hunger

hunger can have a variety of meanings. For example, to a nutritionist it may be used to describe starvation resulting from a lack of food. However, to most of us hunger refers to the sensations we feel when we need food, such as an aching, growling stomach, weakness, or a grumpy mood. Indeed, the Oxford English Dictionary defines hunger as ‘The uneasy or painful sensation caused by want of food.’ This sensation generally motivates an individual to find and to consume food.

Let us start our consideration of hunger with how it feels. Descriptions of hunger come from written reports consisting of checklists of particular sensations, or line scales on which the degree of a sensation is marked. Open-ended verbal reports in which the experience of hunger is described freely are of interest but difficult to quantify. Sometimes the emphasis has been on where in the body an individual experiences hunger. All of these different types of assessments agree that gastric sensations, such as an aching or growling stomach, are most commonly associated with hunger. Other sensations found to occur less frequently are weakness, headaches, pain, dizziness, anxiety, loss of concentration, food craving, thoughts of food, watering of the mouth, discomfort, dry mouth, nausea, and thirst. Large individual differences are seen in both the degree to which hunger is experienced and the way it is perceived. However, in general, when hunger is reported over the entire day it is seen to follow a cyclical pattern in which it rises gradually before meals and falls rapidly as eating proceeds.

An important issue is whether reported hunger can be used to infer how much a person would eat in a given situation. Investigators have found that in some situations hunger does correlate well with food intake, but often it does not. For example, regardless of how hungry someone feels, the amount consumed in a meal can be affected by the palatability, variety, and amount of food offered. The eating environment may also influence intake, in that people often eat more when they are with a group of friends than when eating alone.

Much research has been directed at discovering the role of hunger in the regulation of food intake and food selection. Since hunger is a subjective sensation, strictly speaking it can only be studied in humans. However, experimental animals, particularly laboratory rats, are often used in studies of hunger and the regulation of food intake. Thus, hunger has been defined in ways that do not rely on reports of subjective sensations. Hunger in animals is inferred and measured by the amount eaten. So, for example, if the time since the last meal affects the amount consumed, we assume that this is because food deprivation increases hunger. The assumption in both experimental animals and humans is that, when there is a need for food, the body senses this through a variety of physiological mechanisms, including changes in blood glucose and insulin, and metabolic signals from the liver, all of which are integrated by the brain. When the brain detects that the body needs fuel, a state of hunger develops and the animal eats an amount appropriate to reduce the hunger and reverse the deficit.

While hunger may relate to the physiological signals indicating the body's need for food, learning and environmental influences can influence it as well. A newborn baby apparently experiences hunger in response to cues signalling a need for food, and cries to be fed regularly every few hours. Gradually, through learning, this behaviour changes so that it conforms to imposed meal times. That these times vary widely between cultures — for example, dinner is eaten early in the evening in Norway and late at night in Spain — illustrates the impact of learning on the cyclical pattern of hunger over the day. Studies also indicate that people learn the types and amounts of food required to satisfy their hunger.

Sometimes hunger and food intake are unrelated to the body's need for food. For example, when we are bored or nervous, we may not feel hungry but will nevertheless eat to pass the time or to calm ourselves. Such inappropriate eating can become problematic and contribute to the development of obesity. Some behaviour therapies for obesity emphasize learning to recognize hunger sensations and to eat in response to them. However, some obese individuals complain that they never experience hunger. A critical question which has not yet been clearly answered is whether some overweight individuals have impairments in physiological systems that normally signal hunger.

It is also important to determine whether low food intake, or anorexia, can be due to impaired or reduced hunger in response to physiological needs. For example, hunger may be reduced in chronically ill patients and is reported to be low in individuals with anorexia nervosa. The challenge to their carers is how to induce them to eat. They may need to stimulate ‘appetite’ rather than hunger. While hunger refers to the need to eat, appetite relates to the pleasure of eating. It is appetite that steers us to particular foods. While hunger and appetite are often experienced together, when we are hungry and want a particular food, appetites for foods can occur in the absence of hunger. Thus, we may have an appetite for chocolate or ice cream at the end of a large meal when we are no longer hungry.

Like many other bodily systems, those underlying hunger and the regulation of food intake change with age. Many elderly people do not eat enough to maintain their body weight. While the low food intake may be due in part to poor appetite associated with decreased ability to smell and taste food, there are also changes in hunger. Older individuals not only may report decreased hunger, but studies show that many do not adjust intake appropriately to changes in bodily needs.

Studies of hunger are important clinically in that they may suggest an abnormality of physiological systems related to the detection of signals from normal metabolism of ingested food. Understanding both the biological and behavioural foundations of hunger may help to suggest therapies for inappropriate food intake. For example, some pharmacological agents can reduce both hunger and food intake, which is helpful in the treatment of obesity. When food intake is inappropriately low, treatment may involve stimulating appetite, for example, by increasing the variety and palatability of the available foods, which could increase food intake even when hunger sensations are depressed.

The sensation of hunger links our bodily needs to behavioural food seeking and ingestion. In the wild, animals would be unlikely to survive if the sensation of hunger were abnormal. However, in humans, food is often abundant and culture dictates when and how much should be consumed, so the sensation of hunger has become less crucial for survival. Nevertheless, the high incidence of disorders of body weight indicates that relying on these environmental cues rather than hunger to guide food intake is not an optimal strategy.

Barbara J. Rolls

Bibliography

Bell, E. A. and and Rolls, B. J. (2001). Regulation of energy intake: Factors contributing to obesity. In: Present Knowledge in Nutrition, 8th Edition. Eds B. Bowman and R. Russell. ILSI Press, Washington, D.C., pp. 31–40.
Rolls, B. and and Barnett, R. A. (2000). The Volumetrics Weight-Control Plan: Feel Full on Fewer Calories. HarperCollins Publishers, New York.

See also eating; eating disorders; obesity.