CHAPTER TWENTY- ONE Flavor and Obesity
Does knowledge of the human brain flavor system give insights into practical problems like the current epidemic of obesity in worldwide populations? Let us consider the case of fast food.
A “Normal” Meal? Suppose you’ve taken a bite of French-fried potatoes. This may not have been the kind of food that Jean Anthelme Brillat-Savarin had in mind when he extolled human flavor, but it is probably as near to a universal food as we have. It was brought to the United States from France by returning soldiers after World War I and gained popularity in the 1920s and 1930s. According to Eric Schlosser, French fries are “the most widely sold foodservice item in the United States.” A recent estimate is that 25 percent of vegetables consumed in the United States are French fries. Therefore it might be interesting for a neurogastronome to understand the role of the human brain flavor system in this popularity, especially because it is responsible for many of the culprits leading to obesity. Sensing flavor starts with the sensory receptors. Your first breath carries the smell through the retronasal route to the olfactory sensory receptors inside the nose. Activation of a combination of the receptors creates in the olfactory bulb the image of the smell that is processed in the olfactory bulb and the olfactory cortex to give rise to the smell
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perception in the orbitofrontal cortex. This image will dominate your perception of the flavor of the French fries, because it has been learned by you to be a pleasurable smell image, and because smell is the dominant sense of flavor. At the same time, the French fry will come in contact with the taste buds on your tongue, and you begin to have added to the flavor the taste of the salt on the potatoes. As we have seen, from birth saltiness is built into our taste perceptions as an attractive taste. It is not only essential for life but also enhances smells. Together, a learned smell enhanced by salt begins to be irresistible; as we know, it is hard to eat just one salted peanut. At the same time, we expect a certain sweetness to the potato itself, and the sweetness of sugar is also attractive from birth. In addition to sensing the taste, the tongue begins to do its motor job, moving the French fries to the teeth for mastication, which engages the motor control of both the tongue and the jaw muscles. The mush that is produced builds up in the space between the teeth and the inside of the cheek, so the tongue keeps sweeping it back to the teeth as well as around the inside of the mouth. The motor control of all these muscles is coordinated so that the movements of the tongue, cheek, and jaw take place almost automatically without our being really conscious of them unless we perceive a taste emerging that causes us to slow and examine more carefully the crushed food. All this motor activity adds to the perception that the flavor comes entirely from the mouth. Meanwhile, the tongue is sensing the texture of the French fry. This is actually a complex mixture of senses. The crispness is usually one of the most important characteristics, along with the contrast between the crisp outer edges and the soft interior. Sogginess will be unacceptable, as will too hard a crust. There must be a springiness in the “mouth-sense� of the potato to indicate that it is fresh and has been cooked just right. The temperature also has to be right: still warm from the fryer, but not too hot. Even before eating, we will have judged the French fries by looking at them to be sure the pieces are the right size, with the right color. For example, French fries any color but golden brown would be unacceptable. No one wants to eat a gray or black fry. As we chew, we expect to hear the familiar sound that starts with a sharp crunching and ends with the soft squish of the mash in our mouths.
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Finally, the motor control of swallowing takes over, and the mash goes down to our stomachs (followed by our breathing out for a last enjoyment through retronasal smell). Three other nonpotato components are extremely important to the flavor: meat flavor, condiments, and companion food and drink. Consumption of French fries in the United States really took off after World War II as the central item in the rise of fast-food culture. This was based on the fact that, according to Schlosser, potatoes were the food most consumed by Americans after dairy and wheat products. The originators of the fast-food industry figured out that, apart from starting with acceptable potatoes, what really was attractive was the meat flavor imparted to them by the oil in which they were fried: “For decades, McDonald’s cooked its french fries in a mixture of about 7 percent cottonseed oil and 93 percent beef tallow. The mix gave the fries their unique flavor—and more saturated beef fat per ounce than a McDonald’s hamburger.” After a public outcry against all this fat and cholesterol, McDonald’s changed in 1990 to using a vegetable oil, but with a strong meat flavor due to organic chemical compounds that were produced by the flavor industry. This artificial smell produces the meat flavor that continues to be an attractive part of the French fry’s flavor. In addition to the smell, the flavor industry also contributes other artificial means, through “fats, gums, starches, emulsifiers, and stabilizers” to enhance the mouth-sense (texture) of French fries and other processed foods. Driven by this flavor, a single medium-size bag of French fries delivers 380 calories, almost 20 percent of a day’s needs. The second component is the ketchup. We Americans like ketchup with our French fries. We think of ketchup as being made of tomatoes, but that is only the base. A common brand of ketchup includes the ingredients shown in box 21.1. You couldn’t design a modest sauce to be more flavorful. It stimulates directly three of the five tastes (umami already is stimulated by the meat flavor of the fries). It stimulates retronasal smell with volatiles from tomato concentrate, spices, and onion powder (to say nothing of the unknown artificial compounds hidden under the blanket term natural flavoring). So the ordinary potato is becoming the vehicle for an intense barrage of flavors. A principle of neurogastronomy is that the brain responds to multiple sensory inputs. Satiety to one flavor does not produce satiety to 186
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BOX 21.1 Heinz Tomato Ketchup Tomato concentrate Distilled vinegar High-fructose corn syrup Corn syrup
Salt Spice Onion powder Natural flavoring
other flavors. As we saw in chapter 9, adaptation to smell occurs in the olfactory cortex, but adaptation to one smell does not affect adaptation to other smells. Similar principles apply to the other senses. This means that we can and will eat much more in response to multiple different smells, tastes, and textures. That property of our brains defines what fast foods deliver, as quickly as possible. The third component consists of the other foods and drinks we consume with the French fries. For many people, the main course would be a cheeseburger. This adds an even bigger wallop of flavor. The burger, of course, is attractive because of both its orthonasal and retronasal meat aroma. These are enhanced by the fact that frying the meat produces the Maillard reaction, caramelizing the surface of the meat patty to release especially attractive volatiles. The soft chewiness of the meat makes a comforting, familiar contrast with the crunch of the fries. The cheese delivers its own volatiles. The bun gives a soft reassurance to our touch system. A quarter-pound cheeseburger delivers 510 calories. Some of us like mustard on our burger, which adds the ingredients shown in box 21.2. Again, more taste, smell, and texture are added to our meal. All this needs to be washed down, usually by a carbonated soft drink. This not only delivers about 10 calories of sugar per ounce (for example, 160 calories for a “medium� 16-ounce [453-gram] can of cola), but also a shower of fizz that stimulates the touch receptors in our mouth and all the way up our retronasal tract to our noses. The sensory overload, combined with the activation of stretch receptors in our stomachs, leaves us satisfied (satiated, as the psychologists say). In the olfactory bulb, fibers from deep in the brain stem turn off the 187
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BOX 21.2 Grey Poupon Dijon Mustard Mustard seed Distilled vinegar Salt
White wine Citric acid Tartaric acid
Pectin Spices
mitral cells so that they can no longer be activated by the smell of what we have been eating. Then it is time to go around the corner to the local coffee shop for an espresso, latte, or cappuccino and maybe a chocolate chip cookie. As we enter, we breathe in the aroma of the coffee by the orthonasal route. As mentioned in chapter 4, coffee contains more than 600 volatile smellproducing types of molecules, all of which flood our olfactory receptors with every retronasal breath as we drink it. A small latte is worth 150 calories; the chocolate chip cookie another 160. The sweet of the cookie and the bitter-sweet combination of the chocolate complement the bitter of the coffee in just the way we have learned to crave. This overload of flavor is accompanied by an overload of calories. When the calories are added up, the meal just described is, at minimum, more than 1,100 calories, half a day’s recommended 2,200, and, at maximum, more than 2,000 calories, almost the recommended minimum for an entire day. The danger to health is obvious. Risks in addition to those for obesity are coming into focus, including those for diabetes and diabetesrelated conditions such as insulin resistance, metabolic syndrome, and even increased risks for cancer. The human brain flavor system therefore not only gives us pleasure; it can be hazardous to our health. It is an underappreciated factor in determining the balance between health and disease. Public recognition of this role is needed, as discussed in chapter 26.
Why We Overeat Knowledge of the brain flavor system can help us understand why we overeat on this menu. First is the sensory overload. The food is high in 188
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sensory stimulation and dense in calorie content. A normal diet has more roughage to make us feel full faster, and drinking water with the meal further dilutes the calories, but fast food has too much flavor for too little fill. In addition, we wash down the food with soft drinks that are packed with more calories. Second, fast food contains a variety of food types and flavors. This is called the supermarket, smorgasbord, or buffet effect. This idea actually originated with a blind French scientist named Jacque Le Magnen in Paris, who became a legend in research on feeding. In the 1950s he began detailed studies of laboratory rats fed different kinds of diets. He found that on daily lab chow they showed little weight gain, but if he offered them chow with different flavors they quickly began to gain weight. This effect was rediscovered in 1981 by Barbara Rolls and her colleagues at Oxford, who called it sensory-specific satiety, meaning that with one flavor the animal quickly becomes full and bored with eating more, whereas a new flavor stimulates renewed eating. This is the effect we all experience at Thanksgiving or buffets or banquets when we feel the urge to go on eating every new dish or course. It is an expression of the fact that the brain is always interested in something new or changing, a characteristic we have seen in all the sensory systems. Although the fast-food industry probably did not know of Le Magnen’s research, it designed its foods as if it did. Another reason people overeat may lie in long-term overstimulation of the skin and membranes of the lips and mouth. These of course are activated by food when we are eating, but, surprisingly, they can also be overactive even when we are not eating. This has been shown by brain scans of activity in the somatosensory area of the cerebral cortex (chapter 13) comparing subjects who are lean and subjects who are obese. As figure 21.1 shows, from the study in 2002 by Gene-Jack Wang and his colleagues at the Brookhaven National Laboratory in New York, obese individuals even in the resting state show higher levels of brain activity in the lip, tongue, and mouth regions of the somatosensory area. The authors speculate that this could reflect hypersensitivity of the receptors there to the rewarding value of food, and could be among the factors associated with overeating. There are several theories for why we overeat, which have been summarized by Dana Small and her colleagues in a review in 2009. 189
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FIGURE 21.1 The human somatosensory system with superimposed intensities of brain activity in subjects at rest Higher levels of activity (shaded areas) in the lip, tongue, and mouth regions are present in obese individuals compared with lean individuals. (Adapted from G.-J. Wang et al., Enhanced resting activity of the oral somatosensory cortex in obese subjects, Neuroreport 13 [2002]: 1151–1155)
One theory is based on the observation that even though a rat may have fed to satiety, it can be induced by conditioned cues to keep eating. Small and her colleagues mention an experiment that demonstrates this. In this experiment, rats learn to associate the presentation of food with the sound of a buzzer, much like Ivan Pavlov’s dogs. If the buzzer sounds when they are sated, they will begin to eat again. Instead of buzzers, humans have many other cues that keep them eating flavorful foods. In our example, a burger cues a bag of potato chips, then the ketchup, then the soft drink, then the . . . This kind of feeding has been shown to be dependent on connections between the amygdala, a node in the emotional network, and the hypothalamus, which is involved in activating feeding. These connections become hypersensitive because of long-standing habits. Another idea is that overeating is due to ineffective inhibitory circuits in the prefrontal brain regions, combined with heightened excitability of 190
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the circuits mediating reward from the foods consumed. An analogy is the involvement of these circuits in drug cravings. This is more evidence that a tendency to overeat involves circuits at the highest cognitive, as well as emotional, levels. Another factor is the possibility that overeating occurs because eating itself does not have adequate reward value; the brain does not register enough “pleasure� with lesser amounts of food. In his book The End of Overeating: Taking Control of the Insatiable American Appetite, David Kessler, my former dean at the Yale School of Medicine, has emphasized the combination of salt, sugar, and fat as the main flavor villains to be resisted and controlled. Neurogastronomy supports this conclusion, identifying retronasal smell and its associated multisensory brain mechanisms of flavor as underappreciated major factors. If flavor plays this central role in what we eat, the brain must contain mechanisms for making decisions about whether a food that produces an attractive internal flavor image in the brain is also nutritious. This is a final critical part of the human brain flavor system for determining normal function in healthy people and abnormal function in people who overeat.
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