disappointment in keeping food intake in check, what about leptin combined with other natural suppressants such as CCK? In animal studies conducted in a lab where Morrison did his postdoctoral work, doses of CCK followed by leptin did a better job of curbing appetite than either one alone. "There have been some good data to suggest that the [effectiveness] of short-term CCK signals are influenced by the presence of leptin," Morrison says.

Other, still more direct strategies rely on other, still simpler mechanisms. Most people on diets notice that their hunger pangs seem to diminish over time. Perhaps they're just getting used to living with their cravings, but the recent findings showing that leptin may become more effective as obese people shed fat suggests a biochemical mechanism is at work. While knowing this won't make the weight fall off faster, it does provide one more incentive to stay the dietary course.

Barbara Rolls, a professor of nutritional sciences at Pennsylvania State University, advocates another way to attack hunger even more aggressively. Rolls currently tops the best-seller lists with a book about what she calls the "volumetrics" eating plan—the kind of prefab word that cries out diet fad but in this case describes a sensible idea, provided that it's followed in moderation. The key to volumetrics, Rolls explains, is to consume foods that are high in volume but not in calories in order to stimulate the digestive system's distension nerves. It's the difference between, say, a large, filling salad with a low-calorie load and a small, unfilling brownie with a high one.

"This whole idea of eating smaller portions—I'm really fed up with it," Rolls says. "It's not big portions that make you eat more. It's big portions of calories. If you eat big portions of fruits and vegetables, they displace other foods." Rolls stresses that it's important to eat a variety of tastes and textures. If you overload on one thing—say, the heavy dose of meats that the low-carbohydrate Atkins plan recommends—you're going to crave the sweet or crunchy or doughy experience of the fruits and breads you're forbidden. "It's called sensory-specific satiety," she says, and it's one of the reasons we still have the appetite for a sweet dessert even after we stuff ourselves with a heavy dinner.

The very discordance between a mouthful term like sensory-specific satiety and the uncomplicated joy of a crème brûlée at the end of a meal speaks to the puzzle that is the human appetite. We may always be pleasure-seeking creatures, intoxicated by the very experience of food—with its colors and textures and notes of flavor—but that doesn't mean our ancient impulse to eat whenever we can must always yield to our modern ability to satisfy that urge. The same human brain that invented the food court and the supermarket must now develop ways to control how we use them. Just as when we were learning to eat on the savanna, our health and even survival may be at stake.

—Reported by Dan Cray/Los Angeles, Elisabeth Salemme/New York and Carolyn Sayre/Baton Rouge

Fat chance

Forget Survivor and The Amazing Race and all the other reality shows that feature generally good-looking, generally physically fit people running around doing generally silly things. In the U.S.'s nation of overstuffed couch potatoes, The Biggest Loser hits closer to real reality by combining the TV genre's outlandishness and greed with the more mundane goal of shedding unwanted kilograms. With its $250,000 grand prize and dramatic stories of weight loss, the show is a natural for the nation's chubby, if not huddled, masses yearning to breathe free without having to unbutton their pants after a meal. More than 100,000 people applied to be on the show's fourth season, now in production, and more than 5 million have had their diets assessed on the show's biggestloserclub.com.

But while the message of the show is inspiring, it is also unrealistic. The Biggest Loser achieves rapid transformations—contestants often drop more than 9 kg in a week—through calorie restriction, endless exercise and no small amount of dehydration that occurs behind the scenes. Ryan Benson, 38, an actor who works for a DVD distributor in Los Angeles, lost 55 kg to win the first season in January 2005 but says he regained 14.5 kg within five days simply by drinking water. Matt Hoover, 31, a motivational speaker based in Seattle, had a 7-kg rebound within a day of winning Season 2. Last season's runner-up, Kai Hibbard, 28, an aerobics instructor in Alaska who says she spent the night before her final weigh-in hopping in and out of a sauna for six hours, consumed only sugar-free Jell-O for several days and wolfed down asparagus, which is a natural diuretic. "It's amazing the things you learn in a weight-loss competition," she says.

The show tries to prevent unhealthy behavior by making contestants keep food journals (to make sure they're not starving themselves) and threatening penalties if tests show they are too dehydrated (although an executive producer says no violations have been uncovered yet). But like the $55 billion U.S. diet industry, The Biggest Loser places the bulk of its emphasis on shedding kilograms rather than maintaining the loss. After all, a show called The Biggest Maintainer wouldn't have nearly the same zing. Contestants learn how to make healthy choices, but total-immersion exercise accounts for most of the weight loss. And it's not as hard to work out for four or more hours a day when urged on by professional trainers. It's also easier to resist high-calorie temptations when the cameras are rolling. Two and a half years after Benson's final weigh-in at 94 kg, the new dad has slipped out of the spotlight and into old habits. "No one sees me get an apple pie in the drive-through," says Benson, whose weight now hovers at around 136 kg.

Still, by keeping 14 kg off for more than a year, he's something of a rarity. The U.S. National Weight Control Registry, which tracks the habits of some 5,000 successful maintainers, cites a study showing only a fifth of dieters with a history of obesity sustain a loss of 10% of their body weight for a year or more. "The best predictor of the ones who are not going to regain are the ones who are doing the most physical activity," says Dr. Holly Wyatt, an obesity expert at the University of Colorado. She says most registrants exercise, on average, at least an hour a day.

Who has time for that? Kelly Minner, for one. The first-season runner-up dropped from 110 kg to 74 kg by the finale and now weighs 63 kg. A school administrator in Bethlehem, Pennsylvania, Minner, 31, says she works out from one to four hours a day, six days a week. She exercises while watching TV—and did so throughout our phone interview. For motivation, she keeps a souvenir from the show in her office: a life-size photo of her old fat self. Other winners share this strategy. Hoover, who has gained 24 kg since November 2005, sees his cardboard cutout every day in his garage. Last season's winner, Erik Chopin, 37, who owns a deli in North Babylon, New York, and since December has put on 10 of the 97 kg he lost, keeps a photo on his fridge taken when he weighed 185 kg.

The series added a more realistic component last season when 36 of its 50 contestants competed from home sans trainer. "They really got none of the bells and whistles," says the show's physician, Dr. Robert Huizenga. Well, almost none: they were still eligible to be tapped for the main show, and the at-home winner got $50,000. If the producers proceed with plans for a reunion episode later this year, it will be interesting to see how the contestants have fared since they went off camera and stopped racing for a cash reward.

Notes on a food-free diet

I hate Jeffrey Kluger. In fact, I hate all the editors of this issue who thought it would be funny to find out what would happen if I didn't eat for two days. I'll tell you what happens: I get hungry. And cranky. And cold and light-headed. What's next on your list of investigative journalism? See if a guy bleeds when you stab him? See if sour milk makes him barf? I must have missed the e-mail about how the magazine was taken over by the guys from TV's Jackass.

For my fast, I decided to do the Master Cleanse, a diet from the '70s that's become popular with the type of women who do yoga as exercise, carry Marc Jacobs handbags, buy organic food and don't read the science section of the newspaper. See, now hunger has even made me mean to my wife.

The Master Cleanse has a slight advantage over normal, Gandhi-type fasting in that it allows you to consume one thing: a glass of water mixed with 2 tbsp. of fresh lemon juice, 2 tbsp. of dark amber maple syrup plus a dash of cayenne pepper. I wisely choose two days to do this when I won't be overly tempted to eat. The first is at my father's house in New Jersey, since my dad doesn't care all that much about food and, other than dips and chips, keeps all of it solidly frozen. The second day is on a cross-country flight, when I normally fast anyway. As JetBlue's business plan demands.

The first day, I don't get at all hungry until about noon, when I make my first Master Cleanse beverage. It isn't bad, similar to what I would have expected if I ordered spicy lemonade at a restaurant, which, of course, no restaurant would put on its menu because the drink is gross. When the rest of the family enjoys its lunch of chips and dips, I don't feel that hungry. And at dinner, my dad eats some sort of microwaved leftover pasta at the counter that looks completely unappealing while my grandmother eats something that was clearly once frozen while she watches Lou Dobbs. That dude is a major appetite suppressant.

By 6:30, however, I'm feeling really light-headed. That feeling of clarity that fasters talk about? That's the loss of peripheral vision. I am also cold even though my 86-year-old grandmother is not. This cannot be healthy. Later that night, I have trouble falling asleep, not because I'm hungry but because I'm freezing. An extra blanket takes care of it. The best part of my day of fasting, I realize as I drift off, was not having to floss.

I wake up the next morning and am not particularly hungry. I am also not interested in any more spicy lemonade. But at noon, I force myself to drink some anyway. My grandmother, who never talks about herself, decides to open up and tell me the history of her life. Through food. I now know that when she started working she made meals a night in advance—and what the weekly menu was. This does not make me any hungrier. I credit this not to the fasting but to Jewish cooking.

By late afternoon, I am still not all that hungry. In fact, I'm a little less hungry than I was at this time the day before. I feel superior to food. Seeing people eat on TV makes them look weak. I also have far more time to focus on my work. How could I ever have been mindlessly sucked into this eating thing, like some sort of food junkie. "What's for dinner?" "Where should we eat tomorrow night?" "Want to go to Whole Foods?" I was pathetic.

I was also insane for not realizing a fault in my plan. Not only will flying to L.A. make my day three hours longer but also airports are designed to tempt you to load up on calories. I walk by a Mex and the City, a Dunkin' Donuts, a Cheeseburger Cheeseburger and a Create Your Own Salad. I am, to my surprise, drawn to people's salads far more than to their fries. Maybe my body craves actual nutrition. Or maybe my body is an idiot and doesn't realize how good French fries are.

On the plane, I feel actual hunger pangs. They come in waves and don't really hurt so much as just annoy—not much worse than before a late dinner. I still, however, feel weak. My back hurts, and I'm still light-headed. I'm far too aware of my tongue—like its taste buds are so desperate for stimulation it might slither out of my mouth and find something itself. Luckily, the couple next to me break out a really bad-looking submarine sandwich, and she licks the mayo off her fingers. I go back to feeling superior. Superior, but very interested in her remaining bits of bread.

After a great night's sleep at home, I wake up at 8 and go straight to the kitchen to break my fast. But I'm not really hungry. And nothing looks very tempting: oatmeal seems leaden, raisin bran gloppily suffocating, smoothies mucus-catalyzing. I head back to my office and do some work. Three hours later, I'm still not hungry, but I know I've got to eat something. The only thing I can think of that seems appealing is what my grandmother used to make when I was a kid: poached eggs on toast. They taste like super-eggs—so rich and velvety, the pumpernickel bursting with earthy chewiness. I take my time with them and then, foolishly, eat a banana and some walnuts. I reach that uncomfortable level of fullness, weighty and stomach-focused and stretched, that I realize I usually feel at least once a day. And I think I like it less than the hunger.

What the world eats

Originally the food of emperors, the cuisine known as kaiseki is the pinnacle of Japanese eating—and few restaurants serve a more refined menu than Kikunoi, in the former imperial capital of Kyoto. The experience begins before a single plate is served, with the setting: a tatami-mat room, bare save for the tokonoma, the alcove in the corner that displays a single scroll of calligraphy and a seasonally appropriate flower, today a lily. Then the food begins to flow, course after course of carefully trimmed portions, delivered by a kimono-clad server: a single piece of sea-bream sushi wrapped in bamboo leaf, a tiny grilled ayu fish with water-pepper vinegar, fried prawns and bamboo shoots with an egg-yolk-and-cream sauce. Kaiseki dining is the product of centuries of cultural evolution, but though Kikunoi is high-end—as the bill will indicate—its cuisine is meant to be a grand elaboration of the basic Japanese home meal: rice, fish, pickles, vegetables and miso soup, artfully presented in small, healthy portions.

"I believe that Japanese cuisine is something embedded in Japanese people's DNA," says Kikunoi's owner, Yoshihiro Murata. That may be true, but it's a legacy under assault, increasingly crowded out by fast, convenient, Westernized food. These days, Murata says sadly, his college-age daughter doesn't see much difference between cheap restaurant food and the haute cuisine he makes. "I think that in Japan, people should eat good Japanese food," he says. "But they are far away from it."

Japan is not alone. Food and diet are the cornerstones of any culture, one of the most reliable symbols of national identity. Think of the long Spanish lunch followed by the afternoon siesta, a rhythm of food and rest perfectly suited to the blistering heat of the Iberian peninsula in summer. Think of the Chinese meal of rice, vegetables and (only recently) meat, usually served in big collective dishes, the better for extended clans to dine together. National diets come to incorporate all aspects of who we are: our religious taboos, class structure, geography, economy, even government. When we eat together, "we are ordering the world around us and defining the community most important to us," says Martin Jones, a bioarchaeologist at Cambridge University and author of the new book Feast: Why Humans Share Food.

Even the traditions we learn from others we adopt and adapt in ways that make them our own. Japan received chopsticks from China and tempura from Portugal. Tomatoes, that staple of pasta and pizza, arrived in Southern Europe only as part of the Columbian Exchange (so-called because of Christopher Columbus' journeys to the New World, where tomatoes originated). "A lot of what we think of as deeply rooted cultural traditions are really traceable back to global exchange," says Miriam Chaiken, a nutritional anthropologist at Indiana University of Pennsylvania.

In an era of instant communication and accelerated trade, those cultural exchanges have exploded, leading to something closer to cultural homogenization. That's bad for not only the preservation of national identities but the preservation of health too. Saturated fats and meats are displacing grains and fresh vegetables. Mealtimes are shrinking. McDonald's is everywhere. From Chile to China, the risk of obesity, diabetes and heart disease is on the rise as the idiosyncratic fare that used to make mealtime in New Delhi, Buenos Aires and Sydney such distinctive experiences is vanishing. This, in turn, is leading to a minimovement in some countries to hold fast to traditional food culture, even as their menu grows ever more international. Says Jones: "With every change there's nostalgia for what's gone before."

Such longing for what was may be only natural, but before we get too misty over the way we used to eat, it's important to remember that the first purpose of food is to keep us alive—something that used to be a lot harder than it is today. For thousands of years, humans were chiefly agrarian, which meant that you ate only what you could grow or slaughter yourself or trade for locally. Geography was culinary destiny.

Africa, which strains under so much political and economic hardship, is the place where this ancient reality is in greatest evidence today. Throughout much of the continent, people remain tied to the land and therefore dependent on it. Most meals are keyed around a single calorie-rich starch—in East Africa, it's often cornmeal or flour made into a stiff porridge—with extra food added if available. Meat remains a rare indulgence, something reserved for holidays and feasts. Even relatively well-fed populations like the Iraqw of Tanzania, who typically eat three full meals a day, must brace for periods when that is impossible. "In times of food insecurity—right before a harvest—or in a bad year, they will reduce this to two or one meals," says Crystal Patil, an anthropologist at the University of Illinois in Chicago. "If there are several bad years in a row, it can be devastating." Often, the only sure foods are largely useless ones, those sloshing over from the well-fed world outside. "I've never seen a village where you couldn't find a Coke," says Chaiken.

All human cultures may have started out with this kind of day-to-day, harvest-to-harvest existence, but the better environmental hand that people of other regions drew—richer soil, fewer droughts, milder temperatures—allowed them to tame their land, meaning that the food they ate and the lives they lived could evolve together. In agrarian, preindustrial Europe, for example, "you'd want to wake up early, start working with the sunrise, have a break to have the largest meal, and then you'd go back to work," says Ken Albala, a professor of history at the University of the Pacific. "Later, at 5 or 6, you'd have a smaller supper."

This comfortable cycle, in which the rhythms of the day helped shape the rhythms of the meals, gave rise to the custom of the large midday meal, eaten with the extended family, that is still observed in pockets of Southern and Western Europe. "Meals are the foundation of the family," says Carole Counihan, an anthropologist at Millersville University in Pennsylvania and author of Around the Tuscan Table, "so there was a very important interconnection between eating together" and cementing family ties.

Since industrialization, maintaining such a slow cultural metabolism has been much harder, with the long midday meal shriveling to whatever could be stuffed into a lunch bucket or bought at a food stand. Certainly, there were benefits. Modern techniques for producing and shipping food led to greater variety and quantity, including a tremendous increase in the amount of animal protein and dairy products available, making us more robust than our ancestors. In contemporary China, where tens of millions were starving less than 50 years ago, meat has become far more common, and Chinese youth are on average 2.4 in. taller than they were just three decades ago. "China has gone from a sparse diet to a point where it's got almost too much," says James Watson, professor of Chinese society and anthropology at Harvard University. "As a nutritionist, you have to be outraged. As a historian, you have to consider it one of the biggest success stories on the planet."

Yet plenty has been lost too, even in cultures that still live to eat. Take Italy. It's no secret that the Mediterranean diet—with its emphasis on olive oil, seafood and fresh produce—is healthy, but it was also a joy to prepare and eat. Italians, says Counihan, traditionally began the day with a small meal called colazione, consisting of light baked goods and coffee. The big meal came at around 1 p.m. and included a first course of pasta, rice or soup; a second of meat and vegetables; a third, fruit course and, of course, wine. In between the midday meal and a late, smaller dinner came a small snack, the merenda. Today, when time zones have less and less meaning, there is little tolerance for offices' closing for lunch, and worsening traffic in cities means workers can't make it home and back fast enough anyway. So the formerly small supper after sundown becomes the big meal of the day, the only one at which the family has a chance to get together. "The evening meal carries the full burden that used to be spread over two meals," says Counihan.

South Americans are struggling with similar changes. John Brett, a nutritional anthropologist at the University of Colorado at Denver and Health Sciences Center, says that many Latin Americans too prefer a large family meal at midday, heavy on starchy grains like quinoa or plants like yucca. But migration from the country to the cities has made that impossible. "They don't have the luxury of two hours of lunch," says Brett. "The economy moves on." Not only do these changes add stress for families, but nutritional quality declines as well. "They tend to eat whatever is cheap and quick, " says Chaiken.

Paradoxically, the thing that has contributed the most to the deterioration of food culture may be one of the very things that has helped turbocharge countries in so many other ways: the presence of women in the workforce. "If women are working, they can't shop and cook and prepare a meal," says Counihan. "In the old days, you might have had the grandmother doing it, but family size is shrinking." And the less exposure younger generations have to the food their grandparents ate, the less they develop the sensitive palates that allow them to appreciate it. In Latin America, says Jeffery Sobal, a professor of nutritional science at Cornell University, "parents complain that they make [traditional] dishes, but the kids won't eat them. They want the things they see on television."

It shouldn't be surprising that the societies that have been most successful at retaining food cultures are the ones that have also resisted the pull of Westernization—for better and worse. In many Middle Eastern countries, extended families still live together, and women stay in the home preparing the kinds of traditional meals that women elsewhere no longer can. Diets in the Middle East also show the influence of religion; besides widely observed taboos on pork and alcohol, the fasting month of Ramadan alters Middle Eastern eating habits. While Muslims fast from sunup to sundown, Ramadan nights are marked by calorie-heavy indulgence. "The level of food consumption during Ramadan is much higher than during ordinary months," says Sami Zubaida, co-author of the book Culinary Cultures of the Middle East. Ramadan is "the fasting month that is really a feasting month," Zubaida says, hence the tendency for Ramadan weight gain.

Outside the most conservative nations in the Muslim world, it has proved difficult to hold on to the pleasures of traditional eating. But that's not to say that people don't long for the old ways all the same—inspiring movements in some nations to rediscover how Mom used to prepare a meal. In Europe, Asia and the U.S., the Slow Food movement—a kind of alimentary Greenpeace—campaigns against fast food while championing traditionally prepared meals. Bolivians regularly hold food fairs that celebrate South American staples even as they develop ways to speed up the time-intensive preparation of native meals so that Bolivians can enjoy the dishes of the past at the pace of today. Yet while we might—indeed must—clean up the worst of the fast-food excesses, trying to preserve the diets that keep us both culturally and physically healthier, no one pretends we're ever going to turn back the clock entirely. "Nobody has time anymore," laments Harvard's Watson. "Not even the French."

Nor do the harried Japanese, although there are exceptions. At a trim home in northwest Tokyo, where commuter trains rumble just outside the window, homemaker Estsuko Shinobu, 60, prepares a proper Japanese lunch, using fresh ingredients she bought that morning at the nearby supermarket. The mother of two grown children pads around the kitchen in slippers and a violet kimono, chopping Japanese radishes and carrots, carefully packing a sushi cake with tuna and vinegared rice. She serves dishes arranged on an individual tray just so, down to the direction of each set of chopsticks. She looks happy, even serene as she works, but when asked whether she has passed these skills on to her daughter, she sighs. "Of course not," Shinobu says. "She's far too busy for this."

—With reporting by Elisabeth Salemme/New York, Toko Sekiguchi/Tokyo, Ishaan Tharoor/Hong Kong and Christopher Thompson/London

Eat Fiber - Unrefined foods, especially those that are high in fiber, stimulate appetite-suppressing hormones and make you feel full.