Feeding your Hunger

Flavor: The Science of Our Most Neglected Sense - Bob Holmes 2017

Feeding your Hunger

Dana Small vividly remembers the first and last time she drank Malibu rum and 7UP. “It was a big party, and it was my first time, I think, having alcohol, and I was probably underage,” she recalls. Small is, well, small, with long, bright-copper hair and a barely detectable soft lisp. “I didn’t know what I was doing, and I probably didn’t have that much, but Malibu and 7UP doesn’t really taste like alcohol, it’s like this really sweet kind of . . . Anyway, so I had a few of those, and felt not so good the next day. That was 20 years ago. In those 20 years, I’ve continued to eat a lot of sweet things, but I particularly avoid Malibu and 7UP.”

Most of us can think of a similar event in our past, where a bad experience has permanently scarred our taste for some particular food or drink. But for Small, a neuroscientist at Yale University, the lesson runs much deeper than “Don’t drink this.” She thinks experiences like hers—and the positive ones on the flip side of the coin—are the whole reason our brains assemble a unified perception of flavor from what could have been left as separate senses of taste, retronasal smell, and texture. “The reason that we have flavor, when we already have taste and smell, is for the purpose of associating foods that we encounter in the environment with their post-ingestive effects, because ultimately that’s what it’s all about. That’s really the role of flavor,” she says. Translation: We remember the flavors of what we ate, and what happened afterward, so that the next time we can seek out the good stuff and avoid the bad. “Flavor perception allows us to have a representation of precisely a particular kind of food. So in the case of Malibu and 7UP, there is specific learning to avoid that item. This is really like no other kind of learning. It’s very strong—one trial—and very long lasting. That makes perfect evolutionary sense: you want to only need one trial.”

For our ancient ancestors, omnivore hunter-gatherers that they were, these eating decisions would have been far more than a minor matter of aesthetics. Their choices of what to eat could literally be a matter of life and death. Pick the wrong root to eat, and you poison yourself and your family. Pass up a nourishing root, and you could all starve. More subtly, being a successful hunter-gatherer hinges on finding the foods that deliver the biggest nutritive bang for your hunting, gathering, and chewing buck. In modern terms, if you’re uncertain you’re going to eat tomorrow, then today you damn sure want to eat potatoes or burgers or ice cream, or something else with a lot of calories, rather than wasting your time munching raw celery.

So you’d expect that evolution would have endowed us with a pretty good system for identifying and remembering potential foods and what happens when we eat them. A little bit of this system, as we’ve seen, is built-in: even newborns have an innate liking for sweet tastes. But mostly, we learn through experience. That’s what flavor does for us—and why our brains assemble all the relevant data of taste, texture, retronasal olfaction, and all the rest into a single, unified flavor perception. Thanks to this synthetic perception, we can learn and remember the flavors that made us sick, and we also learn to like the flavors that nourished us. We don’t generally notice that we’re learning about nourishment, because in our everyday world, flavors and calories are inextricably linked. We don’t usually encounter a baked-potato flavor without also ingesting a big slug of carbs, or salmon flavor without protein and fat. Teasing the flavor apart from its nutritional consequences takes careful experimentation, the sort that’s easier to do with rats than with people.

The classic studies here come from Anthony Sclafani, a researcher at Brooklyn College in New York. Sclafani offered rats one of two water bottles to drink from, one grape flavored, the other cherry flavored. Neither contained any sweetener or other nutrients—just flavoring and water. But he also inserted a stomach tube, so that he could deliver sugar solution straight into the rat’s gut when it drank the cherry flavor, but not the grape. Since the sugar never entered the rat’s mouth, it tasted no sweetness. Yet within a few minutes of encountering the two flavors, the rat learned to drink almost exclusively from the cherry-flavored water bottle. What’s happening, says Sclafani, is that nutrient receptors in the rat’s gut quickly signal to the brain that good stuff is coming in. The brain pairs this with flavor information from the nose and mouth, and the rats learn that cherry means calories, even though they never taste the sweetness. Sclafani also reversed the pairings in other rats, just to be sure there wasn’t something special about cherry flavor. Rats that got sugar infused into their stomachs when they drank the grape-flavored water quickly learned to prefer grape instead. And it’s not just sweetness—Sclafani’s rats learn just as well if the calories delivered through the stomach tube come from proteins or fats. It’s exactly the same learning process that Russian biologist Ivan Pavlov used to train dogs to associate the ringing of a bell with imminent food. After not too long, Pavlov could just ring the bell and his dogs would start salivating in anticipation of the meal to come. Sclafani’s rats hit the cherry-flavored water because they expect calories to follow.

By moving the location of the rats’ stomach tube, Sclafani was able to show that the nutrient receptors responsible for the learning are located right at the beginning of the small intestine, just past the stomach. This is the region that surgeons remove when they do gastric bypass surgery on morbidly obese patients. No one knows exactly why gastric bypass surgery works so well, but one reason may be that it eliminates these nutrient receptors and thus prevents the pairing of flavors to their nutritive consequences. Since the flavors of a meal are no longer associated with incoming nutrients, people would gradually lose interest in the flavors and feel less drive to eat them, Sclafani suggests.

This flavor learning—technically known as flavor-nutrient conditioning—is dead easy to demonstrate in rats, as Sclafani’s work shows and others have verified. But it’s a lot harder to prove that the same kind of learning happens in people. For one thing, the whole stomach-tube business is a nonstarter. People also have an annoying habit of eating whenever they feel like it, so that experimenters have a much more difficult time controlling their food intake or ensuring that they haven’t already formed associations with, say, grape and cherry flavors. As a result, studies of flavor-nutrient conditioning in humans have had mixed results. Sometimes it looks like it happens, and other times it doesn’t.

Probably the best evidence that we really do learn to pair flavors with their nutritive rewards comes from Dana Small’s lab at Yale. Small flipped through a flavor-supply catalog to come up with ten really obscure flavors that no normal person had much chance of encountering in daily life. “They’re novel, and you don’t know what they are,” Small told me when I asked her to describe the flavors. One, for example, was called “aloe,” though it tasted nothing like aloe vera. Not surprisingly, people tended not to like the flavors when they first encountered them—our built-in neophobia raising its head again.

Small and her colleagues picked two flavors and used them to make artificially sweetened soft drinks. One of the flavors also got a dose of maltodextrin, a sugar that delivers a full load of calories—it turns into glucose almost immediately when it reaches the stomach—but is devoid of flavor. (A triangle test—which of these three is not like the others?—confirmed that people couldn’t tell the difference between soft drinks with and without the maltodextrin.) Volunteers consumed each drink several times over the course of a few days, using only one kind of drink each day to keep the postingestive consequences separate. And then Small brought them back into her lab to see how they responded to the two flavors. To be sure she was looking at the effects of learning, and not real-time perception, this time neither flavor was spiked with maltodextrin. Sure enough, the people showed a slight tendency to like the high-calorie flavor better than the low-calorie one. In other words, they had learned which flavor delivered the nutritive goods, and they liked it a little better—but not all that much better. The big difference showed up when Small put them in a brain scanner.

Being in Small’s brain scanner is not exactly a fine dining experience. Just like any hospital MRI, you’re flat on your back inside a giant magnet, with your head immobilized. To get a good image of the effect of each flavor on brain activity, she needs to average over multiple sips: on again, off again. She needs to know exactly when the flavor arrives, and she needs to keep stray odors from lingering and confusing the test. “What that means,” Small says, “is that you’ve got a nasal mask, and then this teflon thing that liquids are dripping off onto your tongue.” Charming.

Even in that utterly strange context, the results were dramatic. When people drank the flavor they’d learned to associate with calories, a part of the brain called the nucleus accumbens lit up like a Christmas tree. The nucleus accumbens is a part of what’s often described as the “reward pathway,” the part of the brain where good things begin to feel good, so that you want to do them again. The reward pathway plays a role in making you want more of things like sex, drugs, and rock and roll (literally—music activates the nucleus accumbens). An old study from the 1950s hooked rats up so they could stimulate their nucleus accumbens by pressing a lever; the rats just kept pressing the lever, over and over and over again, not even pausing to eat or drink.

Crucially, the learned flavor-nutrient link swayed the response of people’s reward pathways much more strongly than it affected their conscious liking of the two flavors. Let’s pause for a moment to underscore that point: When Small asked her subjects which flavor they preferred, she didn’t find all that much difference. That might explain why previous studies of flavor-nutrient conditioning in humans haven’t been very convincing. But Small didn’t stop there. Instead, she also let the subjects’ brains tell her which flavor they valued more—and their brains spoke loud and clear. All the real work, it turned out, was happening under the surface, in the unconscious.

Small points to another recent study that reinforces the point. Researchers at her alma mater—McGill University in Montreal—wanted to separate our conscious and unconscious valuations of food items to see how they differed. To do this, they showed pictures of food to hungry volunteers, and asked them to estimate their calorie content. (That’s the conscious valuation.) At the same time, a brain scanner measured the activity in a region of the brain called the ventromedial prefrontal cortex, another area involved in valuation and appetite. (That’s the unconscious valuation.) To top it off, the subjects were also given five dollars and asked how much of it they’d pay to have that food item to eat right now. Remember, these were hungry college students, who presumably cared about getting some calories at the time.

People turned out to be pretty lousy at consciously guessing how many calories the food items contained. Their unconscious brains, however, did much better: Their brain activity matched the real caloric content of the foods, not their estimate of the calories. The interesting result, though, showed up in people’s willingness to pay for the food. You’d think that when people are consciously deciding how much to pay for a snack, they’d base their decision on their conscious estimate of calorie count. But in fact, the amount they paid was a much closer match to the actual calories—the information accurately assessed by their unconscious.

At this point, you might be wondering why people persist in drinking Diet Coke, or continue to put sugar substitutes in their coffee. You’d think that their bodies would learn that those flavors don’t deliver calories and thus aren’t worth craving. One reason we don’t learn to ignore those flavors is that they deliver a jolt of caffeine, which also feels good. Our bodies learn to like the flavors associated with that kick—and with the buzz of alcohol, too. That’s why so many of us so easily develop a predilection for what are, objectively speaking, nasty, bitter, burning flavors.

There’s another point to consider when it comes to fooling the flavor system. You might be a dedicated Diet Coke drinker, but you probably encounter some of the same sweet, citrusy, caramely flavors in other foods, too, where they’re accompanied by real calories. That variability—sometimes sweet citrus means calories, sometimes not—might interfere with flavor-nutrient conditioning and make it harder for our internal calorie counter to keep track of how much we’ve eaten and when to stop. We might even be making matters worse, because we turn the flavors into a caloric slot machine that sometimes pays off and sometimes doesn’t. This sort of “intermittent reinforcement,” to use the technical lingo, is especially good at snaring our reward pathway. (Just look at all the zoned-out people sitting in front of actual slot machines in your nearest casino.) If so, artificial sweeteners might actually increase our attraction toward sweetness and the other flavors that accompany it. This may help explain why artificial sweeteners haven’t exactly been a weight-loss panacea.

It makes good evolutionary sense that all of this sophisticated learning takes place below the threshold of consciousness. Long before human beings ever walked the planet, and even before the first primates picked their way through the trees looking for fruit, our primitive mammalian ancestors would have needed to identify which foods were most nutritious. In short, they would have needed flavor-nutrient conditioning. And they probably had little or no conscious thought to help them with the task. “These circuits evolved so long ago,” says Small. “They were working perfectly well before we had consciousness.” As good mammals, then, we have evolved to want the flavor of calories. Or, to put the matter more precisely, we want the flavors that we’ve learned are accompanied by a dose of calories, while we ignore the flavors that aren’t. And this happens mostly without our conscious awareness.

But modern humans, with very few exceptions, no longer live on the African savannas, digging up roots and picking fruits and running down the occasional gazelle. We’re surrounded by an abundance of foods, and many of them are calorie rich in a way our ancestors rarely experienced. In this new context, our evolved instincts let us down. We no longer benefit by being attracted to high-calorie flavors when they’re always there—and the increased caloric density kicks our flavor-nutrient conditioning into overdrive, making those foods even more attractive. We want those flavors even when getting them is bad for us.

As we’ve seen, a few of our flavor preferences are clearly innate. Even newborn babies like sweet tastes—and they have to, otherwise they might not latch on to the mother’s breast and feed. And those same babies naturally reject bitter tastes, which are often an indication that something is toxic. Once past those few simple cues, though, our flavor preferences are wide open. Each of us has to decide which potential foods we will eat and which ones we will shun. A panda doesn’t have to learn this—it eats only bamboo. A lynx eats rabbits. An anteater eats ants. But people are different: as omnivores, we have to learn the flavors that mark the foods we eat.

That learning starts before birth, as flavor molecules from foods eaten by a pregnant mother pass into the amniotic fluid and are ingested by the developing fetus. In essence, the fetus samples what the mother eats—and, later on, recognizes and likes those flavors. Nursing infants get the same chance to sample mom’s diet through breast milk. The best demonstration of this early learning comes from Julie Mennella at Monell. Mennella asked one group of pregnant women to drink a glass of carrot juice at least four days a week during the last trimester of their pregnancy. Another group drank the carrot juice not during pregnancy but while nursing their infants, while a third group never drank carrot juice. Later, after the infants had begun to try solid food, Mennella watched as they got their first taste of carrot-flavored baby food. Most babies make scrunchy faces when they taste something new, but the babies who tasted carrots in utero or while nursing made fewer expressions of distaste than babies whose mothers had avoided carrot juice during pregnancy. The carroty babies’ mothers also thought they enjoyed the carrot-flavored cereal more. In short, the babies who’d experienced carrot through their mother were more comfortable with the flavor when they first encountered it directly.

And it’s not just carrots. Over and over again, researchers have shown that babies who are exposed to flavors ranging from anise to garlic through the mother’s diet prefer those foods when they first encounter them directly. In short, we learn to like what our mothers eat. “It’s a really beautiful system,” says Mennella. “For the baby to learn to like a food, the mother has to eat it. You can’t pretend to eat it, because the flavors don’t get in.”

Those early, learned preferences can linger for many years. In Germany, for example, almost all infant formula used to be flavored with vanilla. Many years after the practice had stopped, researchers took advantage of this natural experiment by comparing taste preferences of children who were infants just before the change—and who therefore almost certainly drank vanilla-laced formula—with those born just a few years later who drank vanilla-free formula. Sure enough, the children who had tasted vanilla as infants liked it better, years later, than those who hadn’t.

The odd bit of vanilla flavoring notwithstanding, formula-fed infants don’t get the same exposure to the flavors of foods their mother eats. Instead, they get exactly the same set of flavors with every bottle, unless the parents switch brands now and then. A formula-fed infant, then, arrives at weaning with little or no experience of the flavors he or she will soon experience firsthand. That’s especially true of infants who feed on cow milk formula or soy-based formula, which tend to be sweet and bland. In contrast, formulas made of hydrolyzed protein have bitter and sour flavor notes, so that infants who drink them get some familiarity with those “difficult” tastes. That familiarity helps those babies—like their breast-fed neighbors—be more accepting of vegetable flavors when they eat their first solid food, Mennella finds.

There seems to be a window of opportunity during the first few months of life when babies will accept almost anything they’re given—even hydrolyzed protein formula, which most adults find nasty tasting. But even after weaning, toddlers and older children gradually learn to accept new foods if they try them often enough. Toddlers and young children tend to be wary of new flavors and will often reject a new food the first few times they sample it. After eight or ten tries, though, most kids will begin to accept it in their diet—although they’ll often continue making skeptical faces well after that. (Parents should ignore the faces and pay attention to what their child actually eats, Mennella says.) Simple variety seems to matter, too, so children who have the opportunity to sample many different foods are more likely to accept new flavors. And they learn by watching what their parents and older siblings eat.

The lesson for parents is very clear: Eat what you’d like your children to eat. “Children learn through repeated exposure, variety, and modelling. I don’t know what more I can say,” says Mennella. “It’s just the basic tenets of learning—and learning as a family. Foods identify what that family is. Eat the healthy foods that you enjoy and like. Offer them to your kids in a positive context. Kids will learn.”

Perhaps the most vivid demonstration that it’s easier to learn to like something if you grew up with it comes from the high Arctic. The Chukchi and Yupik people of the Bering Strait region traditionally lived on a diet of fish and walrus, and many favorite dishes involve burying meat, blood, and fat to ferment for months and become what the locals call “tastily rotten.” To someone who hasn’t grown up with the practice, such foods can be hard to take, even with all the open-minded goodwill in the world. Here’s how one anthropologist, eager to try indigenous foods, described her first encounter with aged walrus meat:

What a shock! The smell of the thoroughly aged meat permeated my senses. My only thought was that as a guest I should not be rude. I must finish this piece of meat. I chewed and chewed and chewed. . . . Finally [one of the hosts] said quietly, still smiling, “You know, Carol, you are turning green!”

Even among the Chukchi and Yupik themselves, their fondness for these foods depends on their childhood experiences. Elders raised on traditional diets generally love them and still go out of their way to eat them. However, a generation of children raised during the 1960s—1980s, when the Soviet government actively discouraged traditional foods, often struggle to eat these foods. Their aversion was so strong that many still refused to eat them even when they had little choice, after outside food became very scarce after the collapse of the Soviet Union. And anthropologists report that many young people today are happy to honor and spend time with their grandparents—but only if they can leave before dinnertime. Even those who now eat these tastily rotten foods sometimes do so wearing latex gloves to avoid the lingering aroma.

A few preferences, in contrast, don’t seem to be open to learning. Newborn babies suckle more and make happy faces when they have something sweet in their mouth. And there’s not much that parents can do to change that preference. Gary Beauchamp—the same flavor researcher I gargled with in the Monell boardroom—tried to get people used to eating less sugar. Decades earlier, Beauchamp had found that putting people on a reduced-salt diet for a few weeks shifted their tastes so that they preferred the less salty food over what they used to eat, which they now regarded as too salty. But when Beauchamp tried to do the same experiment for sweetness, he found that people didn’t respond the same way. After three months on a low-sugar diet, his test subjects preferred exactly the same sweetness of vanilla pudding or raspberry drink as people who ate their usual diet. To Beauchamp’s knowledge, no one else has tried a similar study, and he cautions against concluding too much from a single experiment. However, if he’s right—and if children respond the same way as adults—then parents might be able to relax a bit about sugar. “Every person on the face of the Earth, almost, believes that if you feed kids lots of sugar, they’re going to like it more. There’s no evidence of that,” he says. Nor does a sugar-free diet keep kids from craving sweetness. (Beauchamp recalls one child he tested whose parents were fanatical about avoiding processed sugar and other sweets. The kid told him that at school, he got his sweet fix by pulling used chewing gum off of chairs and chewing that.) Sweet is sweet, and it always tastes good—and there’s nothing parents can do about that.

In most societies today, the most pressing question about food is how we can nudge people to eat less of it. As everyone knows, Americans have been gaining weight for decades, and more than two-thirds of American adults now weigh enough to be classified as overweight or obese. Europeans and even Chinese and Indians are now starting to join them. Worldwide, 39 percent of adults are overweight or obese, and overnutrition now kills more people each year than undernutrition.

Since this book is about flavor, we’re going to look at just one tiny piece of the puzzle: how the flavor of our food helps determine what we choose to eat and—more critically for weight control—how much of it we pack away. Even that little piece of the diet pie gets tricky. The first complication arises because we generally stop wanting something once we’ve had enough. The clearest illustration of this is the phenomenon of sensory-specific satiety that we touched on in the last chapter. The more you experience the flavors of a single food item during a meal, the less your brain’s reward system responds to the sensory input, no matter how many calories it packs. Even if you really like something, you enjoy it less with each bite.

This is a big reason why high-end chefs gravitate toward tasting menus that feature a long progression of tiny dishes. At Chicago’s Alinea restaurant, for example—generally rated among the world’s best—you can expect a meal of more than a dozen tiny courses, each consisting of just a few bites. The chef, Grant Achatz, learned much of his craft at the renowned French Laundry in California’s Napa Valley, another restaurant famous for serving a long succession of “small plates.” Here’s how Achatz’s mentor there, chef Thomas Keller, described why he does that:

Most chefs try to satisfy a customer’s hunger in a short time with one or two dishes. They begin with something great. The initial bite is fabulous. The second bite is great. But by the third bite—with many more to come—the flavors begin to deaden, and the diner loses interest.

The same principle applies to any meal. If your holiday meal consisted of nothing but mashed potatoes, you’d almost certainly eat a lot less than you do when you’ve also got turkey, stuffing, green beans, and brussels sprouts on your plate. Sensory-specific satiety kicks in within fifteen to twenty minutes after you start eating a food, so it may help turn off your appetite for what’s on your plate even before other satiety signals like stomach fullness kick in. (The effect wears off within an hour or so, so other satiety mechanisms must be more important in determining when we start thinking about eating again.)

Some people have suggested that more highly flavored foods might be better at inducing sensory-specific satiety. If so, people might be able to lose weight by maximizing the flavor per bite. For example, Edmund Rolls—who discovered sensory-specific satiety—notes that in most cultures, the staple foods, the ones that people eat the most of, tend to be relatively bland starches like rice, potatoes, or bread; the more highly flavored meats and vegetables usually take a smaller share of the plate. On the other hand, there’s only a little evidence to date to back the notion that maximizing flavor could keep us thinner.

Perhaps the clearest support comes from a recent Dutch study where researchers used plastic tubes threaded up the back of volunteers’ throats to deliver either a strong or a weak tomato soup aroma. Either way, the volunteers drank the same bland, flavorless soup, but they perceived it as either a richly flavored or mildly flavored tomato soup, depending on the intensity of the piped-in aroma. Sure enough, they ate about 9 percent less of the more highly flavored soup, as long as the more intense aromas were present more than fleetingly.

If sipping soup with a hose up your nose sounds unpleasant, just be thankful you didn’t end up in the experiment that some other young Dutch men volunteered for a few years ago. Researchers wanted to sort out whether we stop eating because our stomachs are full or because we’ve had enough flavor. To answer that question, they needed to separate chewing and flavor release from swallowing and filling the stomach. The way they did it wasn’t pretty.

If you’d been part of that experiment, you would have come in to the lab sometime after eating a normal breakfast, and a technician would have threaded a skinny stomach tube, thin enough to poke into the headphone jack on your phone, up your nostril, down the back of your throat, and into your stomach. After an hour of sitting around and filling out paperwork, you’d have been given a half pound of cake. You’d have been told to chew the cake normally, but then, just when you were about to swallow it, spit it out into a cup instead. Take another bite, chew and spit, over and over, for either one or eight minutes. When you were done, a technician—presumably the one who drew the short straw—collected the contents of the cup, dried them, and weighed them to make sure you hadn’t swallowed any cake on the sly. While you chewed, the stomach tube flavorlessly deposited into your stomach ninety-nine calories’ worth of the same cake, pureed in a blender with either a small amount of water (one hundred milliliters, which is around three ounces) or a larger, more belly-filling amount (eight hundred milliliters, around three cups). A half hour after all the spitting and pumping (and after the stomach tube was snaked out of your nose again), you would have been given a sandwich lunch and instructed to eat your fill.

Sounds miserable, all this chewing and spitting and slithering of tubes. And apparently, the study participants thought so, too. Of the forty-three young men who signed up for the job, eight walked away once they found out what was involved, even though they would have been paid for their time. Five were sent packing for failure to spit out all their cake, and four washed out for other reasons, leaving just twenty-six to finish the experiment.

After all that, it turned out that when it comes to satiating us, flavor in the mouth matters at least as much as fullness in the stomach. On the days when the volunteers spent eight minutes chewing and spitting out their cake, they later ate 10—14 percent less of their sandwiches than when they chewed and spat for just one minute or not at all. In contrast, their sandwich consumption barely went down at all on days when they had more fluid pumped into their stomachs.

In another, slightly less unpleasant experiment, the same researchers pumped tomato soup into people’s mouths, either as big squirts separated by twelve-second pauses, or a series of small squirts separated by three-second pauses. Either way, the eaters got the same amount of soup per minute, but they got less flavor from the big gulp, because they spent less time with soup in their mouths. Sure enough, when people got the little doses—hence the more flavor exposure—they ate less soup before deciding they were full.

All of this would seem to suggest that advocates of thorough chewing may have a point: The more you chew, the more you’re exposed to food flavors, and therefore the quicker satiety may set in. In one study, people felt fuller after eating pasta with a small spoon, dutifully chewing each mouthful twenty or thirty times, than when they used a large spoon and ate as quickly as they comfortably could. (Unfortunately, the science isn’t quite clear-cut yet—though the long chewers felt fuller, they didn’t actually eat any less of the pasta despite feeling more sated.) Even if you buy into the notion that chewing more means swallowing less, you don’t have to take this to fanatic extremes—as advocated, most notoriously, by Horace Fletcher early in the twentieth century, who sparked a brief fad for “Fletcherizing,” or chewing every mouthful hundreds of times. Instead, you may be able to achieve the same end without obsessing over chews by playing with texture. Foods that are thicker, chewier, or crunchier force eaters to take smaller bites and chew longer, which slows the eating rate and ups mouth time.

More to the point, liquids such as soft drinks, juices, and beer go down much more quickly than chew-and-swallow food—as much as ten times faster, according to some studies. We get less exposure to their flavor in the mouth, which may explain why we tend to overconsume liquid calories: They don’t trigger our internal calorie meter as strongly as calories from solid foods do. And in fact, people find identical amounts of soup more filling if they eat it from a spoon, slowly, than if they drink it from a mug more quickly.

Of course, another way to experience more flavor from your food is simply to eat more flavorful food. No one knows whether tastier meals will make you feel full more quickly, though many of the experts I spoke to said they wouldn’t be surprised if that was the case. And a few experiments hint at the possibility. People take smaller bites of more highly flavored vanilla custard, for example, and they ate less of a saltier tomato soup than of a less salty one that they rated equally desirable.

It’s tempting to think that the immense variety of foods available today might contribute to overeating, because when sensory-specific satiety kicks in we can just switch to a different food. When obesity first appeared on the social issues radar, back in the 1970s, a lot of people worried that the root cause was our modern “cafeteria” diet, in which we’re constantly exposed to a huge variety of potential foods. With so much to choose from, people worried, we’d hop from one food to the next and end up eating too much. It rapidly became clear that variety, per se, was not the culprit. In the early 1980s, researchers at Monell bought commercial flavors in a dozen rat-friendly flavors. (In case you ever want to charm a rat, the flavors were peanut, bread, beef, chocolate, nacho cheese, cheese paste, chicken, cheddar cheese, bacon, salami, vanilla, and liver.) Then they fed the rats standard rat chow spiked either with the same one flavor over and over again, or else a constantly changing smorgasbord of varied flavors. If variety causes overconsumption, the latter rats should have blown up like happy little blimps.

But they didn’t. Over the three weeks of the experiment, the smorgasbord rats ate no more food, and gained no more weight, than the boring-food rats. What really mattered, it turned out, was how much sugar and fat was in the rats’ diet. A third group of rats that got a much higher-fat, higher-sugar diet (let’s call it the fast-food diet) did indeed balloon up, regardless of whether they got a diet with monotonous or varied flavor. In other words, it’s not about the variety. It’s about the reward-pathway pull of concentrated calories.

Even so, many people manage to navigate past the siren calls of fast-food calories without gaining weight. That’s because we have a separate system for regulating the total amount of food we take in. A complex network of hormones with names like leptin, ghrelin, and neuropeptide Y regulates our levels of hunger and satiety to keep calorie input roughly equal to output in the long run. When I eat a varied holiday dinner, I’m likely to eat a little bit more than usual. Most people do—not just because of the variety, but also because the social context says we’re supposed to. But we compensate for that later, by eating a little less the next day, or by skipping a snack or two. And you can forget the old saying, “Never trust a skinny chef”—as a general rule, people don’t gain weight just because their diet is especially tasty. “There’s nothing out there—and I’ve asked around—that really shouts that if you make food as palatable as you can, people overeat,” says Mark Friedman, a researcher who’s been studying flavor and appetite for decades. “For something that everybody believes, there’s not much there.”

Nor does bad-tasting food make you eat less. (Just ask any college student on a meal plan.) If you make rats’ chow less palatable, such as by adding a bitterant to it, they’ll avoid it for a little while, but if they have no other options, hunger kicks in eventually and they scarf the stuff down anyway. Likewise, people who suddenly lose their sense of smell—a malady we discuss later in this chapter—usually don’t end up losing weight. For that matter, most of us can think of someone we know who cooks boringly, or even badly, and they’re generally not emaciated.

Further evidence that we probably can’t fix obesity by tinkering with flavor comes from the field of genetics. You’ll recall that each of us carries a unique set of genetic variants in our taste and odor receptors, and as a result no two of us perceives the flavor world in exactly the same way. If our flavor perception was an important cause of obesity, then you’d expect that people with some flavor-gene variants would be more likely to be obese than others. For example, people with my sweet-receptor variant tend to prefer sweeter flavors, which might put us at greater risk of eating too many sugary treats and gaining weight. Or people who are especially sensitive to bitter tastes might eat high-calorie french fries in preference to lower-calorie broccoli.

One way geneticists look for patterns like this is through something called a genome-wide association study (GWAS), which is often used to identify genetic diseases. Researchers compare the genomes of people with and without the disease—Alzheimer’s disease, say, or a strong family history of cancer—and look for parts of the genome that differ in the two groups. The disease-related genes they’re looking for must lurk somewhere within those regions of difference. In the case of obesity, a GWAS would compare the genomes of overweight people against their normal-weight peers. Sure enough, those studies do turn up regions of difference, suggesting that there must be genes that affect obesity. However, not a single one of those regions of difference includes any taste receptor or odor receptor genes. How we perceive the flavor world doesn’t seem to matter at all in determining our risk of obesity.

And there’s yet another reason for suspecting that flavor, by and large, doesn’t have a lot to do with how much we eat. If our overeating was driven by delicious flavors, then people who lose their sense of flavor—especially smell—should lose interest in food, and they should have a hard time eating enough. This was the question that led me to Monell’s boardroom to eat tasteless hamburgers with Gary Beauchamp. And to see what happens after long-term loss of flavor senses, I headed just a few blocks down the street to the University of Pennsylvania Medical Center and Richard Doty’s clinic on smell and taste disorders to meet some of his patients.

Patricia Yager had never had a serious health problem. “I go to Antarctica for a living. They don’t let you go there if you aren’t healthy,” says Yager, an oceanographer who studies climate change and the oceans. In January of 2014, though, Yager—a slender woman with a broad face, heavy-lidded eyes, and long hair streaked lightly with gray—noticed a persistent metallic taste in her mouth. As scientists do, she worked through the possibilities: acid reflux, menopause, diabetes. Nothing fit. Her doctor found a little fluid in her middle ear and suggested decongestants, but the metallic taste persisted.

Then one day she was cooking in her kitchen when her preteen son came rushing in saying there was a terrible smell in the house. It turned out that some cheese had bubbled over in the oven and was burning. “I didn’t smell the smoke,” Yager recalls. “I thought, oh my gosh, something serious is going on here!” An ear, nose, and throat specialist guessed that her sense of smell was damaged, and the likeliest causes were permanent nerve damage or a brain tumor—not what she wanted to hear. “So I ended up in a puddle on the floor.” Fortunately, an MRI ruled out the latter, scarier option, which is how Yager ended up in Richard Doty’s clinic in Philadelphia.

Doty, a neuropsychologist, directs the Smell and Taste Center at the University of Pennsylvania, which is widely regarded as the best place in North America for diagnosing and treating smell and taste disorders. “We’re a unique center in the world, really,” says Doty. By the time patients make it to his center, many have already seen several doctors without understanding their condition, and they’re desperate for Doty’s specialized knowledge—even though he often can’t fix the problem. “Much of what we do is correct misinformation and put people at ease,” says Doty. “One of the things I like about this job is that most people are thankful that they came to see us, since we understand their problem.”

A few days each month, he sees patients in his small, crowded office at the center. It’s a classic academic office: books, papers, and binders cover his desk and side tables in teetering stacks a foot and a half high. The desk alone has six of these piles, and he and Yager have to peer between the towers to see each other. At first, Yager tells him, things either had no smell at all, or else they all had the same, unpleasant smell. Lately, though, she’s noticed that she can sometimes distinguish among different smells. “None of them smell good yet, and none of them smell like they used to smell. Watermelon doesn’t smell like watermelon, but it has a very distinctive, unpleasant smell.” Vanilla now has an odd, turpentiney smell.

Most likely, Doty tells her, a viral infection has killed some of the nerve cells that carry odor receptors. We start out with a few million of these cells, each one carrying just one of our four hundred or so odor receptor molecules. A severe viral infection of the nasal passages can sometimes kill enough cells that some—or, in extreme cases, all—of the odor receptors effectively go extinct in the nose. The effect is like progressively cutting some strings on a piano: the chord begins to sound dissonant, then unrecognizable. Cut enough strings and the piano falls silent. It’s also possible that Yager’s smell loss is the result of a head injury, since she did hit her head in a fall while roller-skating a few weeks before she noticed the loss. That fall—mild though it seemed at the time—could have severed the connection between the olfactory epithelium and the brain.

Doty sends Yager off for a battery of tests to measure her senses of smell and taste. His associates measure the shape of her nasal cavity and the airflow through it; they test her sense of taste by dripping sweet, salty, sour, or bitter test solutions on each quadrant of her tongue, and by stimulating the tongue with an electric probe; they give her the University of Pennsylvania Smell Identification Test—the same scratch-and-sniff test that Doty had given me a few months earlier in Florida. The test is multiple choice, which avoids the complication of recognizing a smell but being unable to name it. That also makes it easier for Doty to spot malingerers who are faking smell loss in the hope of profiting from a juicy lawsuit.

While Yager is off doing the tests, Doty explains that viral infections are one of the three most common causes of smell and taste problems, along with head trauma and chronic nasal and sinus inflammation. Many of the patients he sees arrive in his office complaining of a loss of taste, but most of the time his tests reveal that the problem is actually with their sense of smell—further proof that most people can’t really distinguish between their two main flavor senses. Olfactory defects are surprisingly common, affecting one in five people by most estimates, and about one person in twenty has no sense of smell at all. Often, people are completely unaware that they have a problem—in fact, one study found that asking people if they have a defective sense of smell tells you nothing useful about whether they actually do. (Oddly, older people tend to have lost olfactory ability without knowing it, whereas younger people tend to underestimate their sense of smell.) “Every time you get a bad cold, or are exposed to pollution, it takes a toll on the olfactory epithelium,” he says.

Sometimes—as, apparently, with Yager—a single infection is enough to push a person “over the waterfall,” as Doty puts it. Other times, the damage accumulates gradually, and our ability to smell slips away bit by bit, without our noticing, as we age. (Taste may fade a little bit with age, as well, but not enough for most people to notice.) Most of us, if we live long enough, will eventually have trouble with smell: Almost 30 percent of seventy-year-olds and about 60 percent of people over the age of eighty have significant impairments of their sense of smell, with men more likely to lose function than women. Surprisingly, scientists haven’t tracked enough people through their lifetimes to be sure whether age-related loss creeps up slowly, or whether we reach a threshold where problems suddenly become much more common. Often, studies simply compare a group of older people with younger ones and report that the older ones have a worse sense of smell.

One sparkling exception to that rather dismal track record came in 1986, when every one of National Geographic magazine’s 10.5 million subscribers received a scratch-and-sniff smell survey with their September issue. For each of the six odors, the subscribers were asked to rate the odor’s intensity and pleasantness, and pick the best description of the odor from a list of twelve possibilities. They also answered some questions about themselves, so that the masterminds behind the survey—Monell researchers Charles Wysocki and Avery Gilbert—could make sense of their responses.

The survey was a huge success, with more than 1.2 million readers returning their questionnaire. When Wysocki and Gilbert tabulated the results, they found—as expected—that more older people than younger ones had trouble detecting some or all of the odors. Surprisingly, though, people’s sense of smell didn’t fade uniformly—they lost some odors faster than others. Virtually everyone could smell the banana, clove, and rose odors right up into their sixties, and even after that age, the ability to smell those odors trailed off slowly. Even among ninety-year-olds, 90 percent of men and almost 95 percent of women could still smell the clove and rose odors, and the success rate for banana was only a few percent lower. In contrast, the ability to smell mercaptans—the stinky chemicals added to natural gas to make people aware of leaks—began to drop off when people were in their forties.

Scientists aren’t sure exactly why our sense of smell often fades as we age, but most of them think it’s just part and parcel of our body’s diminished ability to repair itself. The cells of the olfactory epithelium are among the few nerve cells that the body regularly replaces during adult life. As with other regularly replaced cells—skin and hair follicles are obvious examples—problems accumulate over time. The olfactory epithelium of a newborn infant is a nice smooth, solid sheet of cells, but it gets more ragged and patchy as we age.

But something else may be going on, too. Even as the olfactory epithelium breaks down, the responses of its remaining cells may start to blur. To show this, a team led by Beverly Cowart, yet another Monell researcher, collected biopsy samples from the olfactory epithelium of elderly and middle-age volunteers. This is as creepy a procedure as it sounds. Under local anesthetic, doctors thread a fiber-optic scope into one nostril and insert something called a “Kuhn-Bolger giraffe forceps”—a scissorslike clamp with a long, offset neck—up the other nostril to grab a little pinch of olfactory epithelium. The resulting cells can be grown in petri dishes to see which odors—or, in Cowart’s case, odor mixtures—they respond to. Each cell from middle-aged noses responded to just one of the two odor mixtures Cowart used. By contrast, about a quarter of the cells from elderly noses responded to both. That suggests that older people’s noses blur together details they once could have resolved—an olfactory analogue of cataracts, perhaps.

The healthier we stay, though, the more likely we are to keep our sense of smell intact: The “successfully aged elderly” often continue just fine. Smell loss, in fact, can be an early warning sign of more serious medical problems such as Alzheimer’s disease and Parkinson’s disease. That’s not surprising: the olfactory system is basically part of the brain, so many degenerative brain diseases should affect the sense of smell, as well. Oddly, some oncologists also report that one of the first warnings of a developing cancer is that food doesn’t taste right—even when the tumor is in the breast or prostate or some other organ unrelated to flavor perception. Indeed, seniors who have lost their sense of smell are four times as likely to die within the next five years, compared with people of the same age with good olfaction. (It’s important to note that olfactory loss is not a death sentence—most people who lose their sense of smell still survive just fine.)

Whatever the cause, loss of smell can bring major problems. In one study, nearly half of the patients with smell disorders reported experiencing depression and anxiety, and more than half felt isolated and had difficulties relating to other people. The effect on flavor is even worse, with 92 percent of people reporting less pleasure in eating—and that brings social difficulties of its own. “Most of our social interactions involve food,” says Cowart. “It becomes very difficult to justify going out to eat and paying a lot of money for food they can’t taste, or going to a friend’s house and not being able to tell the host that the meal tasted great.”

You could hear that loss in the voices of the patients in Doty’s office—though, being nonspecialists, they tended to talk about failures of taste, not smell. “I don’t taste my food at all,” said one elegantly dressed older woman. “It tastes like eating sawdust when I eat a cracker.” Or, as another put it, “The only reason I know what I ate is that I’m looking at it, and I remember what it tasted like.”

Despite complaints like this, most people find ways to cope, and about two-thirds manage to maintain their usual weight. Only a small minority—one expert puts it around 10 percent—of people with a damaged sense of smell actually lose weight as a result. And those tend to be people who suffer not total loss, but distortion of smell, like Yager’s turpentiney vanilla and stinky watermelon. Often, patients report that everything has the same “burnt-chemical” smell—probably the best they can do at describing something unfamiliar and unpleasant. Olfactory bugaboos like this can actively turn people off their food. Among the elderly, people who have lost their ability to smell are far more likely to be undernourished—but that may be because loss of smell is linked to other health problems, rather than because they find their food less appealing.

On the other hand, a few people who suffer a sudden loss of smell actually gain weight as a result. These tend to be people who were already susceptible to food cravings, which are often more about habit than about deliciousness. (One researcher who tried to create cravings for other foods by putting people on a boring diet of vanilla-flavored meal-replacement drinks found that the subjects actually began to crave the boring drinks. “They tried to scam cans of this stuff off the technicians in the lab,” she recalls.) The craving gives them a sensory template, a hole waiting to be filled—and with no sense of smell, they keep eating in the futile hope of finding satisfaction.

When Yager returns to Doty’s office that afternoon, he gives her the results of the tests: no problems at all in her sense of taste, but on her smell tests she scored no better than if she’d been guessing randomly. Clearly, what little smell she has left is so badly degraded that she can’t tell the difference between familiar odors like grape and peanut butter.

And unfortunately, says Doty, medical science can’t do much to fix the problem. About half of the people who suffer smell impairment get some function back within a few years, but less than a quarter recover fully. Among people with complete loss of smell, like Yager, the odds of full recovery drop to just 8 percent.

There might be ways to improve that discouraging prognosis, he notes. There are a few reports that a supplement, alpha-lipoic acid, might help. And some studies suggest that even a failing sense of smell might improve with practice, because nerve cells are more likely to be replaced or regrow if they’re being used. Grab bottles of spices—“Anything that says McCormick”—and keep them beside your bed, he tells Yager. Sniff through them three or four times first thing in the morning and before you go to bed for the next three or four months, and see if it helps. She brightens at the thought that she might be able to do something about the problem.

A year later, I checked in with Yager to see whether the exercises helped. No luck, she reported—she still can’t smell anything. “I’ve grown accustomed to it, I suppose,” she says. She’s learning to cope by making sure her food is well seasoned with salt, pepper, and lemon, which don’t depend on smell for their flavor impact, and admits that “Sriracha has become a close friend.” (The chili burn uses a different nerve to reach the brain, so she still gets full value for that flavor component.) She rarely drinks wine any more, except to be sociable, since it doesn’t offer much to interest her. These days, her preferred tipple is gin and tonic, which is still exciting in her mouth, thanks to its pronounced hit of bitterness.

It’s hard to come away from Doty’s clinic with the feeling that flavor makes much difference in maintaining body weight. But what does, then? Why do some people gain weight while others don’t? And, the big question: What can we do about it? Unfortunately, scientists still haven’t agreed on an answer. Mark Friedman thinks—and he has some research to back him up—that overweight people’s metabolism has shifted so that the energy they take in from meals is more likely to get stored as fat, and less likely to be available for the daily needs of living and moving around. “You’re losing energy internally that you can’t get to, so you eat more,” he explains. “Essentially, you’re overeating because you’re getting fat.”

On the other hand, Dana Small thinks—and there’s a lot of evidence to back her up—that overweight people are less sensitive to their body’s satiety signals, so that they’re less likely to shut down their food intake when they should. Instead, they tend to eat from habit, because it’s time, or they walk through the kitchen, or because they drive near the golden arches. In the absence of a good satiety signal, they’re more vulnerable to the pull of their reward system, even when they’re not actually hungry. Even rats sometimes overeat merely because the food is there. In one study, just putting extra containers of sugary or fatty food in a rat’s cage was enough to get it to eat more of the abundant calories. Similarly, rats that can choose among six different water bottles get almost twice as fat if five of the bottles contain sugar water than they do when just one bottle has sugar water. They could have drunk just as much sugar water from the single bottle—the researchers kept it full—but something about having so many sugary options made a difference.

The bottom line here seems to be that flavor does make a difference to what we eat, and indirectly to how much we eat. Flavor-nutrient conditioning pulls us toward wanting the calorie-laden foods that are so readily available these days. But even though flavor is part of the overeating equation, tinkering with flavor may not be part of the solution. Making food more or less flavorful probably won’t change consumption in the long run. No matter how enticingly you flavor that no-fat rice cake, your body will still know there’s nothing much there—and will quickly learn that those flavors aren’t worth liking. Given a choice between a full-fat cheese and a reduced-fat version, the reward systems in our brains will clamor for the full-fat one. Any food company that tries to sell the reduced-fat version has to fight this innate emotional pull—usually by appealing to reason and prudence, which have a hard time competing against yearning. Often, food companies don’t even try, and when they do they frequently lose. That’s why frozen pizzas have as much cheese as they do, and why French fries remain so popular on fast-food menus. They’re the intensely researched result of the flavor companies that design, make, and test the flavors in processed foods.