Why do we eat what we eat, how much we eat, when we eat, and with whom we eat? Food choice is influenced by many interrelated factors. The key drivers for eating are hunger and satiety, but what we choose to eat is not determined solely by physiological or nutritional needs.1 Other factors that influence our food choice are taste, smell, and appearance of food, likes and dislikes, knowledge and attitudes related to diet and health, habit, social context when eating, cooking skills, beliefs regarding organic diet, and other cultural, religious and economic factors. Eating occurs within a “zone of biological indifference,” in which the individual is neither genuinely hungry nor genuinely sated.2 People confess to eating all kinds of unusual food combinations when eating alone, such as fried Spam and grape jelly, ice cream with sea salt, and mashed potatoes with brownies bites.3 In his book, The End of Overeating: Taking Control of the Insatiable American Appetite, David Kessler describes how the restaurant industry has used the addictive powers of the combination of sugar, fat, and salt to get people hooked on foods they can't resist.4 According to Brian Wansink at Cornell University, “We are a nation of mindless eaters.”5 He has shown that adding food coloring to every tenth Pringle potato chip, to suggest a portion, cuts potato chip intake in half. In another study, he gave free, stale popcorn in either a medium or large sized container to people going for a movie. A majority of the eaters thought the popcorn was “stale,” “soggy,” or “terrible” but the people receiving the large bucket still ate 34% more than the people who got the smaller container.6 Studies have shown that people don't even realize differences in the size of portions served.7
The brain has a lot to do with eating habits. One brain region involved in monitoring nutrition signals is the hypothalamus. If the hypothalmic system senses a need for nutrition, it will send out hormone signals that make a person hungry. However, there are other players in the drive to eat that come from emotional centers and decision-making centers in the brain. This is what separates humans from other animals. Human behaviors aren't as “hard-wired” so they are more likely to eat inappropriately just because they want to, not because they need to.8 One of the first lessons that an infant learns is that breast milk has a slightly sweet taste. The response to sweetness is so instinctual, it is impossible to put aside.9 Higher brain centers that control the desire to eat can trump the influences of the hypothalamus. The dopamine reward system is one such brain center.10 Calorie load can affect the brain dopamine reward system independent of taste. In one study, genetically altered mice without functional sweet taste receptors showed an increase in dopamine with a sugar load, indicating that caloric load (and not the taste of sweetness) was triggering their dopamine reward system.10
Radiologic imaging is becoming an important tool in researching how the brain controls eating. Positron emission tomography (PET) scanning has shown that men, but not women, are able to control their brain's response to their own favorite foods.11 In one such study, subjects were presented with their favorite foods—from bacon-egg-and-cheese sandwiches to pizza, cinnamon buns, barbecued ribs, and chocolate cake—warmed, if appropriate, to enhance the enticing aromas and taste. During the scan, subjects were asked to smell, taste, observe, and react to the food, but not eat it. On another day, they were instructed to inhibit their desire for food before being tempted with the same foods. A control scan with no food was performed on another day. The subjects were asked to rate the foods and describe their feelings of hunger and their desire to eat during the scans when food was presented. In both men and women, brain areas associated with emotional regulation, conditioning, and motivation “lit up,” indicating increased metabolic activity in those regions, in response to the tempting foods when compared with the no-food scans. Only the men showed a relative decrease in activity in the food-activated brain regions during the scan when they were asked to inhibit their response. Another study using PET scanning12 looked at brain activity in volunteers as they ate chocolate to beyond satiety. As an aside, this same study revealed that of 20 kinds of chocolate offered, Lindt bittersweet (50% cocoa) and Lindt milk chocolate were consistently ranked as the most pleasant.
Functional MRI (fMRI) studies have shown that peptide YY (PYY), a naturally occurring hormone that regulates appetite, operates through 2 areas of the brain.13 In a food-deprived state, brain activity within the hypothalamus predicted how much food the subjects ate. However, in the presence of increased PYY levels, mimicking a meal, there was a switch in the circuits controlling eating so that brain activity within the orbitofrontal cortex predicted feeding behavior. It is hoped that studies using fMRI-based physiological approaches may help determine whether pharmacologic or other interventions target appetite-regulating regions.
This issue's Guest Editor, Dr. Rebecca Cornelius, has gathered some of the brightest minds in neuroradiology to write chapters that provide an overview of neuroimaging. One chapter, devoted to fMRI, further discusses the potential of this powerful imaging tool to positively affect patient care and aid understanding of neurophysiology and pathology. As stated by Dr. Shah and her colleagues, “the practice of neuroradiology has gone beyond mere structural and anatomic assessment of the brain to the subtle nuances in the function of the brain.” I'm sure you will enjoy reading their chapter and all the chapters in this issue. I thank Dr. Cornelius and all the authors for their exceptional contributions.
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