Evidence suggests that our dietary habits and the decisions we make about what we eat are acquired over several years, but what psychological processes underpin this kind of learning? And why do preferences vary across different groups of people, such as those who consciously restrict their intake?
Dr. Jeff Brunstrom, currently working in the Department of Experimental Psychology at Bristol University, UK, has extensively studied the links between cognition and dietary control such as the relationship between attention and meal size, our reactions to food’s sight and smell, and the decisions we make about portion size.
“There is every indication that most of our flavor preferences and dietary behaviors are learned,” he said. “Despite this, we know very little about the underlying mechanisms.” He explains that several types of dietary learning have been identified, such as linking flavors with other flavors, and associating flavors with bodily sensations like fullness. Some of these processes may take place outside awareness, he suggests, as a form of automatic learning.
But being aware of our preferences and choices can help us alter learned patterns. Doing so may help in treating eating disorders, according to Brunstrom.
Meal size is a large determinant of energy intake. “After we consume a novel food an association can form between its sensory characteristics (e.g. taste properties) and the effect it has on the body (rewarding),” Brunstrom said. “Associations of this kind explain much of our everyday dietary behavior.” This is because they affect both our flavor preferences and the amount we choose to consume.
Brunstrom also has investigated whether food-related learning is less likely to occur in people who are more restrained with food. He gave 44 women two differently flavored desserts three times on separate days. One was 1,882 kilojoules (a unit of energy that can be translated into dietary calories) and the other226 kilojoules. “We found little evidence for learned satiation,” Brunstrom said. “However, we did observe flavor-preference learning.” The “unrestrained eaters” showed a greater difference in their response to the two desserts than did the nonrestrained eaters.
In another set of experiments, Brunstrom found that distraction can significantly reduce eating satisfaction. Participants ate five Jaffa Cakes (a chocolate-and-orange cakelike cookie popular in the United Kingdom and Ireland) either while distracted by a computer game or while sitting in silence. Distracted participants reported a smaller reduction in fullness than did nondistracted participants.
This suggests that distraction causes a reduced sensitivity to the physiological and sensory cues that signal when to end a meal. A further experiment showed that distracted participants maintain a desire to eat after consuming food, whereas nondistracted people report a reduction in their desire to keep eating. This effect persists at least for a brief period after eating.
But under most circumstances, exposure to the sight and smell of food can trigger a desire to eat it. Experiments were carried out to explore this phenomenon. Exposure was found to increase the amount of food that people actively plan to eat, and increase their actual intake. Interestingly, exposure to one food also increases the chosen portion sizes of other foods.
Brunstrom also found evidence that restrained eaters react less strongly to exposure than do unrestrained eaters. It is therefore possible that heightened ‘cue reactivity’ represents a risk factor for obesity.
Finally, Brunstrom explored the idea that people with a higher body weight choose larger portion sizes. Participants were shown a picture of food portions of 12 commonly consumed foods and asked whether it is larger or smaller than their usual portion. From this, an estimate of everyday portion size was calculated.
Portion sizes were linked to hunger, gender, and level of dietary restraint, as expected. But no link was found with body mass index (BMI). Brunstrom suggests that the difference in total energy intake of individuals with a higher and lower BMI is too small to be detected as a difference in portion size.
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Brunstrom, J.M. (2007). Associative learning and the control of human dietary behavior. Appetite, 49, 268-271
Brunstrom, J. M. & Mitchell, G.L. (2007). Flavor-nutrient learning in restrained and unrestrained eaters. Physiology and Behavior, 90, 1, 133-141.
Brunstrom, J. M. & Mitchell, G.L. (2006). Effects of distraction on the development of satiety. British Journal of Nutrition, 96, 761-769.
Brunstrom, J. M. & Shakeshaft, N. G. Measuring affective (liking) and non-affective (expected satiety) determinants of portion size and food reward. Appetite. 2008 Sep 10.
Ferriday, D. &, Brunstrom, J. M. How does food-cue exposure lead to larger meal sizes? British Journal of Nutrition 2008 May 9:1-8.
Brunstrom, J. M. et al. Estimating everyday portion size using a ‘method of constant stimuli’: in a student sample, portion size is predicted by gender, dietary behaviour, and hunger, but not BMI. Appetite. 2008 Sep;51(2):296-301.
Brunstrom, J. M., Shakeshaft, N. G. & Scott-Samuel, N. E. Measuring ‘expected satiety’ in a range of common foods using a method of constant stimuli. Appetite. 2008 Nov;51(3):604-14.