Few foods inspire such passion in people as chocolate. This love affair goes far beyond the typical fondness for sweets: after all, we’re not likely to head out into a snowy night, panic-stricken after finding that we are out of lemon-crème pie or bubble gum. There is something special about chocolate that drives us to extraordinary lengths. Chocoholics find nothing strange in spending a small fortune for even a sampler box of champagne truffles. No, a simple sweet addiction is not the same as a chocolate addiction—indeed, many connoisseurs prefer the darkest, most bitter variety.
The history of the chocolate bean is a story riddled with desire that transcends cultural distinctions. The roots of chocolate go back some twenty-six hundred years to the great Olmec and Mayan civilizations that flourished throughout southern Mexico, Belize, Guatemala, and Honduras. Spouted, teapot-shaped vessels have been excavated from towns such as Colha in northern Belize, and found to contain residue of ancient chocolate. The Mayan drink was very different from the watery, sugar-laden version of hot chocolate that dominates modern society. The journals of Spanish conquistadors are filled with descriptions of middle Mayan culture that include the preparation of dried cacao beans ground into a powder and mixed with water, honey, chili pepper, and sometimes maize. The liquid would then be heated and repeatedly poured from one vessel to another to produce a thick head of rich chocolate foam that was the most coveted part of the drink.
A reverence for chocolate was also present in Aztec culture throughout the region. The great Aztec emperor Moctezuma reportedly drank up to fifty flagons of chocolate per day, believing it to have restorative and even aphrodisiac powers. Within this culture, the cacao bean became the primary form of currency, and folklore has it that when the Spanish conquistadors stormed Moctezuma’s temple, they found beans in place of gold.
After conquering the Aztecs, Hernando Cortés returned to Spain and brought King Carlos treasures of cacao beans and a recipe for making
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, which was sweetened with sugar by members of his court. Today this recipe lives on, and the domesticated cacao tree grows on farmlands near the equator in a number of regions including the Caribbean, Africa, southeastern Asia, and in several South Pacific islands such as Samoa and New Guinea.
Whereas modern processing and distribution technology has made chocolate a more common item on the food landscape, its seductive properties have remained a mystery to science. It has only been in the past few years that neuroscientists and biochemists have begun to get a handle on why we find chocolate so pleasurable.
Chocolate contains more than 350 known compounds, several of which activate three important brain systems that contribute to the experience of pleasure.The first ingredient that gives chocolate its wide fan base is plain old sugar, an underappreciated compound these days. Considering our modern tendency to detest all that is carbohydrate and the epidemic-like rates of diabetes in many subpopulations, it is easy to understand why sugar is seen as something to avoid if you consider yourself a health-conscious individual. But in reasonable doses, sugars have a profound and positive impact on our physiology, most notably in the form of a calming effect. Placing a small amount of liquid sweetened with either glucose or sucrose on the tongue of a crying newborn has an immediate calming effect that can last for several minutes. Sugars, in their varying chemical structures from lactose to sucrose, have been shown to activate the brain’s opioid system, a set of circuitry that plays a prominent role in regulating the body’s stress response.
In addition to sucrose, chocolate contains small amounts of theobromine (a mild stimulant) and phenylethylamine, a substance that is chemically similar to amphetamine. Once in the brain, each of these ingredients has an effect on the dopamine and noradrenergic neurotransmitter systems, which are implicated in attention and general arousal. These compounds are thought to provide the “boost” we all experience after eating chocolate.
But chocolate gives us more than a mere boost; most people crave the sense of euphoria that lingers long after the treat is gone. A recently discovered trio of chemicals has been identified in chocolate that seems to be at the heart of this feeling of well-being that is familiar to all chocoholics. Anandamide is a chemical messenger in the brain that binds to the same nerve cell receptors that are activated by tetrahydrocannabinol (THC)—that’s right, the active compound in marijuana. Anandamide, it turns out, is released in small quantities during times of stress and provides a calming and analgesic effect; however, it is quickly broken down by naturally produced enzymes, so there is never very much of the substance in the brain under normal circumstances.The buzz one gets from marijuana is another story altogether—in this case a deluge of THC enters the brain, overwhelming the ability of the enzymes to break it down, so it has a prolonged and more intense effect than the naturally occurring version. The “THC buzz” is essentially an exaggeration or amplification of normal cannibinoid brain system functioning.
The chocolate buzz occurs through a slightly different mechanism. Small amounts of anandamide are present in chocolate (darker chocolates have larger quantities), but not so much that would activate the brain’s cannibinoid system above normal. The key to unraveling this mystery came when two additional anandamide-like compounds were identified in chocolate and found to be present in fairly large quantities. While these related compounds don’t activate THC receptors directly, they increase the effect of naturally occurring anandamide by blocking the enzymes that usually break it down.This means that even small amounts of naturally occurring anandamide or that ingested while eating chocolate will stay in the brain for a prolonged period of time, since it is not metabolized as quickly as normal.The result is that blissed-out feeling of calm that we experience after downing a hot chocolate or going through a few Droste pastilles.
It is easy to see why the depressed and stressed among us self-medicate with chocolate. It quenches the pleasure instinct by activating three key brain transmitter systems that are involved in reward, although they have evolved as adaptations to very different environmental circumstances.
The sucrose in chocolate is just a “souped-up” version of fructose—a form of sugar that is naturally present in most fruits that were widely available to early hominid hunter-gatherers. Sugars are a critical component of life because they provide metabolic energy in the form of ATP that powers the many biochemical reactions within every cell of our body. For the average hunter-gatherer, fruits were a very good nutritional choice, since ounce for ounce they offer a rich source of energy with virtually no exposure to dangerous horns, teeth, or claws. The only problem is in identifying fruits as a desirable substance to eat.
Early in our hominid evolution, the brain opioid system became very important for controlling our eating behavior, mainly in functioning to make sure that certain foods seemed more palatable than others. During this point in the evolutionary history of humans, opioid system activation and the pleasurable sensations that result became associated with the consumption of foods that have relatively high concentrations of sugar. This association was strictly in terms of alterations in brain wiring—some hominids evolved changes in their opioid system that made its indirect activation possible through receptors that were sensitive to the presence of sugar. In a very real sense, the opioid system, which until then probably played a role mainly in sexual reproduction, was co-opted by selection factors that made it very cost-effective (energywise) for hominids to be able to find and want to eat sugar-rich fruits. Hominids with a tendency to experience pleasure when eating something sweetened by natural sugar had a clear survival advantage over their peers who were not “afflicted” with this important mutation to opioid system wiring. Similar mutations may have occurred within the dopamine reward system and the cannibinoid system, making the taste of chocolate a “triple threat.”
Sugar and Health
Modern societies have very different survival pressures, and hence selection factors, than those of early hominids. Not only do we have access to all the fruits we want, we have also perfected the packaging and delivery of refined sugar such as sucrose in a staggering variety of forms that include processed foods and candies. Our opioid systems are awash in a sea of sweet-tasting stimulants, and this has serious consequences for societal health.
Study after study has shown that in humans, the most palatable foods release the highest levels of beta-endorphins into our bloodstream. Endorphins are the brain’s natural opioids that are typically released during stress. When this system is rendered inactive by administering an opioid antagonist (drugs that bind to opioid receptors but do not activate them) such as naltrexone or naloxone, subjects report that foods taste less palatable and food consumption is often substantially reduced. Thus there is very clear evidence that the opioid system is involved in the hedonic experience of food.
In the past decade research has shown that obese people often have a different opioid system response compared with nonobese individuals. At least two independent studies have found that obese subjects produce up to three times as much beta-endorphin in their blood plasma after consuming a palatable meal when compared to their skinnier counterparts. One interpretation of this finding is that some individuals may be predisposed to obesity because they have hyperactivated opioid systems and literally experience more intense pleasure in response to opioid-system-activating foods than others. It is currently unknown, however, whether this change in opioid system functioning is a cause or a result of obesity.
The opioid system also plays an important role in attachment behaviors. In mouse pups, the response to being separated from their mother consists of ultrasonic vocalizations accompanied by a brief period of hyperactivity until the two are reunited. Mice that have their opioid system blocked by chemical agents or through genetic manipulations fail to display the same plaintive calls as normal mice. They do, however, protest to other events such as sudden changes in temperature or the introduction of an adult male. Hence, attachment behaviors depend on opioid system activation.
The fact that attachment behaviors seem to involve the opioid system is interesting when one considers that breast milk is rich in lactose, a sugar that serves to stimulate the activation of this system. Newborns and infants are innately attracted to the smell (see chapter 5) and taste of breast milk, and they have an uncanny ability to identify milk from their own mother. By the end of the first week of life, newborns prefer the taste of their mother’s breast milk over cow’s milk. There are many compounds in mother’s milk that may account for this preference. Besides being sweetened with the sugar lactose, it is rich in essential fatty acids that we will see are sought out by virtually all humans, from newborns to adults. Additionally, mother’s milk contains a number of important immune factors and whole immune cells (one reason that synthesizing human breast milk has not been possible for manufacturers of baby formula) that may be critical in helping the infant identify and develop a preference for its own mother’s milk over milk from an unrelated lactating mother.
Hence, newborns that are allowed to breast-feed will naturally self-stimulate their own opioid system, which itself may be a necessary component for the development of normal attachment. The timing of this sequence of behaviors is important, since the ingestion of lactose occurs coincidentally with other forms of stimulation that are known to activate the opioid system, such as the sensation of being touched and the familiar smell of Mom. All of these stimuli activate the opioid system at a highly opportune time for the development of maternal-offspring attachment—during feeding behavior.A biological mechanism such as this, which uses the experience of pleasure to prod newborns toward behaviors that at once maximize both attachment and the intake of nutrition, is likely to have tremendous survival value.
Getting Wired for Taste
Modern science has identified five basic food tastes: sweet, sour, bitter, salty, and the latest entry, umami, which is caused by the presence of monosodium glutamate (MSG). The first step in the path toward taste perception begins with the humble taste bud. Under an electron microscope, a taste bud has a shrublike appearance—think rhododendron—that is shaped by forty or so elongated epithelial cells. Each epithelial cell has receptors that preferentially respond to the presence of compounds affiliated with one of the five taste groups. As I eat my lunch of stir-fried vegetables, the natural sugars in the carrots and snow peas that pass by the approximately five thousand taste buds that line the perimeter of my tongue will activate groups of cells that are most sensitive to sweet-tasting food; salt-sensitive cells will become activated in response to the presence of sodium and potassium in the veggies and sauce added for seasoning; still other cells will be excited by the MSG.
The collective ensemble of activated cells sends this taste information on to the next stage of processing in the medulla and other nearby brain-stem structures that control the automatic behaviors involved in feeding such as sucking, salivation, and swallowing. From the brain stem, this signal makes its way to the thalamus, and finally branches out to cortical gustatory areas where the conscious perception of taste occurs and to various limbic nuclei where taste information can be integrated with memory, emotions, and motivation centers that regulate our desire to eat.
From the basic physiology and anatomy of the two chemical senses, we know that taste and smell are processed in very different ways. Epithelial cells in the olfactory mucosa respond to thousands of different types of odorants, while those that make up the gustatory system seem to have evolved a preference for basically five dominant taste classes. Besides this difference, however, there are remarkable similarities in the evolution of these systems in the human species and their development in the individual.