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Hunger
Hunger J Pickering and J Halford, University of Liverpool, Liverpool, UK ã 2016 Elsevier Ltd. All rights reserved.
Glossary Hedonic hunger A drive to consume palatable food, which is driven by pleasure and not a need for energy. Homeostatic hunger The drive to restore the body to a state of homeostatic equilibrium. That is, when the body is in energy-deficit (more energy has been expended than energy that has been taken in), the body will respond by creating a strong motivational drive for the individual to seek and consume food.
Definition The term ‘hunger’ is used in many different ways. It can be used figuratively as a motivational driver (e.g., ‘She’s hungry for success’), to refer to cravings for specific foods (e.g., something sweet), and to refer to malnutrition and starvation (i.e., ‘world hunger’). In a scientific sense, hunger has a more precise definition. Here, we refer to hunger as the motivational force that drives people to seek and consume food. There are two forms of hunger: firstly, homeostatic hunger, which is the body’s drive to increase energy intake in response to an energy deficit, in order to restore energy balance or equilibrium (this is an important biological expression of the body’s need for energy to sustain growth and life) and secondly, hedonic hunger, a powerful psychological driver to eat foods known to be pleasurable and rewarding. These expressions of hunger are not always distinct and it is not always possible to identify which type of hunger a person may be experiencing. For example, a person who has not eaten for several hours and whose upper digestive tract is empty may experience not only homeostatic hunger but also hedonic hunger triggered by others nearby eating palatable foods. Hunger can be inferred in two distinct ways within nutritional science. Firstly, it can be inferred through directly observable behavior, such as the willingness to work for food or through observable events such as a long period of food deprivation (this is the method used in animal studies). In this sense, hunger becomes a mediating concept or an intervening variable. Secondly, subjective feelings or sensations of hunger can be measured through rating scales and other measurement devices. The subjective perception of hunger is often accompanied by easily identifiable bodily sensations in the stomach, mouth, throat, and head, for example, increased saliva production, stomach rumbling, and preoccupation with food-related thoughts. It is worth noting at this point that eating food ordinarily decreases the sensation of hunger and increases the sensation of fullness. However, although hunger and fullness are related concepts, they are not opposite dimensions of the same concept. For example, fullness can be increased via gastric
Encyclopedia of Food and Health
Hunger A motivational force to find and consume food, which often (but not necessarily) precedes an eating episode. Satiation The process that develops during eating that generates negative feedback to end the eating episode. This determines meal size (energy, weight, or volume). Satiety A state of noneating, characterized by the absence of hunger, that follows at the end of a meal and that arises from the consequences of food ingestion. This determines the length of time between eating episodes.
distention (e.g., by inflating a balloon in the stomach or through ingesting large quantities of water) but these methods only marginally reduce hunger. This suggests that physical sensations of emptiness within the gastric tract are only one aspect of hunger sensations.
Measurement of Hunger Hunger is an internal subjective state and there is not an objective ‘standard’ by which internal experiences can be calibrated. There are therefore several difficulties in attempting to objectively quantify and measure hunger. Firstly, there is an inherent mistrust of subjective reports of appetite, which are often varied, and therefore, the accuracy of which is often doubted. Another difficulty is that the term ‘hunger’ is not only a scientific concept but also a commonly used colloquial word and people may have their own individual interpretations of what the word refers to. These difficulties are somewhat illustrated by the large amount of between-subject variation in hunger reports.
Measurement Scales The two most common methods for quantifying hunger are fixed-point rating scales and visual analog scales (Figure 1). Fixed-point rating scales are quick and simple to use and provide numeric data, which are easy to analyze. These scales can be affected by the word choice and the number of points chosen. Furthermore, past examples of these scales show that they vary considerably in complexity. In considering the appropriate number of points to be included in this type of scale, the freedom to make a range of possible responses must be balanced against the precision and reliability of the device. Research indicates that scales with an insufficient number of fixed points can be insensitive to subtle changes in subjective experience. In addition, the fixed points affect how people interpret the scales and distribute their ratings. One way of overcoming some of these failings is to remove these points.
http://dx.doi.org/10.1016/B978-0-12-384947-2.00381-0
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(A) How hungry do you currently feel? A little full Neither hungry nor full Very full 2 3 1 (B) How hungry do you currently feel? Peckish Full Indifferent 2 0 1 (C) How hungry do you currently feel? 0 1 2 3 Not at all hungry
A little hungry 4
Hungry 3
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Starving/ravenous 4
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(D) How hungry do you currently feel? Not at all hungry
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Figure 1 Examples of different types of scales used to determine hunger: fixed-point scales with points defined (a and b), fixed-point Likert scale (c), and visual analog scale (d).
Visual analog scales are normally 100 mm horizontal lines, which allows data to be easily converted into percentage scores (they can be longer to allow more freedom of responses). Participants are asked to mark the line at the point that most accurately reflects their current feelings of subjective hunger. The researcher then measures the distance from the negative end of the scale (not hungry) to the participant’s mark, yielding a score of 0–100 (in the case of 100 mm lines). The scales are unbroken and unmarked, which has the advantage of making it difficult for participants to remember exactly where they may have marked the line in previous hunger questions or assessments. This helps to reduce a cross-checking response bias, whereby participants amend some question answers to be consistent with previously answered questions, which can overly inflate the reliability of some scales. Scales can be computerized or presented via pen and paper. Computerized scales remove human error in recording the data and can ensure that scales are completed at precise time points in the day. By removing verbal labels (except the end anchors), visual analog scales retain the advantages of fixedpoint rating scales while avoiding many of the problems with uneven response distributions and are therefore often the preferred scale in many experiments. It is important to interpret these scales’ results properly. For example, it cannot be assumed that the difference between 20 and 30 mm on a visual analog scale is perceptually the same as the difference between 80 and 90 mm. Nor can a hunger rating of 80 mm be said to represent a feeling of hunger that is twice the intensity of one rated as 40 mm. The variation of scores on these scales represents fluctuations in hunger perception and not absolute change of some physical commodity. Individuals will use these scales idiosyncratically (each individual may interpret the scale and gauge their response differently). For example, a minority of subjects are reluctant to make ratings away from the upper or the lower end points of the scale, despite clear instructions. This leads to the problem of ‘end effects.’ However, participants are generally consistent on repeated occasions (participants will use their own parameters, producing a reliable record of alterations in an individual’s
appetite). Despite their limitations, data from such scales are often analyzed using parametric statistics, such as analysis of variance, and in general, this appears to be satisfactory.
Hunger and Eating Behavior The subjective experience of hunger is a biologically useful indicator of a depleted nutritional state and often precedes an eating episode. However, hunger and eating behaviors, although closely linked under many circumstances, are not always entirely dependent upon each other. Hunger is neither a necessary nor sufficient condition for eating to occur. There are many examples of people who are hungry but do not eat, for example, those who fast for political or moral reasons, those who deliberately restrict their energy intake for weight loss, or even those who do not want to spoil their appetite before a planned evening meal. Hunger is therefore not a sufficient condition to cause food intake, even when food is readily available. People can, and regularly do, resist the drive to consume. Hunger is also not a necessary condition to cause food intake. Often, people will overeat at special occasions such as weddings and Christmas, will eat a desert despite having just consumed a filling meal, and finish a large meal in the spirit of economic prudence. Laboratory-based research notes that increasing the palatability and variety of food can encourage eating beyond the limits of normal satiety. It takes longer to achieve sensoryspecific satiety (where one becomes bored of a particular flavor) with highly palatable and energy-dense foods. It is also easy to overconsume energy-dense foods because they can be eaten more quickly. As satiety signals are not instantaneous, eating quickly makes it more likely that a person will overconsume. Therefore, it is misplaced to criticize the validity of hunger ratings due to a discrepancy with recorded food intake, as the previous examples show that in certain circumstances, the two can be disengaged. Proximity to food cues can have dramatic, instantaneous effects on appetite expression. For some individuals,
Hunger
responsiveness to food cues overwhelms normal appetite control and the normal rhythm of hunger. For others, eating in the ‘absence of hunger’ is a component of their natural eating repertoire. It is clear that hunger ratings cannot be used simply as a proxy measure for food intake. Equally, when such factors are taken into account, there is good evidence that self-report ratings of hunger correlate statistically and meaningfully with eating. By questioning the relationship between hunger and eating, we are also forced to place the action of hunger within a broader context of social and psychological variables that moderate food choice and eating behavior. Eating patterns and food intake are affected by habits, attitudes, and opinions about the value and suitability of foods and an overall liking for them. Food can be avoided, even when an individual is hungry, for health, moral, or political reasons: for example, reducing sugar intake to protect against diabetes, eating a vegetarian diet in support of animal rights, and avoiding palm oil to reduce the impact of deforestation. These factors, derived from the cultural ethos, largely determine the range of foods that people consume and sometimes the timing of consumption. In addition, recent research suggests that social modeling and mimicry can have an important impact upon food intake. Laboratory research suggests that a participant’s ad libitum food intake can be manipulated by how much a stooge eats and that even food-deprived participants will eat less if a stooge only eats a little. Fluxes in hunger are therefore only one aspect of the range of eating determinants.
Hunger, Satiation, and Satiety Hunger is a motivational driver to seek and consume food. When a meal is initiated, the process of satiation will begin, which generates negative feedback to end the eating episode. Hunger declines as satiation develops and usually reaches its lowest point at the end of a meal. Satiation therefore determines meal size (energy, weight, or volume). Satiation can be measured via eating rate within a meal. Satiety is defined as a state of noneating, characterized by the absence of hunger, which follows at the end of a meal and arises from the consequences of food ingestion. This determines the length of time between eating episodes. The intensity of satiety can be measured by the duration of time until the next eating episode, by the amount consumed at the next meal, or by how long hunger is suppressed. Satiety does not refer to a single event or process, but rather an interconnected series of sensory, cognitive, postingestive, and postabsorptive signals. By definition, satiety is not an instantaneous event but occurs over a considerable time period. In the very short term, hunger is inhibited by sensory effects generated through the smell, taste, temperature, and texture of food. Hunger is inhibited for a little longer by cognitive beliefs about the properties of foods and appropriate portion sizes. Hunger is increasingly suppressed by postingestive processes such as gastric distension, rate of gastric emptying, and the release of hormones such as cholecystokinin and peptide YY. These factors will inhibit the rebound of hunger during the early postprandial period. In the postabsorptive phase of satiety, metabolites are absorbed from the
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intestine into the bloodstream. These signals will continue to suppress hunger late into the postprandial period. Hunger and satiety are affected by the nutritional composition and structure of foods; therefore, some foods have a greater capacity to maintain suppression over hunger than others. This will depend on the food’s sensory impact, physical characteristics, energy density, and macronutrient composition. The satiating potential of foods is often tested via a preload design, whereby isocaloric ‘preloads’ (usually soup or milk shakes) are consumed before an ad libitum meal and their effects on intake are assessed. Research suggests that per unit of energy proteins are the most satiating macronutrient, then carbohydrate, and then fat. The satiating effect of protein may depend on the preabsorptive factors of elevated cholecystokinin and glucagon-like peptide-1 release and the postabsorption of circulating amino acids. The satiating effects of protein are prevalent in short-term studies in both lean and obese participants. Emerging evidence suggests that high-protein diets can have medium-term effects on satiety. Current research is investigating the long-term satiating effects of high-protein diets. Fats are the most energy-dense of the macronutrients at 9.5 kcal per gram and high-fat diets (in comparison with low-fat and high-carbohydrate diets) have a disproportionately weak action on satiety. Dietary fiber is highly satiating for several reasons: It takes longer to chew (sensory satiety), soluble fiber binds to water and swells causing bulk (mechanical satiety), it slows down gastric emptying and remains in the stomach longer (mechanical satiety), and soluble fiber prolongs the intestinal phase of nutrient digestion and absorption (thereby prolonging the postabsorptive satiety signal). Fiber-rich foods can therefore suppress hunger for longer. Hunger is also reduced more by solid foodstuffs than energy-loaded drinks. Sugar-sweetened beverages provide energy without many of the cues for satiety (such as mastication, bulk, and cognitive beliefs about the drink being filling).
Hunger: Physiological Determinants Hunger often begins with signals from the environment, such as the sight and smell of food and learned contextual cues such as location and time of day. Environmental cues of highly palatable foods can instigate hedonic hunger, which is a drive to consume for pleasure and not energy. Hedonic hunger is underscored by the endogenous opioid system, such as betaendorphins released in the hypothalamus. Research suggests that opioid antagonists, such as naltrexone, reduce food intake by lowering the perceived pleasantness of foods. It should therefore be noted that the biological mechanisms critical to the expression of hunger are not independent of psychological ones. Indeed, the sensory and cognitive cues that stimulate hunger produce physiological changes that anticipate the ingestion and metabolism of energy and subsequently aid these processes. This gives rise to the psychological factors critical in the expression of appetite. The ability for external stimuli to incite an anticipatory hunger is known as the cephalic response, which prepares the body for digestion. The cephalic response is expressed in several parts of the gastrointestinal tract and acts to anticipate food ingestion and optimize nutrient absorption. Cephalic
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responses include increased release of saliva, stomach acid, and digestive enzymes. Upon food intake, stomach distension and the detection of macronutrients within the gut act as powerful satiety cues, bringing a meal to an end and temporarily inhibiting further consumption. Eventually, hunger again prevails and food intake follows. The flux between hunger and satiety is episodic and underpins the expression of our eating behavior throughout the day. In addition to episodic hunger signals, tonic signals such as a reduction in blood glucose levels (glucoprivation or hypoglycemia) or low levels of the hormone leptin (produced by the adipose tissue) indicate a deficit in available energy and stored energy reserves and therefore promote feelings of hunger. Other tonic factors that indicate the body’s energy status, such as adiponectin, cytokines, and gonadal hormones, appear to act on energy regulator centers within the brain, particularly the hypothalamus, mainly to suppress hunger. Again, their absence serves to promote appetite. However, not all physiological signals, episodic or tonic, inhibit hunger. The peptide hormone ghrelin is a potent stimulator of food intake. It is released by the stomach and the brain and is detected by receptors in the upper intestine, the mesolimbic system, and various orexigenic circuits in the hypothalamus. Blood levels of ghrelin increase with fasting, reduce after meals, and are correlated with self-reported hunger levels. Further studies demonstrate that ghrelin infusions increase food intake. It may be due to this hormone’s role in appetite that most people have a three meal per day macrostructure of food intake, as an individual’s preprandial ghrelin levels tend to be similar before breakfast, lunch, and dinner, suggesting that this hormone is a major instigator of meals. The rare disorder Prader–Willi syndrome is a genetically determined conditioned characterized by chronically high ghrelin levels and insatiable hunger. Most people with the disorder are obese and their excessive levels of food intake are associated with both a delayed reduction in hunger while eating and a more rapid return to premeal states when eating has finished. However, mice with a knockout of the ghrelin gene eat almost normally, suggesting that appetite is too important to entrust to only one mechanism. Other hunger signals from the digestive tract include the lack of satiety hormones (such as cholecystokinin, peptide YY, amylin, and glucagon-like peptide-1). Finally, in further support of the assertion that the body’s energy regulatory system is far too important to be entrusted to any singular mechanism, many neurotransmitters and neuropeptides have been identified that stimulate appetite, including neuropeptide Y, agouti-related peptide, melanin-concentrating hormone, orexin A, melanocortinconcentrating hormone, glucocorticoids, endocannabinoids, and galanin. It should be noted that the hypothalamus is a key location for the release of such appetite-promoting neuropeptides (particularly the lateral hypothalamus and the arcuate nucleus of the hypothalamus) and the ventrolateral medulla is important for receiving hunger signals transmitted to the brain via the vagus nerve from the rest of the body.
Hunger: Cognitive and Affective Consequences It is a common observation among the lay public that hunger affects concentration, attention, and memory. Recent research
supports these observations. Systematic reviews indicate that children who eat breakfast have superior cognitive function and academic performance in comparison with those who do not eat breakfast. Such research has led to health and wellbeing initiatives introducing breakfasts in schools. Furthermore, bodily sensations of hunger often accompany diffuse feelings of restlessness and excitability. The consumption of food changes both the pattern of physical sensations and the accompanying emotional feelings. Classic drive reduction theory suggests that hunger is associated with unpleasant feelings, and food consumption eases these. Thus, for example, after eating, an aching stomach becomes relaxed and feelings of excitement and irritability become contented. People frequently make judgments regarding their own hunger state using these sensations. However, there is much within- and between-subject variability. In other words, not every person sitting down to eat nor every act of eating by an individual will necessarily be accompanied by bodily sensations of hunger or aversive emotional states.
Conditioned Hunger As the classical work of Pavlov demonstrates, hunger responses can be easily entrained to specific stimuli. Omnivores, like carnivores, are meal eaters (in contrast to grazers); however, their dietary decisions are more complex. One of the essentials for an omnivore faced with a variety of new and different foods is the capacity to learn. It is not possible for an innate preference or aversion to guide the choice of every possible food. Therefore, we learn which foods are beneficial (and which are not) through experience. Emerging research suggests that some food preferences can be acquired prenatally and during lactation. This is because flavonoids from the pregnant woman’s food are passed to the fetus via the amniotic fluid and to infants via breast milk. Babies are often averse to bitter-tasting vegetables (which is an evolutionary advantage, as many bitter alkaloid plants are poisonous to humans). However, babies who are exposed to high levels of bitter vegetables in the womb are more likely to find vegetable juices pleasant in comparison with babies who were exposed to lower levels. This emphasizes how exposure to different flavors is essential to developing our taste preferences. This principle extends throughout our lifetimes. Through our experiences, we learn that ingesting food creates pleasant postingestive sensations of satiety that serve as a positive experience, leading to learned preferences, as studied by Birch. Alternatively, if consumption leads to an association between a food and negative gastrointestinal consequences such as nausea, then a conditioned aversion to that food (or even other similar tastes and flavors) is likely to develop. The learning process involves the association between the sensory and the postabsorptive characteristics of foods. In this way, the sensory characteristics of foods act as cues and come to predict the impact that foods will later have. Consequently, these cues should suppress hunger according to their relationship with subsequent physiological events. The potency of taste aversions following vomiting or illness indicates the strength of cue–consequence learning. Food intake depends upon the stability and reliability of the
Hunger
relationship between the taste (sensory cues) and physiological effect (metabolic consequences) of food. It is possible to demonstrate experimentally how human beings adapt their eating to a food’s energy content. A distinctively flavored food that contains ‘extra’ hidden energy, presented on several occasions, will result in a change in eating and in preference. When deprived of food, the subjects’ preference for the taste increases with gained experience. If presented when satiated, preference for the taste decreases. This process is also observable in young children, who eat smaller meals following a taste previously associated with a high-energy snack and larger meals following a taste previously associated with a low-energy snack. Our hunger can thereby be potently reduced by the learned associations of satiety. This relates to the cognitive factors discussed earlier regarding a person’s beliefs about foods and their satiating potential. In environments where food sources remain limited and dietary variety is controlled, learned responses may effectively control subjective feelings of appetite. However, in wealthy countries where there is an abundance of food, there is distortion, variation, and extreme complexity in the relationship between sensory characteristics and nutritional properties. The conditioned control of hunger is therefore weakened or lost in such rich environments. In such a scenario, hunger may become less controllable. The variety of foods available has many different sensory characteristics. Furthermore, some ingredients preserve sensory qualities while altering the nutritive value (such as diet products that substitute sugar for sweeteners or that remove certain ingredients and replace them with artificial bulking agents). Learned hunger therefore is a relatively less important factor when the food supply contains many food items with identical tastes but differing metabolic properties. In addition to learning associations between the sensory properties of foods and their satiating potential, people also learn association between food cues and future food intake. Food cues are plentiful and researchers have found that food intake increases after television viewing when food adverts are shown and also after playing advert games (online games often designed and produced by food manufacturers, where food products are highly prominent). Some individuals are more receptive to the effects of food cues than others and demonstrate an attentional bias toward them (their attention is more easily ‘grabbed’ and maintained by food-related stimuli). Reactivity to food cues emphasizes the importance of conditioned hunger and how associating cues with the naturally rewarding properties of energy intake serves to promote hunger and eating behaviors. The naturally rewarding properties of food consist of two distinct psychological components, wanting and liking, that are controlled by separate neurobiological systems. Wanting is primarily mediated by dopamine, whereas liking is mediated by opioids in hedonic hot spots. Some research has noted that obese individuals are more sensitive to the rewarding properties of food than lean individuals, promoting greater hunger. While other research suggests that obese individuals have greater hunger to compensate for reward deficiency due to reduced dopamine function; more research is needed to resolve this debate.
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Disorders of Hunger The clinical eating disorders anorexia nervosa and bulimia nervosa are commonly believed to encompass major disturbances of hunger. Yet the role that hunger may play is not entirely clear. Contrary to the literal meaning of the term, ‘anorexia’ is not experienced as a loss of appetite. Rather, clinicians recognize that anorexics may endure intense periods of hunger during their self-restricted eating. For some, their strength in resisting intense episodes of hunger provides a feeling of self-mastery and control that is absent in other areas of their lives. Research suggests that restricting anorexics (compared with those who binge) have the greatest blunting of hunger responses and that this disturbance in hunger is not a product of other areas of perceptual confusion. There is evidence that under conditions of total starvation, hunger may become temporarily diminished. This circumstance is extremely rare and obviously relatively brief. Once eating is recommenced, hunger returns rapidly and with extreme intensity. The accounts of the male volunteers who submitted to a 6-month period of semistarvation during World War II (the ‘Minnesota Starvation Experiment’) are a testament to the extreme power of hunger. Referred to as semistarvation neurosis, the behavior of these men was shaped by their need for food, and their hunger experience was extreme. Nearly two-thirds reported feeling hungry all the time and a similar proportion experienced physical discomfort due to hunger. Participants described a marked increase in what was referred to as ‘hunger pain.’ For some, this was mildly discomforting and vaguely localized in the abdomen. For others, it was extremely painful. This account is especially useful in reminding us why energy-reduced diets aimed at achieving weight loss are often difficult to maintain and easy to abandon. Bulimia literally means ‘ox hunger’; however, the term is imprecise. Close analysis of the precursors of binge episodes shows hunger to be lower than it is before a typical meal. In addition, although the urge to eat may be strong during a binge, the large amount of food consumed implies some defect in satiation rather than in hunger. Moreover, binging is often a well-practiced behavior that develops and changes with time. Similarly, binge-eating disorder (binging without the purging behaviors that are present in bulimia) is characterized by binging even in the absence of hunger and continuing until uncomfortably full. For all three of these disorders, it is likely that a stable eating pattern is necessary in order to normalize the experience of hunger, a process that may take a long time to establish. The question of whether obesity reflects a disorder of hunger is now regarded as largely redundant. Obesity is strictly a disorder of energy balance and partitioning. There is limited evidence that heightened hunger contributes to excessive energy input.
See also: Appetite Control in Humans: A Psychobiological Approach; Eating Disorders; Energy: Intake and Energy Requirements; Food Environment; Gut Hormones; Obesity Management; Obesity: The Role of Diet; Pituitary Gland: Pituitary Hormones; Satiety; Snack Foods: Role in Diet.
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Further Reading Birch LL (1999) Development of food preferences. Annual Review of Nutrition 19(1): 41–62. Blundell JE (1979) Hunger, appetite and satiety-constructs in search of identities. In: Turner M (ed.) Nutrition and lifestyles, pp. 21–42. London: Applied Sciences Publishers. Blundell JE, De Graaf K, Finlayson G, et al. (2009) Measuring food intake, hunger, satiety and satiation in the laboratory. In: Graves K (ed.) Handbook of methods for eating behaviors and weight-related problems: measures, theory and research, pp. 283–326. London: Sage Publications Ltd. Flint A, Raben A, Blundell JE, and Astrup A (2000) Reproducibility, power and validity of visual analogue scales in assessment of appetite sensations in single test meal studies. International Journal of Obesity and Related Metabolic Disorders: Journal of the International Association for the Study of Obesity 24(1): 38–48. Halford JC and Harrold JA (2008) Neuropharmacology of human appetite expression. Developmental Disabilities Research Reviews 14(2): 158–164. Halmi KA and Sunday SR (1991) Temporal patterns of hunger and fullness ratings and related cognitions in anorexia and bulimia. Appetite 16(3): 219–237.
Hill AJ, Rogers PJ, and Blundell JE (1995) Techniques for the experimental measurement of human eating behaviour and food intake: a practical guide. International Journal of Obesity and Related Metabolic Disorders: Journal of the International Association for the Study of Obesity 19(6): 361–375. Keys A, Brozˇek J, Henschel A, Mickelsen O, and Taylor HL (1950) The biology of human starvation. Minneapolis: University of Minnesota Press. Kirkmeyer SV and Mattes RD (2000) Effects of food attributes on hunger and food intake. International Journal of Obesity and Related Metabolic Disorders: Journal of the International Association for the Study of Obesity 24(9): 1167–1175. Kissileff HR (1984) Satiating efficiency and a strategy for conducting food loading experiments. Neuroscience and Biobehavioral Reviews 8(1): 129–135. Lowe MR, Butryn ML, Didie ER, et al. (2009) The power of food scale. A new measure of the psychological influence of the food environment. Appetite 53(1): 114–118. Ogden J (2011) The psychology of eating: from healthy to disordered behavior. Oxford: Wiley. Rogers PJ (1999) Eating habits and appetite control: a psychobiological perspective. Proceedings of the Nutrition Society 58(01): 59–67. Womble LG, Wadden TA, Chandler JM, and Martin AR (2003) Agreement between weekly vs. daily assessment of appetite. Appetite 40(2): 131–135.
Glossary Hedonic hunger A drive to consume palatable food, which is driven by pleasure and not a need for energy. Homeostatic hunger The drive to restore the body to a state of homeostatic equilibrium. That is, when the body is in energy-deficit (more energy has been expended than energy that has been taken in), the body will respond by creating a strong motivational drive for the individual to seek and consume food.
Definition The term ‘hunger’ is used in many different ways. It can be used figuratively as a motivational driver (e.g., ‘She’s hungry for success’), to refer to cravings for specific foods (e.g., something sweet), and to refer to malnutrition and starvation (i.e., ‘world hunger’). In a scientific sense, hunger has a more precise definition. Here, we refer to hunger as the motivational force that drives people to seek and consume food. There are two forms of hunger: firstly, homeostatic hunger, which is the body’s drive to increase energy intake in response to an energy deficit, in order to restore energy balance or equilibrium (this is an important biological expression of the body’s need for energy to sustain growth and life) and secondly, hedonic hunger, a powerful psychological driver to eat foods known to be pleasurable and rewarding. These expressions of hunger are not always distinct and it is not always possible to identify which type of hunger a person may be experiencing. For example, a person who has not eaten for several hours and whose upper digestive tract is empty may experience not only homeostatic hunger but also hedonic hunger triggered by others nearby eating palatable foods. Hunger can be inferred in two distinct ways within nutritional science. Firstly, it can be inferred through directly observable behavior, such as the willingness to work for food or through observable events such as a long period of food deprivation (this is the method used in animal studies). In this sense, hunger becomes a mediating concept or an intervening variable. Secondly, subjective feelings or sensations of hunger can be measured through rating scales and other measurement devices. The subjective perception of hunger is often accompanied by easily identifiable bodily sensations in the stomach, mouth, throat, and head, for example, increased saliva production, stomach rumbling, and preoccupation with food-related thoughts. It is worth noting at this point that eating food ordinarily decreases the sensation of hunger and increases the sensation of fullness. However, although hunger and fullness are related concepts, they are not opposite dimensions of the same concept. For example, fullness can be increased via gastric
Encyclopedia of Food and Health
Hunger A motivational force to find and consume food, which often (but not necessarily) precedes an eating episode. Satiation The process that develops during eating that generates negative feedback to end the eating episode. This determines meal size (energy, weight, or volume). Satiety A state of noneating, characterized by the absence of hunger, that follows at the end of a meal and that arises from the consequences of food ingestion. This determines the length of time between eating episodes.
distention (e.g., by inflating a balloon in the stomach or through ingesting large quantities of water) but these methods only marginally reduce hunger. This suggests that physical sensations of emptiness within the gastric tract are only one aspect of hunger sensations.
Measurement of Hunger Hunger is an internal subjective state and there is not an objective ‘standard’ by which internal experiences can be calibrated. There are therefore several difficulties in attempting to objectively quantify and measure hunger. Firstly, there is an inherent mistrust of subjective reports of appetite, which are often varied, and therefore, the accuracy of which is often doubted. Another difficulty is that the term ‘hunger’ is not only a scientific concept but also a commonly used colloquial word and people may have their own individual interpretations of what the word refers to. These difficulties are somewhat illustrated by the large amount of between-subject variation in hunger reports.
Measurement Scales The two most common methods for quantifying hunger are fixed-point rating scales and visual analog scales (Figure 1). Fixed-point rating scales are quick and simple to use and provide numeric data, which are easy to analyze. These scales can be affected by the word choice and the number of points chosen. Furthermore, past examples of these scales show that they vary considerably in complexity. In considering the appropriate number of points to be included in this type of scale, the freedom to make a range of possible responses must be balanced against the precision and reliability of the device. Research indicates that scales with an insufficient number of fixed points can be insensitive to subtle changes in subjective experience. In addition, the fixed points affect how people interpret the scales and distribute their ratings. One way of overcoming some of these failings is to remove these points.
http://dx.doi.org/10.1016/B978-0-12-384947-2.00381-0
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(A) How hungry do you currently feel? A little full Neither hungry nor full Very full 2 3 1 (B) How hungry do you currently feel? Peckish Full Indifferent 2 0 1 (C) How hungry do you currently feel? 0 1 2 3 Not at all hungry
A little hungry 4
Hungry 3
4
5
Very hungry 5
Starving/ravenous 4
6
7 Very hungry
(D) How hungry do you currently feel? Not at all hungry
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Figure 1 Examples of different types of scales used to determine hunger: fixed-point scales with points defined (a and b), fixed-point Likert scale (c), and visual analog scale (d).
Visual analog scales are normally 100 mm horizontal lines, which allows data to be easily converted into percentage scores (they can be longer to allow more freedom of responses). Participants are asked to mark the line at the point that most accurately reflects their current feelings of subjective hunger. The researcher then measures the distance from the negative end of the scale (not hungry) to the participant’s mark, yielding a score of 0–100 (in the case of 100 mm lines). The scales are unbroken and unmarked, which has the advantage of making it difficult for participants to remember exactly where they may have marked the line in previous hunger questions or assessments. This helps to reduce a cross-checking response bias, whereby participants amend some question answers to be consistent with previously answered questions, which can overly inflate the reliability of some scales. Scales can be computerized or presented via pen and paper. Computerized scales remove human error in recording the data and can ensure that scales are completed at precise time points in the day. By removing verbal labels (except the end anchors), visual analog scales retain the advantages of fixedpoint rating scales while avoiding many of the problems with uneven response distributions and are therefore often the preferred scale in many experiments. It is important to interpret these scales’ results properly. For example, it cannot be assumed that the difference between 20 and 30 mm on a visual analog scale is perceptually the same as the difference between 80 and 90 mm. Nor can a hunger rating of 80 mm be said to represent a feeling of hunger that is twice the intensity of one rated as 40 mm. The variation of scores on these scales represents fluctuations in hunger perception and not absolute change of some physical commodity. Individuals will use these scales idiosyncratically (each individual may interpret the scale and gauge their response differently). For example, a minority of subjects are reluctant to make ratings away from the upper or the lower end points of the scale, despite clear instructions. This leads to the problem of ‘end effects.’ However, participants are generally consistent on repeated occasions (participants will use their own parameters, producing a reliable record of alterations in an individual’s
appetite). Despite their limitations, data from such scales are often analyzed using parametric statistics, such as analysis of variance, and in general, this appears to be satisfactory.
Hunger and Eating Behavior The subjective experience of hunger is a biologically useful indicator of a depleted nutritional state and often precedes an eating episode. However, hunger and eating behaviors, although closely linked under many circumstances, are not always entirely dependent upon each other. Hunger is neither a necessary nor sufficient condition for eating to occur. There are many examples of people who are hungry but do not eat, for example, those who fast for political or moral reasons, those who deliberately restrict their energy intake for weight loss, or even those who do not want to spoil their appetite before a planned evening meal. Hunger is therefore not a sufficient condition to cause food intake, even when food is readily available. People can, and regularly do, resist the drive to consume. Hunger is also not a necessary condition to cause food intake. Often, people will overeat at special occasions such as weddings and Christmas, will eat a desert despite having just consumed a filling meal, and finish a large meal in the spirit of economic prudence. Laboratory-based research notes that increasing the palatability and variety of food can encourage eating beyond the limits of normal satiety. It takes longer to achieve sensoryspecific satiety (where one becomes bored of a particular flavor) with highly palatable and energy-dense foods. It is also easy to overconsume energy-dense foods because they can be eaten more quickly. As satiety signals are not instantaneous, eating quickly makes it more likely that a person will overconsume. Therefore, it is misplaced to criticize the validity of hunger ratings due to a discrepancy with recorded food intake, as the previous examples show that in certain circumstances, the two can be disengaged. Proximity to food cues can have dramatic, instantaneous effects on appetite expression. For some individuals,
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responsiveness to food cues overwhelms normal appetite control and the normal rhythm of hunger. For others, eating in the ‘absence of hunger’ is a component of their natural eating repertoire. It is clear that hunger ratings cannot be used simply as a proxy measure for food intake. Equally, when such factors are taken into account, there is good evidence that self-report ratings of hunger correlate statistically and meaningfully with eating. By questioning the relationship between hunger and eating, we are also forced to place the action of hunger within a broader context of social and psychological variables that moderate food choice and eating behavior. Eating patterns and food intake are affected by habits, attitudes, and opinions about the value and suitability of foods and an overall liking for them. Food can be avoided, even when an individual is hungry, for health, moral, or political reasons: for example, reducing sugar intake to protect against diabetes, eating a vegetarian diet in support of animal rights, and avoiding palm oil to reduce the impact of deforestation. These factors, derived from the cultural ethos, largely determine the range of foods that people consume and sometimes the timing of consumption. In addition, recent research suggests that social modeling and mimicry can have an important impact upon food intake. Laboratory research suggests that a participant’s ad libitum food intake can be manipulated by how much a stooge eats and that even food-deprived participants will eat less if a stooge only eats a little. Fluxes in hunger are therefore only one aspect of the range of eating determinants.
Hunger, Satiation, and Satiety Hunger is a motivational driver to seek and consume food. When a meal is initiated, the process of satiation will begin, which generates negative feedback to end the eating episode. Hunger declines as satiation develops and usually reaches its lowest point at the end of a meal. Satiation therefore determines meal size (energy, weight, or volume). Satiation can be measured via eating rate within a meal. Satiety is defined as a state of noneating, characterized by the absence of hunger, which follows at the end of a meal and arises from the consequences of food ingestion. This determines the length of time between eating episodes. The intensity of satiety can be measured by the duration of time until the next eating episode, by the amount consumed at the next meal, or by how long hunger is suppressed. Satiety does not refer to a single event or process, but rather an interconnected series of sensory, cognitive, postingestive, and postabsorptive signals. By definition, satiety is not an instantaneous event but occurs over a considerable time period. In the very short term, hunger is inhibited by sensory effects generated through the smell, taste, temperature, and texture of food. Hunger is inhibited for a little longer by cognitive beliefs about the properties of foods and appropriate portion sizes. Hunger is increasingly suppressed by postingestive processes such as gastric distension, rate of gastric emptying, and the release of hormones such as cholecystokinin and peptide YY. These factors will inhibit the rebound of hunger during the early postprandial period. In the postabsorptive phase of satiety, metabolites are absorbed from the
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intestine into the bloodstream. These signals will continue to suppress hunger late into the postprandial period. Hunger and satiety are affected by the nutritional composition and structure of foods; therefore, some foods have a greater capacity to maintain suppression over hunger than others. This will depend on the food’s sensory impact, physical characteristics, energy density, and macronutrient composition. The satiating potential of foods is often tested via a preload design, whereby isocaloric ‘preloads’ (usually soup or milk shakes) are consumed before an ad libitum meal and their effects on intake are assessed. Research suggests that per unit of energy proteins are the most satiating macronutrient, then carbohydrate, and then fat. The satiating effect of protein may depend on the preabsorptive factors of elevated cholecystokinin and glucagon-like peptide-1 release and the postabsorption of circulating amino acids. The satiating effects of protein are prevalent in short-term studies in both lean and obese participants. Emerging evidence suggests that high-protein diets can have medium-term effects on satiety. Current research is investigating the long-term satiating effects of high-protein diets. Fats are the most energy-dense of the macronutrients at 9.5 kcal per gram and high-fat diets (in comparison with low-fat and high-carbohydrate diets) have a disproportionately weak action on satiety. Dietary fiber is highly satiating for several reasons: It takes longer to chew (sensory satiety), soluble fiber binds to water and swells causing bulk (mechanical satiety), it slows down gastric emptying and remains in the stomach longer (mechanical satiety), and soluble fiber prolongs the intestinal phase of nutrient digestion and absorption (thereby prolonging the postabsorptive satiety signal). Fiber-rich foods can therefore suppress hunger for longer. Hunger is also reduced more by solid foodstuffs than energy-loaded drinks. Sugar-sweetened beverages provide energy without many of the cues for satiety (such as mastication, bulk, and cognitive beliefs about the drink being filling).
Hunger: Physiological Determinants Hunger often begins with signals from the environment, such as the sight and smell of food and learned contextual cues such as location and time of day. Environmental cues of highly palatable foods can instigate hedonic hunger, which is a drive to consume for pleasure and not energy. Hedonic hunger is underscored by the endogenous opioid system, such as betaendorphins released in the hypothalamus. Research suggests that opioid antagonists, such as naltrexone, reduce food intake by lowering the perceived pleasantness of foods. It should therefore be noted that the biological mechanisms critical to the expression of hunger are not independent of psychological ones. Indeed, the sensory and cognitive cues that stimulate hunger produce physiological changes that anticipate the ingestion and metabolism of energy and subsequently aid these processes. This gives rise to the psychological factors critical in the expression of appetite. The ability for external stimuli to incite an anticipatory hunger is known as the cephalic response, which prepares the body for digestion. The cephalic response is expressed in several parts of the gastrointestinal tract and acts to anticipate food ingestion and optimize nutrient absorption. Cephalic
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responses include increased release of saliva, stomach acid, and digestive enzymes. Upon food intake, stomach distension and the detection of macronutrients within the gut act as powerful satiety cues, bringing a meal to an end and temporarily inhibiting further consumption. Eventually, hunger again prevails and food intake follows. The flux between hunger and satiety is episodic and underpins the expression of our eating behavior throughout the day. In addition to episodic hunger signals, tonic signals such as a reduction in blood glucose levels (glucoprivation or hypoglycemia) or low levels of the hormone leptin (produced by the adipose tissue) indicate a deficit in available energy and stored energy reserves and therefore promote feelings of hunger. Other tonic factors that indicate the body’s energy status, such as adiponectin, cytokines, and gonadal hormones, appear to act on energy regulator centers within the brain, particularly the hypothalamus, mainly to suppress hunger. Again, their absence serves to promote appetite. However, not all physiological signals, episodic or tonic, inhibit hunger. The peptide hormone ghrelin is a potent stimulator of food intake. It is released by the stomach and the brain and is detected by receptors in the upper intestine, the mesolimbic system, and various orexigenic circuits in the hypothalamus. Blood levels of ghrelin increase with fasting, reduce after meals, and are correlated with self-reported hunger levels. Further studies demonstrate that ghrelin infusions increase food intake. It may be due to this hormone’s role in appetite that most people have a three meal per day macrostructure of food intake, as an individual’s preprandial ghrelin levels tend to be similar before breakfast, lunch, and dinner, suggesting that this hormone is a major instigator of meals. The rare disorder Prader–Willi syndrome is a genetically determined conditioned characterized by chronically high ghrelin levels and insatiable hunger. Most people with the disorder are obese and their excessive levels of food intake are associated with both a delayed reduction in hunger while eating and a more rapid return to premeal states when eating has finished. However, mice with a knockout of the ghrelin gene eat almost normally, suggesting that appetite is too important to entrust to only one mechanism. Other hunger signals from the digestive tract include the lack of satiety hormones (such as cholecystokinin, peptide YY, amylin, and glucagon-like peptide-1). Finally, in further support of the assertion that the body’s energy regulatory system is far too important to be entrusted to any singular mechanism, many neurotransmitters and neuropeptides have been identified that stimulate appetite, including neuropeptide Y, agouti-related peptide, melanin-concentrating hormone, orexin A, melanocortinconcentrating hormone, glucocorticoids, endocannabinoids, and galanin. It should be noted that the hypothalamus is a key location for the release of such appetite-promoting neuropeptides (particularly the lateral hypothalamus and the arcuate nucleus of the hypothalamus) and the ventrolateral medulla is important for receiving hunger signals transmitted to the brain via the vagus nerve from the rest of the body.
Hunger: Cognitive and Affective Consequences It is a common observation among the lay public that hunger affects concentration, attention, and memory. Recent research
supports these observations. Systematic reviews indicate that children who eat breakfast have superior cognitive function and academic performance in comparison with those who do not eat breakfast. Such research has led to health and wellbeing initiatives introducing breakfasts in schools. Furthermore, bodily sensations of hunger often accompany diffuse feelings of restlessness and excitability. The consumption of food changes both the pattern of physical sensations and the accompanying emotional feelings. Classic drive reduction theory suggests that hunger is associated with unpleasant feelings, and food consumption eases these. Thus, for example, after eating, an aching stomach becomes relaxed and feelings of excitement and irritability become contented. People frequently make judgments regarding their own hunger state using these sensations. However, there is much within- and between-subject variability. In other words, not every person sitting down to eat nor every act of eating by an individual will necessarily be accompanied by bodily sensations of hunger or aversive emotional states.
Conditioned Hunger As the classical work of Pavlov demonstrates, hunger responses can be easily entrained to specific stimuli. Omnivores, like carnivores, are meal eaters (in contrast to grazers); however, their dietary decisions are more complex. One of the essentials for an omnivore faced with a variety of new and different foods is the capacity to learn. It is not possible for an innate preference or aversion to guide the choice of every possible food. Therefore, we learn which foods are beneficial (and which are not) through experience. Emerging research suggests that some food preferences can be acquired prenatally and during lactation. This is because flavonoids from the pregnant woman’s food are passed to the fetus via the amniotic fluid and to infants via breast milk. Babies are often averse to bitter-tasting vegetables (which is an evolutionary advantage, as many bitter alkaloid plants are poisonous to humans). However, babies who are exposed to high levels of bitter vegetables in the womb are more likely to find vegetable juices pleasant in comparison with babies who were exposed to lower levels. This emphasizes how exposure to different flavors is essential to developing our taste preferences. This principle extends throughout our lifetimes. Through our experiences, we learn that ingesting food creates pleasant postingestive sensations of satiety that serve as a positive experience, leading to learned preferences, as studied by Birch. Alternatively, if consumption leads to an association between a food and negative gastrointestinal consequences such as nausea, then a conditioned aversion to that food (or even other similar tastes and flavors) is likely to develop. The learning process involves the association between the sensory and the postabsorptive characteristics of foods. In this way, the sensory characteristics of foods act as cues and come to predict the impact that foods will later have. Consequently, these cues should suppress hunger according to their relationship with subsequent physiological events. The potency of taste aversions following vomiting or illness indicates the strength of cue–consequence learning. Food intake depends upon the stability and reliability of the
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relationship between the taste (sensory cues) and physiological effect (metabolic consequences) of food. It is possible to demonstrate experimentally how human beings adapt their eating to a food’s energy content. A distinctively flavored food that contains ‘extra’ hidden energy, presented on several occasions, will result in a change in eating and in preference. When deprived of food, the subjects’ preference for the taste increases with gained experience. If presented when satiated, preference for the taste decreases. This process is also observable in young children, who eat smaller meals following a taste previously associated with a high-energy snack and larger meals following a taste previously associated with a low-energy snack. Our hunger can thereby be potently reduced by the learned associations of satiety. This relates to the cognitive factors discussed earlier regarding a person’s beliefs about foods and their satiating potential. In environments where food sources remain limited and dietary variety is controlled, learned responses may effectively control subjective feelings of appetite. However, in wealthy countries where there is an abundance of food, there is distortion, variation, and extreme complexity in the relationship between sensory characteristics and nutritional properties. The conditioned control of hunger is therefore weakened or lost in such rich environments. In such a scenario, hunger may become less controllable. The variety of foods available has many different sensory characteristics. Furthermore, some ingredients preserve sensory qualities while altering the nutritive value (such as diet products that substitute sugar for sweeteners or that remove certain ingredients and replace them with artificial bulking agents). Learned hunger therefore is a relatively less important factor when the food supply contains many food items with identical tastes but differing metabolic properties. In addition to learning associations between the sensory properties of foods and their satiating potential, people also learn association between food cues and future food intake. Food cues are plentiful and researchers have found that food intake increases after television viewing when food adverts are shown and also after playing advert games (online games often designed and produced by food manufacturers, where food products are highly prominent). Some individuals are more receptive to the effects of food cues than others and demonstrate an attentional bias toward them (their attention is more easily ‘grabbed’ and maintained by food-related stimuli). Reactivity to food cues emphasizes the importance of conditioned hunger and how associating cues with the naturally rewarding properties of energy intake serves to promote hunger and eating behaviors. The naturally rewarding properties of food consist of two distinct psychological components, wanting and liking, that are controlled by separate neurobiological systems. Wanting is primarily mediated by dopamine, whereas liking is mediated by opioids in hedonic hot spots. Some research has noted that obese individuals are more sensitive to the rewarding properties of food than lean individuals, promoting greater hunger. While other research suggests that obese individuals have greater hunger to compensate for reward deficiency due to reduced dopamine function; more research is needed to resolve this debate.
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Disorders of Hunger The clinical eating disorders anorexia nervosa and bulimia nervosa are commonly believed to encompass major disturbances of hunger. Yet the role that hunger may play is not entirely clear. Contrary to the literal meaning of the term, ‘anorexia’ is not experienced as a loss of appetite. Rather, clinicians recognize that anorexics may endure intense periods of hunger during their self-restricted eating. For some, their strength in resisting intense episodes of hunger provides a feeling of self-mastery and control that is absent in other areas of their lives. Research suggests that restricting anorexics (compared with those who binge) have the greatest blunting of hunger responses and that this disturbance in hunger is not a product of other areas of perceptual confusion. There is evidence that under conditions of total starvation, hunger may become temporarily diminished. This circumstance is extremely rare and obviously relatively brief. Once eating is recommenced, hunger returns rapidly and with extreme intensity. The accounts of the male volunteers who submitted to a 6-month period of semistarvation during World War II (the ‘Minnesota Starvation Experiment’) are a testament to the extreme power of hunger. Referred to as semistarvation neurosis, the behavior of these men was shaped by their need for food, and their hunger experience was extreme. Nearly two-thirds reported feeling hungry all the time and a similar proportion experienced physical discomfort due to hunger. Participants described a marked increase in what was referred to as ‘hunger pain.’ For some, this was mildly discomforting and vaguely localized in the abdomen. For others, it was extremely painful. This account is especially useful in reminding us why energy-reduced diets aimed at achieving weight loss are often difficult to maintain and easy to abandon. Bulimia literally means ‘ox hunger’; however, the term is imprecise. Close analysis of the precursors of binge episodes shows hunger to be lower than it is before a typical meal. In addition, although the urge to eat may be strong during a binge, the large amount of food consumed implies some defect in satiation rather than in hunger. Moreover, binging is often a well-practiced behavior that develops and changes with time. Similarly, binge-eating disorder (binging without the purging behaviors that are present in bulimia) is characterized by binging even in the absence of hunger and continuing until uncomfortably full. For all three of these disorders, it is likely that a stable eating pattern is necessary in order to normalize the experience of hunger, a process that may take a long time to establish. The question of whether obesity reflects a disorder of hunger is now regarded as largely redundant. Obesity is strictly a disorder of energy balance and partitioning. There is limited evidence that heightened hunger contributes to excessive energy input.
See also: Appetite Control in Humans: A Psychobiological Approach; Eating Disorders; Energy: Intake and Energy Requirements; Food Environment; Gut Hormones; Obesity Management; Obesity: The Role of Diet; Pituitary Gland: Pituitary Hormones; Satiety; Snack Foods: Role in Diet.
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Further Reading Birch LL (1999) Development of food preferences. Annual Review of Nutrition 19(1): 41–62. Blundell JE (1979) Hunger, appetite and satiety-constructs in search of identities. In: Turner M (ed.) Nutrition and lifestyles, pp. 21–42. London: Applied Sciences Publishers. Blundell JE, De Graaf K, Finlayson G, et al. (2009) Measuring food intake, hunger, satiety and satiation in the laboratory. In: Graves K (ed.) Handbook of methods for eating behaviors and weight-related problems: measures, theory and research, pp. 283–326. London: Sage Publications Ltd. Flint A, Raben A, Blundell JE, and Astrup A (2000) Reproducibility, power and validity of visual analogue scales in assessment of appetite sensations in single test meal studies. International Journal of Obesity and Related Metabolic Disorders: Journal of the International Association for the Study of Obesity 24(1): 38–48. Halford JC and Harrold JA (2008) Neuropharmacology of human appetite expression. Developmental Disabilities Research Reviews 14(2): 158–164. Halmi KA and Sunday SR (1991) Temporal patterns of hunger and fullness ratings and related cognitions in anorexia and bulimia. Appetite 16(3): 219–237.
Hill AJ, Rogers PJ, and Blundell JE (1995) Techniques for the experimental measurement of human eating behaviour and food intake: a practical guide. International Journal of Obesity and Related Metabolic Disorders: Journal of the International Association for the Study of Obesity 19(6): 361–375. Keys A, Brozˇek J, Henschel A, Mickelsen O, and Taylor HL (1950) The biology of human starvation. Minneapolis: University of Minnesota Press. Kirkmeyer SV and Mattes RD (2000) Effects of food attributes on hunger and food intake. International Journal of Obesity and Related Metabolic Disorders: Journal of the International Association for the Study of Obesity 24(9): 1167–1175. Kissileff HR (1984) Satiating efficiency and a strategy for conducting food loading experiments. Neuroscience and Biobehavioral Reviews 8(1): 129–135. Lowe MR, Butryn ML, Didie ER, et al. (2009) The power of food scale. A new measure of the psychological influence of the food environment. Appetite 53(1): 114–118. Ogden J (2011) The psychology of eating: from healthy to disordered behavior. Oxford: Wiley. Rogers PJ (1999) Eating habits and appetite control: a psychobiological perspective. Proceedings of the Nutrition Society 58(01): 59–67. Womble LG, Wadden TA, Chandler JM, and Martin AR (2003) Agreement between weekly vs. daily assessment of appetite. Appetite 40(2): 131–135.