by John McQuaid
The neurons in hedonic hotspots respond to several different endorphins, suggesting that pleasure is complicated, the result of many brain systems interacting at once. One such endorphin is orexin, a comparatively rare substance also connected to appetite, arousal, and wakefulness; another is anandamide, named for the Sanskrit word ananda, which means “bliss.” It plays a role not only in pleasure but pain, memory, and higher thought processes. Orexin and anandamide activate opioid and cannabinoid receptors, respectively, which also respond to heroin and marijuana.
The anatomy of pleasure bridges the visceral and the brain’s higher functions, placing hedonic hotspots smack in the middle. They work something like circuit boards. There are two hotspots, as well as a “coldspot” nearby that sparks disgust. The coldspot is nestled in an area rich in dopamine neurons that inspire intense wanting: when the two were stimulated together, Berridge made a rat yearn for something that tasted terrible. Removing one hotspot reduced pleasure but didn’t eliminate it, but removing the other made sweet things taste terrible. That could mean this hotspot’s job is to inhibit disgust and enhance pleasure at the same time.
The simple delight of sugar dissolving on one’s tongue appeared to be the product of certain neurons tucked into structures deep inside the brain percolating in a cocktail of the body’s most intoxicating hormones. Yet however detailed these anatomical maps of the origins of pleasure became, they could not explain its purpose. The role of the cravings caused by dopamine was also up in the air. As he learned more, Berridge formulated a theory he hoped would fill this void. Like many behavioral models, it was blunt, reducing the vast diversity of human decisions and actions down to the shape of a triangle.
The triangle’s sides are named “wanting,” “liking,” and “learning.” It can describe all behavior, but applies particularly to taste and to flavor. Wanting is a state of desire and heightened focus before food is eaten. Liking is the pleasure of a good taste, a reward for doing the work of obtaining food. Wanting and liking work in tandem to forge learning. The human brain very quickly picks up on how to gratify itself, learning where the tastiest food is and how to get it.
In the 1990s, Cambridge University neuroscientist Wolfram Schultz did a series of groundbreaking experiments that dramatized this dynamic. Schultz also showed that dopamine was the hidden hand in cravings: it is what powers “wanting.” In one test, monkeys were placed in front of a computer screen that displayed geometric patterns. One pattern flashed two seconds before sugar syrup was dispensed from a bottle; the other appeared randomly. Electrodes measured the activity of a single dopamine neuron in the monkeys’ brains. At first, this neuron fired when a monkey sipped. But as the cycle repeated and the monkeys picked up on the signals, the neuron adapted. It began to fire before the treat arrived—predicting a good taste was coming and sharpening the anticipation and craving for it. When kitchen smells make the mouth water, that’s dopamine setting the sensory table. And if learning can be tracked in a single neuron, imagine billions of neurons in the human brain doing the same over the course of a lifetime.
Having identified building blocks of pleasure, Berridge pondered what the fleeting “goodness” in a sweet taste really was. Clearly, it was distinct from the feelings that accompany listening to a favorite song, or seeing an old friend. But deep down, these states might be the same—formed in the same areas of the brain, reliant on the same patterns of firing hedonic hotspots and hormonal flux. Evidence from fMRI scans suggests there’s something to this idea—the different forms of pleasure have patterns of brain activity that closely overlap. As humans evolved and culture made its imprint on the human brain, perhaps the ancient neural circuitry responsible for sweetness was adapted as the template for more exalted pleasures, and maybe even happiness itself. “Final happiness may be a state of liking without wanting,” Berridge said. “That may be a Buddhist sense of happiness.”
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Overindulging in sugar disrupts the normal rhythms of wanting, liking, and learning. Humans evolved eating just enough food to sustain big brains and lithe, active bodies. The stomach can hold only so much, and the gut and brain engage in a continual dialogue to ensure a balance is struck. Powerful hormones excite the dopamine-sensitive parts of the brain, spurring humans to seek food when hunger strikes. Pleasure peaks at the start of a meal, when hunger is keenest, then declines—one reason why no one eats the entire contents of a sugar bowl. But persistently overdose this system, and the signals start going awry. Fructose appears to raise levels of the hormone ghrelin, for instance, which stimulates hunger; instead of satisfying, eating sugar leaves us wanting more.
Science has only begun to trace these easily corruptible pathways running between body and brain. Thanks to our expanding knowledge of taste genes, lab mice and rats can be genetically engineered with specific genetic traits for experiments. Ivan de Araujo, a neuroscientist at Yale, fed both plain water and sugar water to a type of mouse engineered to have no taste for sweetness. They should not have been able to sense the difference, yet they strongly preferred the sugar water.
Ordinarily, the tongue’s sweet receptors signal the brain that a delicious reward is on the way. With that signal absent, de Araujo suspected that the sugar was still making its presence known through an unknown back channel, making the mice crave sugar with no conscious awareness of it. To test this hypothesis, he implanted probes in the mice’s brains that measured their dopamine levels; the sugar water produced a dopamine rush. Somehow, de Araujo thought, the body sensed the sugar—perhaps through taste receptors lining the gut—and signaled the brain it was there, triggering the yen for more. When the experiments were repeated with human subjects—their sweet taste blocked with a drug—they described a hazy sense of satisfaction after sipping sugar water.
These urges resist both willpower and medications. Appetite suppressants reduce hunger, but craving and pleasure are more complicated phenomena. Dopamine-blocking drugs shut down the hankering for sugar, but extinguish all motivation at the same time. A drug aimed at suppressing the pleasure of food might kill all joy along with it.
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As more of sugar’s insidious effects were discovered, people began getting the message. In the first few years of the 2010s, sales of soft drinks—the single largest source of dietary sugar in the United States—leveled off, and then decreased for the first time in their century-long history. Overall high-fructose corn syrup consumption fell, too. Obesity rates plateaued, though at high levels nutritionists still found alarming. Diabetes rates, however, continued to rise. It will take years to assess the public health toll.
The ideal solution would be a sugar substitute that perfectly mimics the taste and poses no health risks. But this is the oldest unsolved taste problem in the world. The Romans boiled crushed grapes in lead vessels to make a syrup called sapa, used to sweeten wine, stews, and other dishes. The active ingredient was lead acetate, also known as “sugar of lead,” created by a chemical reaction between the grape juice and the containers. It was also toxic. Some have claimed Rome fell because its entire ruling class suffered from sapa-induced lead poisoning (historians are skeptical of this explanation). Lead acetate was still used for centuries afterward as a wine sweetener; among its possible victims were wine-drinkers Pope Clement II, who mysteriously dropped dead in 1047, and, eight hundred years later, Beethoven.
Modern sugar substitutes such as saccharin, the active ingredient in the pink packets of Sweet’N Low, and aspartame, used in diet soda, have their own problems. They don’t taste like sugar. Table sugar, made from sugarcane or sugar beets, is made of sucrose, a molecule in turn made of two sugars, fructose and glucose. High-fructose corn syrup is a physical mixture of those two, with slightly more fructose. Of all the sugars, fructose is the sweetest. The uncannily precise bond between sweet receptor and fructose molecule means there is no other substance capable of exactly mimicking its taste.
The molecules of substitutes bond to sweet receptors, but don’t fit perfectly, like a key that slides into a lock but won’t turn all the way. They also bond to other types of receptors, including those for pungency and bitterness. The result is odd, off flavors, such as aspartame’s faint metallic aftertaste, that don’t fully ignite the brain’s pleasure circuitry. Most sugar alternatives don’t dissolve well in water, either, and will cling to the tongue rather than remain in solution. This means they pack a sensory punch—aspartame is about two hundred times as sweet as table sugar—but it can also make their tastes linger too long. The differences in chemical structure also make them poor baking ingredients. Sugar is not just sweet but versatile. Heat it and complex flavors emerge, with hints of acidity and bitterness. It can take multiple forms and consistencies, from crystals to caramels, that no substitute can touch.
Today’s dominant artificial sweeteners are also lab-engineered industrial chemicals. Saccharin is a coal-tar derivative accidentally discovered at Johns Hopkins University in 1878. Aspartame was found in 1965 when a lab scientist at the pharmaceutical company Searle absentmindedly licked his index finger, which was dusted with an ingredient from an ulcer drug. Sucralose, the active ingredient in Splenda, was found while researchers for the sugar giant Tate & Lyle were studying ways to turn sucrose derivatives into an insecticide. The health concerns over sweeteners are more ambiguous than those surrounding sugar. Aspartame produces trace amounts of methanol in the intestine, a form of alcohol the body transforms into formaldehyde—the chemical used in embalming fluid, and a carcinogen—before it breaks down again. But so do oranges and tomatoes. The Food and Drug Administration banned saccharin in 1976 because of a tentative link to cancer in lab animals, but later unbanned it because the evidence was scant. Sucralose is not broken down in the body. However, recent studies ominously show that artificial sweeteners may contribute to diabetes.
Shell-shocked consumers are rejecting artificial ingredients on general principle. In 2013 alone, sales of Diet Coke and Diet Pepsi each plunged by 7 percent. Since the early 2000s, food and soft-drink makers have spent tens of millions of dollars in a race to find natural sugar substitutes. Many plants produce sweetish substances. But these don’t taste exactly like sugar either. Thaumatin, a protein found in Thaumatococcus daniellii, a plant that grows in West African rain forests, is the sweetest substance known—three thousand times more potent than sugar. It lingers on the tongue for minutes, leaving licoricey aftertastes. Stevia, made from the leaves of the South American Stevia rebaudiana plant, has a bitter edge.
Homaro Cantu believed the frustrations of big food companies offered an opening for miracle berries, though he faced his own set of obstacles. In 1974, the FDA had classified miraculin as a food additive, meaning it would have to go through extensive testing before it could gain approval as an ingredient. Its advocates claimed that the US sugar industry, which wields substantial clout in Washington, had lobbied for this behind the scenes. Miraculin is currently classified as a dietary supplement. By the time Cantu found it, several startups had sprung up to market and sell miracle berry extracts. The price was still high—a single pill cost $1.50—but researchers had found ways to transfer miracle berry genes to tomatoes and lettuce, which can produce much larger amounts of miraculin than berries. Chemically, miraculin is not even a sweetener. Its flavor is mild. Instead, it alters other flavors, sometimes unpredictably. This may not be enough to start a diet revolution. But it does show that there are new frontiers of sweetness that have yet to be fully explored.
CHAPTER 6
Gusto and Disgust
As HMS Beagle sailed along the South American coast toward Tierra del Fuego in 1833, Charles Darwin had a series of scientific adventures. He had been recruited by the Beagle’s captain, Robert FitzRoy, as a geologist to aid the ship’s principal mission, mapping the South American coastline and seafloor. In a few months, they would reach the Galápagos Islands, off the coast of Peru, where Darwin would find the strange flora and fauna on which he would base his theory of evolution. As the Beagle sailed south, he spent most of his time on land, observing and collecting geological specimens. At Bahía Blanca, Argentina, he rode with gauchos into the pampas and dined with them on roast armadillo. In Uruguay, he bought the skull of an extinct rodent, the size of a hippo’s, from a farmer for eighteen pence. At Punta Alta, on the coast of Patagonia, he found bones from a megatherium, a huge, extinct armored sloth.
Darwin was both fascinated and repelled by the indigenous people living at the southernmost tip of the continent. He was twenty-three, on his first voyage, and had plunged into an alien world full of strange sensations. The people he met were the strangest of all. The Yahgan tribe lived a marginal existence as hunter-gatherers, roaming the archipelago near Cape Horn in dugout canoes. Most had long hair and wore little clothing, even in frigid weather. When the Beagle rounded the cape, he observed some of them rowing canoes. They struck Darwin as strange and degraded examples of humanity. “These poor wretches were stunted in their growth, their hideous faces bedaubed with white paint, their skins filthy and greasy, their hair entangled, their voices discordant, and their gestures violent. Viewing such men, one can hardly make one’s self believe that they are fellow-creatures, and inhabitants of the same world.”
Their food was vile. “If a seal is killed, or the floating carcass of a putrid whale is discovered, it is a feast; and such miserable food is assisted by a few tasteless berries and fungi,” he wrote in his journal. The Yahgan would render the carcasses of beached whales, burying their meat and blubber in the sand. Without oxygen, it would ferment rather than decay. A few months later they would unearth it and feast. A shipmate who had spent time in the region filled Darwin in on the worst of it: cannibalism. When famine struck, the Yahgan ate their old women before their dogs. A native boy had explained the rationale: “Doggies catch otters, old women no.” The unfortunate grandmas would sometimes escape into the mountains, then be captured and brought back to the hearth, where they were suffocated with smoke, then butchered for the choicest parts. (This was almost certainly a rumor; anthropologists have found no evidence that the Yahgan practiced cannibalism.)
On January 19, 1834, the ship anchored at the midpoint of the Beagle Channel (so named after the same ship explored it six years earlier), a one-hundred-mile stretch of water north of Cape Horn. A party of twenty-eight disembarked, including FitzRoy, Darwin, and three Yahgan, who had been captured on a previous voyage and were returning home after three years in England. They took four boats and rowed along the eastern bank. The following day, they entered an inhabited area; surprised Indians lit signal fires up and down the shore, and some followed the boats. The Beagle party came ashore near a Yahgan camp, and a tentative meeting took place. Initially hostile, the Indians warmed as the crew handed out gimlets (small tools for boring holes) and stretches of red ribbon that they tied around their heads. One of the three returning Indians, Jemmy Button, “was thoroughly ashamed of his countrymen, and declared his own tribe were quite different,” Darwin wrote, “in which he was woefully mistaken.”
As they all sat around campfires, Darwin opened a tin of preserved beef and began to eat. Canning had been invented only twenty years earlier, and canned meat had only recently become standard shipboard fare in the British Empire. Its taste was passable at best, something like the canned corned beef sold today, but a vast improvement over the smoked and salted meats in use a decade earlier, which rotted on journeys of any length.
“They liked our biscuit,” Darwin wrote. “But one of the savages touched with his finger some of the meat preserved in tin cases which I was eating, and feeling it soft and cold, showed as much disgust at it, as I should have done at putrid blubber.”
The next day, the party rowed to Wulaia Cove, where they left their “civilized” Yahgan companions, and continued to explore the area before returning to the ship a week later.
But the meat-tin incid
ent stuck in Darwin’s head. His observations of the Yahgan forced him to confront his prejudices. Unlike many educated Europeans of the time, he believed that the absence of civilization, not a savage nature, was responsible for the natives’ bizarre tastes and abject circumstances. If this were true, the extremes he observed meant human behavior and sensibilities were even more malleable than he had imagined.
Almost forty years later, after On the Origin of Species had secured his place in history, Darwin wrote about this encounter in his new book, The Expression of the Emotions in Man and Animals. The book’s main argument was a controversial, though logical, extension of his theory of natural selection: mankind’s infinitely subtle emotional expressions, thought to be reflections of the soul, had evolved from those of animals. Both the Yahgan man’s and his own reaction to the meat tin exemplified disgust, an emotion that had originated as a response to noxious foods but had evolved into something more complicated:
The term “disgust,” in its simplest sense, means something offensive to the taste. It is curious how readily this feeling is excited by anything unusual in the appearance, odour, or nature of our food. In Tierra del Fuego a native touched with his finger some cold preserved meat which I was eating at our bivouac, and plainly showed utter disgust at its softness; whilst I felt utter disgust at my food being touched by a naked savage, though his hands did not appear dirty.
Each man had been repelled not by the meat’s taste or smell but by some ephemeral quality springing from a mixture of the sense of touch and imagination. For the Yahgan tribesman, it was the feel of this strange new food on his fingertip and the thought of it on his tongue. For Darwin, it was the idea of eating something touched by such a degraded example of humanity, who had, perhaps, consumed human flesh.
