Hungry? The question "why do we feel hungry?" seems to be very obvious to answer. It is because we need to get nutrients to survive. Hunger is the motivation for us to be able to know that we need to get the nutrients in our body. But how do we really know that we are hungry? The answer can be analyzed by three different components: biological, learned, and cognitive.
Many theories of hunger are historically discussed from the biological component. Cannon and Washburn (as cited in Coon, 1995) came up with the stomach contraction theory which states that we know we are hungry when our stomach contracts. In the notorious balloon study, Washburn trained himself to swallow a balloon which was attached to a tube, then the balloon was inflated inside of his stomach. When the balloon was inflated, he did not feel hungry. Later this theory was opposed by the fact that people whose stomach was removed still felt hungry. Glucose theory states that we feel hungry when our blood glucose level is low. Bash (as cited in Franken, 1994) conducted an experiment transfusing blood from a satiated dog to a starved dog. The transfusion resulted in termination of stomach contraction in the starved dog, and supported the glucose theory. But as LeMagnen (as cited in Kalat, 1995) suggests that blood glucose level does not change much under normal conditions. Insulin theory states that we feel hungry when our insulin level increases suddenly in our bodies (Heller, & Heller, 1991). However, this theory seems to indicate that we have to eat to increase our insulin level in order to feel hungry. Fatty acid theory states that our bodies have receptors that detect an increase in the level of fatty acid. Activation of the receptor for fatty acid triggers hunger (Dole, 1956, Klein et al., 1960 cited in Franken, 1994). Heat-Production theory suggested by Brobeck (as cited in Franken, 1994) states that we feel hungry when our body temperature drops, and when it rises, the hunger decreases. This might be explain that we tend to eat more during winter.
Hunger cannot truly be explained only by the biological component. As human beings, we cannot ignore our psychological part, the learned and cognitive components of hunger. Unlike any other beings, we humans use an external clock in our daily routine, including when to sleep and when to eat. This external time triggers our hunger. For instance, when the clock says 12 pm, lunch time, many people feel hungry just because it is lunch time. This hunger is triggered by learned behavior. In addition, the smell, taste, or texture of food also triggers hunger. For instance, if you like french fries, the smell of frying potatoes may trigger your hunger. However, this preference of taste, smell, or texture is a culturally learned preference. If one does not like sushi, the smell of sushi does not trigger hunger. Interestingly, people also feel hungry for a particular taste, more specifically, the four basic tastes: sweet, sour, bitter, and salty. For example, an often heard expression is "I am hungry for something sweet." People keep feeling hungry until these four tastes are satisfied.
Colors also contribute to hunger. Looking at a yellow banana makes one to want to eat it, but a red banana does not. Similarly, red or green can trigger hunger for an apple, but not blue. It is hard to find natural food with blue color, because mother nature does not produce blue food. Blue is said to be an appetite suppressant. Color greatly affects our hunger.
Many people eat foods base on their knowledge of what foods are good for them. For example, low fat, low sugar, and low sodium food are said to be good. Eventually people learn to change their preference and want to eat "good food" only (Franken, 1994).
The mechanism of hunger and satiety are not necessarily the same. There are two mechanisms for satiety. One is at the brain level, the other is at the gastrointestinal tract level. There are two places in the hypothalamus, part of the brain, that controls hunger and eating. The Ventromedial Nuclei gives a signal when to stop eating, and the Lateral hypothalamus gives a signal to start eating (e.g.,Coon 1995). We feel satiety at the brain level because of the function of the Ventromedial Nuclei. On the other hand, at the level of the gastrointestinal tract, Koopmans (1985) states that satiety signals come from the stomach, which controls short-term eating.
Obesity is defined as exceeded the average weight for one's height, bone structure, age, and sex by a given percentage, above 25% (Franken 1994). The question of why some people are obese can be answered in different ways. Is it because obese people have a different hunger and satiety mechanism from people who are not?
Obesity can be caused biologically. Many studies show that twins who grew up apart still weigh about the same. Also, adopted children's weights are similar to their biological parents, not their adopted parents (Stunkard et al., 1986). But this does not explain all cases of obesity.
Set point theory by Keesy and Powley (as cited in Franken, 1994) states that we have a predetermined weight, set by the hypothalamus, that the body attempts to maintain. According to this theory, diet does not work because the individual has his or her own set point weight, and the body works to maintain that set point. Thus the more one tries to intake less calories, the more the body wants to keep the weight that is set by the hypothalamus. For obesity, this set point is too high due to damage to the Ventromedial Hypothalamus.
Stanley Schachter (1971) came up with the internal-external theory of hunger and eating of the obese. They ran an experiment in which subjects were measured by the amount of crackers eaten during the time when the real time was manipulated by a faster clock or a slower clock. They hypothesized that if the obese person is more affected by the clock time than the real time, then, he or she should eat more when the clock shows it is close to dinner time. The results were consistent with the hypothesis. Schachter concluded that obese people respond to external cues of hunger, such as time, more than non-obese people who tend to respond more to internal cues of hunger.
Rodin (1981) connected the external cues of hunger to insulin, and hypothesized that people (whether obese or not) who respond to external cues of hunger tend to increase the level of insulin in the blood more than people who respond to internal cues. In Rodin's experiment, hungry subjects who are external cue respondents were gathered, around noon, where steaks were grilled. After they smelt and heard the steak, their insulin levels were measured. As expected, the smell and sound of cooking increased the insulin level of those subjects.
The boundary theory of hunger (Herman & Polivy, 1984) has a cognitive perspective about hunger of the obese. According to this theory, there are boundary lines of hunger and satiety determined biologically. The space between those two boundaries is determined cognitively. In the space between those two boundaries, people set how much they think they should eat, and if one sets a satiety boundary cognitively lower (like diet) than one that is biologically predetermined, the body tries to compensate food intake to meet the biologically determined boundary level by triggering hunger. For the obese, this biologically determined satiety boundary is higher than for the non-obese.
Many theories point out that obese people have a strong biological component of hunger and eating. What about people with eating disorders? What is the mechanism of hunger and eating for people with eating disorders? There are mainly three kinds of eating disorders; Binge Eating, Anorexia Nervosa, and Bulimia. Binge eating is characterized by one's eating a very large amount of food until she or he feels uncomfortably full. This binge eating is done when one is not hungry. According to the DSM-VI, Anorexia Nervosa has two types; restricting type, and binge-eating/purging type (American Psychiatric Association, 1994). Anorexia Nervosa restricting type is when one extremely restricts food intake, and it is not followed by binge-eating or purging behavior. On the other hand, Anorexia Nervosa binge-eating/purging type was described as one engaged in purging and binge-eating regularly. A common symptom of Anorexia is one's putting her or himself on self-starvation to avoid feeling fat or gaining weight. Although people with this disorder weigh far below normal, they still think they are overweight. Eventually they are at risk of losing their lives due to malnutrition.
People with this disorder still feel hungry, yet they cannot eat because they are too afraid of gaining weight. Physiological causes of this disease are not yet clear, although there are some findings showing a connection with serotonin and norepinephrine. The learned component of Anorexia cannot be ignored. Studies show that there is more Anorexia in westernized cultures than other cultures, (e.g., Suematsu, 1986), because the social value of slimness pushes people to be thinner. Cognitively, these people have a distorted body image of themselves, and dissatisfaction with their own body image, which is influenced by the cultural value of slimness, and leads to eating disorders (Mumford, Whitehouse, & Choudry, 1992).
Bulimia Nervosa is a condition of binge eating followed by purging and use of laxatives (American Psychiatric Association, 1994). Unlike Anorexia, people with this disorder are normal or above weight. Psychologically, having quilt and shame are common symptoms among people with Bulimia. Unlike anorexic people who put absolute control over restricted eating, bulimic people cannot control their eating. The physiological cause of Bulimia is still unclear. Psychologically, Bulimia is said to be linked to depression and anxiety, but clear evidence of causation has not yet been found. Cognitively, people with Bulimia are said to be motivated to escape from reality by binging. It is possible that those people were given food by their caretakers to lift their mood in their childhood. Like Anorexia, cultural learning that one needs to be thin to be accepted may also contribute to the cause.
Hunger is a primary motivation. Despite strong beliefs that
hunger is caused biologically, this motivation is controlled not
just by physiology, but also psychology as well. There are two
kinds of hungers; one is caused physiologically, and the other is
caused psychologically. What makes human beings different from
animals is we eat not only to feed our bodies to satiate
physiological hunger, but also to feed our minds to satiate
psychological hunger as well. Although these two kinds of hunger
interchangeably cause hunger by affecting one another, putting some
food in our mouth is not necessarily the right way to feed our
psychological hunger. Problems like eating disorders and obesity
could occur because we mistakenly keep tying to satiate our
psychological hunger by eating food. Until we realize that we need
to feed our mind with something, rather than eating, we can not
feel satiated. Until we recognize it is our mind, not our body
which needs food, we cannot be satisfied with what we put in our
mouth. Thus, hunger is not only about how the body changes
physiologically, it is about how our body and mind together are
well fed, not just by the food that one can put in their mouth, but
also by the whole environment around us.
A university lecture that discusses the physiological basis of hunger and eating.
This site lists a general outline of the biological component of hunger and eating.
An article from the American Psychiatric Association on eating disorders. It contains examples and theories about causes and discusses treatments and medications.
A psychoanalytic approach to eating and eating disorders.