More Than a Matter of Taste: The Science Behind Food Preferences
You might remember being a little kid and refusing to eat the broccoli that was put on your plate. But now you can’t get enough of the delicious green veggies. Why is it that your food preferences change over time? Why do you love some foods and despise others? What causes one person to love liver while you gag at the thought? The answers might be a little more complicated to explain than simply a difference in taste buds.
This excerpt is from Nourishment by Fred Provenza. It has been adapted for the web.
Let’s look at an example: To survive a rare lung disease, American dancer Claire Sylvia underwent a heart-and-lung transplant in 1988. Her chest was sawed open, her diseased organs removed, and in their place, doctors inserted the heart and lungs of a donor. Following surgery, she was certain that her harrowing journey was over. In reality, it was just beginning. New organs weren’t the only thing she inherited. During her recovery in intensive care, Claire began to feel the presence of someone else inside her. Initially frightened and then captivated, she realized that some of her attitudes and behaviors had changed. In A Change of Heart, Claire describes how she was shocked to discover that her food preferences had changed following her organ transplant. The cells and organs from her donor had a “mind” of their own that was now influencing what she chose to eat. After an extraordinary dream, she began to search for the family of her donor.
Claire’s experience raises intriguing questions. Why do we prefer to eat some foods but not others? What are we feeding? Most people don’t give such questions a bit of thought, and there’s really no need to do so. The wisdom of the body takes care of these matters—without a bit of thought. As a result, people really don’t know why they prefer to eat certain foods or what they are feeding—stomach or maybe intestines—when they eat. Can cells and organ systems influence the food choices of a human or an herbivore? What kinds of learning and memory exist in cells and organ systems, and can cellular memory outlive physical death?
Liking for foods is typically thought to be influenced by palatability. Webster’s dictionary defines palatable as pleasant or acceptable to the taste and hence fit to be eaten or drunk. Animal scientists usually explain palatability, though, as a liking influenced by a food’s flavor (odor and taste) and texture, or the relish an animal shows when eating a food. Plant scientists describe palatability as attributes of plants that alter an herbivore’s preference for consuming them, such as physical and chemical composition and associated plants.
Affective and Cognitive Processes
The body integrates information about foods through affective processes and cognitive processes. Taste plays a prominent role in both processes. Receptors for taste are situated like a Janus head placed at the gateway to the body, with an “affective face” that looks at what’s happening inside and a “cognitive face” that looks at what’s happening outside.
Affective processes integrate taste with post-ingestive feedback. The type of change in liking for and intake of a food item depends on whether feedback is aversive or positive. Food likes and dislikes mediated by sensations such as hunger, satiety, and nausea are how the gut tells the brain of a body’s well-being—gut feelings. The result is incentive modification.
Cognitive processes integrate the odor and visual appearance of a food with its taste. Animals use smell and sight to select foods whose post-ingestive feedback has been positive; they avoid foods that have produced aversive feedback. Cognitive processes are strongly influenced by social models such as mother and other herd mates. The result is behavior modification.
Affective and cognitive processes are mediated by dorsal and ventral vagus nerves from the gut that converge with nerves for taste in the brain stem. From the brain stem, these nerves converge with other nerves as they relay from the brain stem to the limbic system and then to the cortex. The gut sends an enormous amount of information about the well-being of the body to the brain, including sensations that arise from digestive functions as well as stress and illness. Far more neurons ascend from the gut to the brain (afferent neurons) than descend from the brain to the gut (efferent neurons). For every message sent by the brain to the gut, about nine are sent by the gut to the brain. Because of the extensiveness of the enteric nervous system (afferent neurons), the gut has a great deal of sovereignty. As neurogastroenterology expert Michael Gershon notes in The Second Brain, the enteric nervous system has a mind of its own that continually informs the central nervous system about the status of the body. Following Gershon’s discovery of the gut as a complex neural system, researchers have discovered that every neurotransmitter in the brain also operates in the gut. The enteric nervous system has more neurotransmitters and neuromodulators than any other part of the peripheral nervous system.
Discoveries in other fields have helped to deepen the understanding of the results we found in our research, which showed that what makes a food “taste good” to animals is not as simple as was once thought. Bodies are integrated societies of cells and organ systems, including the microbiome, each with nutritional needs. What I call “myself” is a conduit through which the cells and organ systems that make up my body meet their nutritional needs.
I can still recall how my understanding changed the day I realized that palatability and preference are not based entirely on cognitive-rational analytical thought, but also on noncognitive-emotive-synthetic feedback that arises from within a body. The awareness that palatability is unconsciously influenced by cells and organ systems, including the microbiome, stopped me in my tracks. Until then, I’d been trained to think about foraging in a cognitive-rational-analytical sense in terms of the costs and benefits of selecting various foods.
Years later I came to appreciate that the most intriguing part of the tale reveals a dichotomy between two different kinds of rationalities. One has to do with conscious thought and the rational mind, and the other has to do with emotional experiences and the wisdom body. That’s illustrated if you ask a person why they prefer anything from foods to mates to cars. While people can tell you what they prefer, we don’t necessarily understand how their preferences originate. We don’t need to think about that any more than we have to think about which enzymes to release to digest food. The body takes care of that—without a thought.
Learned Preferences in Humans
Like herbivores, people acquire likings through flavor-feedback associationsns, where the flavor of food and positive consequences of nutrient ingestion lead to an acquired liking for the flavor of the food. These associations are influenced by the novelty of food, the amount of nutrients in the food, and the needs of an individual for nutrients in the food.
Unlike with herbivores, few studies have focused on flavor-nutrient learning in humans, and they don’t always show changes in preference or liking after flavor-nutrient pairings.39 Two challenges arise when researchers attempt to study humans. First, accounting for experiences in utero and early in life is virtually impossible, yet those experiences profoundly influence familiarity with flavors and foods and the choices people make. That’s why young children more reliably increase preference in studies of flavor- nutrient learning. Adults show less consistent responses due to their long and unknown (to researchers) histories of experience eating different foods. With children and adults, food novelty decreases preference and intake because people prefer familiar to unfamiliar foods.40 Second, researchers typically have little short- or long-term control over appetitive (nutritional) states in humans, yet different nutritional states strongly influence the outcomes of studies of food selection and satiation.
Most studies of food preferences in humans attempt to create an effect by using energy, but people whose daily diets are already high in energy show only weak preferences for novel-flavored, high-energy foods. They lack familiarity with the novel flavor, and they don’t need more energy. That was evident in a study that compared people in the UK with Samburu people, a native group of seminomadic pastoralists in Kenya who are lean and food stressed.41 In the study, the participants ate nearly a pound (400 g) of either an energy-dense version (1.57 kcal/g) of a novel food or a less energy-dense version (0.72 kcal/g) of the novel food.
The researchers found no evidence of flavor-nutrient learning between the two versions of the novel food. As expected, the fifty-two people from the UK reported feeling greater fullness after eating the high-energy version than after the low-energy version. Their sensitivity to the energy-dense food is a result of their high intake of energy-dense foods. Conversely, the sixty- eight Samburu people didn’t report a different fullness response between the high and the low version of the food, a reflection of their overall poorer nutritional status and the relatively small differences in the energy content of the foods. The Samburu also experienced a return to premeal hunger within an hour after consuming a test food, and they reported having a capacity to consume three times more food than people in the UK. In a food-stressed environment such as the Samburu’s, it makes little sense to reject food even if the food has a low-energy density.
Good examples of appetitive state-dependent learning occur when people experience nutrient de cits in real life conditions.42 People develop cravings for fat when they are stuck eating lean-meat diets; cod liver oil when they are suffering from rickets; fruits when suffering from scurvy; calcium when vitamin D deficient; and salt when they are salt-deprived. When people are deficient in minerals, they exhibit pica, as livestock do, and may crave specific, often unusual “foods” such as soil, a practice referred to as geophagy. Compelling evidence for flavor-nutrient learning in humans also comes from studies of flavor compounds in tomatoes and other fruits. The flavors people find most appealing are related with the essential nutrients— including essential amino acids, carotenoids, and omega-3 fatty acids—a body needs to function. 43 The delicious flavor of a phytochemically rich tomato is directly linked with feedback from the primary and secondary compounds that make tomatoes healthy.
These findings would not have surprised Curt Richter, America’s foremost psychobiologist in the twentieth century.44 As the first empirical scientist in the eld of psychobiology, he influenced every facet of the discipline. Richter and his colleagues deprived animals of nutrients essential to survival or manipulated their hormone levels and showed that these needy states generate appetites and behaviors precisely fitting the animal’s need. Richter’s work was in the tradition of Claude Bernard, who emphasized regulation of the internal milieu, and Walter Cannon, who emphasized homeostasis. While both Bernard and Cannon were concerned with physiological mechanisms that underlie maintenance of the body, Richter emphasized the role of behavior. Early in Richter’s career, one of his colleagues at Johns Hopkins University, noted nutritionist E. V. McCollum, showed that changes in diet invoke compensatory behavioral responses, a demonstration of Cannon’s concept of “the wisdom of the body.” By that time, Clara Davis had already done her studies that suggested children could maintain nutritional balance by self-selecting a diet from a variety of foods. Richter, more than anyone else, went on to show that animals could maintain homeostatic balance when offered selections of energy, protein, minerals, and vitamins, including the B-complex, A, D, and E.
Richter studied a range of behaviors, including ingestion of minerals during pregnancy and lactation. He charted mineral intake during the reproductive cycle and found that a variety of minerals essential in pregnancy and lactation were ingested in large amounts, serving both the mother and the o spring. Richter discovered many other behavioral adaptations, including coprophagia (feces ingestion) in nutritionally deprived rats, a practice that procures required nutrients. Richter also investigated bait shyness, the reluctance of rats to eat unfamiliar foods (food neophobia). He was one of the first to describe learned poison avoidance during his e orts to control rat populations in Baltimore during the Second World War. Years later, learned poison avoidance (conditioned taste aversions) became of central importance in the psychology of learning. As Richter concluded in a talk titled “Total Self- Regulatory Functions in Animals and Human Beings,” “Thus, we believe that the results of our experiments indicate that in human beings and animals the e ort to maintain a constant internal environment or homeostasis constitutes one of the most universal and purposeful of all behavior urges or drives.”45
39. Martin R. Yeomans, “Flavour-Nutrient Learning in Humans: An Elusive Phenomenon?” Physiology and Behavior 106 (2012): 345–55; Per Møller, “Satisfaction, Satiation and Food Behaviour,” Current Opinion in Food Science 3 (2015): 59–64; Barbara V. Andersen and Grethe Hyldig, “Consumers’ View on Determinants to Food Satisfaction: A Qualitative Approach,” Appetite 95 (2015): 9–16; Barbara V. Andersen et al., “Integration of the Sensory Experience and Post-Ingestive Measures for Understanding Food Satisfaction. A Case Study on Sucrose Replacement by Stevia rebaudiana and Addition of Beta Glucan in Fruit Drinks,” Food Quality and Preference 58 (2017): 76–84; E. Boelsma et al., “Measures of Postprandial Wellness a er Single Intake of Two Protein-Carbohydrate Meals,” Appetite 54 (2010): 454–65.
40. L. L. Birch and D. W. Marlin, “I Don’t Like It; I Never Tried It: Effects of Exposure on Two- Year-Old Children’s Food Preferences,” Appetite 4 (1982): 353–60; C. Sanudo et al., “Regional Variation in the Hedonic Evaluation of Lamb Meat from Diverse Production Systems by Consumers in Six European Countries,” Meat Science 75 (2007): 610–21.
41. J. M. Brunstrom et al., “In Search of Flavour-Nutrient Learning. A Study of the Samburu Pastoralists of North-Central Kenya,” Appetite 91 (2015): 415–25.
42. Davis, “Results of the Self-Selection of Diets by Young Children”; Lind, A Treatise on the Scurvy; M. G. Tordo , “The Case for a Calcium Appetite in Humans,” in Calcium in Human Health, eds. C. M. Weaver and R. P. Heaney (Totowa, NJ: Humana Press, 2006), 247–66;
D. A. McCarron et al., “Can Dietary Sodium Intake Be Modi ed by Public Policy?” Clinical Journal of the American Society of Nephrology 4 (2009): 1878–82; E. A. Rose et al., “Pica: Common but Commonly Missed,” Journal of the American Board of Family Practice 13 (2000): 353–58.
43. S. A. Go and H. J. Klee, “Plant Volatile Compounds: Sensory Cues for Health and Nutritional Value?” Science 311 (2006): 815–19.
44. Jay Schulkin et al., “Curt P. Richter 1894–1988,” Biographical Memoirs, National Academy of Sciences 65 (1994): 311–20.
45. Richter, “Total Self-Regulatory Functions in Animals and Human Beings”; S. C. Woods and D. S. Ramsay, “Homeostasis: Beyond Curt Richter,” Appetite 49 (2007): 388–98.
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