Hunger is influenced by appetite and feeling of fullness, but is it the main reason for eating?

Hunger certainly initiates feeding and is a complex sensation stimulated by changes in the state of the body's energy stores, being inhibited by a feeling of fullness. Hunger is influenced by appetite and satiety.

Appetite is controlled by the brain - in particular, the hypothalamus. Observations and studies have demonstrated that if parts of the brain are damaged, overeating and obesity occur.

Over the years, it has become evident that appetite control is not simply and directly under the control of the central nervous system. It is influenced by intrinsic factors, feeding back information to the central nervous system, and extrinsic factors, our food choice and  behaviour.

Extrinsic factors - influence appetite

Most of the time, we eat not because we are hungry but out of habit and because we enjoy food. Our traditions, beliefs, social environment and choice all influence appetite and the amount consumed.

Physical and social environment
Different circumstances influence appetite. Socialising and entertaining encourage eating; and eating on the move, as part of a hectic lifestyle, promotes convenience-snack consumption rather than healthy eating.

Traditions and beliefs
Our habit of eating at certain times of the day, our religious beliefs that determine what and when to eat, and our educational and cultural background all help dictate our feeding patterns.

Dining with the family used to be an occasion in itself. Changes in work patterns and greater youth independence has changed this tradition, and eating patterns along with it. As a result, convenience foods and fast foods form the mainstay of the diet of many households.

We have our own psychological reason for eating, an attitude to food. Certain foods, whether it be chocolate, crisps or bread, are comfort foods and some initiate binging. Some people find they have to eat just because others around them are.

Nature and quality of food  
Certain foods are more filling than others, due to their nutrient composition. We will feel fuller after consuming complex carbohydrates from a jacket potato than from fatty foods such as a chocolate bar. The whole make-up of the food has to be considered - the time it takes to leave the stomach for complete digestion will influence satiety - the slower the time, the greater the satiety.

The pleasurable properties of food, the sight and the smell, encourage us to believe we are hungry. The production of saliva in the mouth is stimulated, as is the secretion of gastric juices. We tend to stop eating when it is no longer pleasurable i.e. feel too full, not when the messages from blood glucose levels register.

Intrinsic factors - directly affect appetite

Physiological mechanisms regulate energy by controlling its inputs and outputs from the centres in the central nervous system, in a similar way to body temperature via a feedback mechanism.

Nervous stimuli

Under the control of the nervous system, our stomach contracts when hungry, signalling awareness.

Chemical stimuli

A decrease in blood glucose levels is associated with feelings of hunger, and cessation of eating increases glucose levels. An increase in blood glucose levels decreases the free fatty acids circulating in the blood. Sensations of hunger and satiety are closely associated with blood free-fatty-acid levels, a possible determinant of feeding behaviour.

The rate of glucose uptake dictates the sensation of satiety; foods which initiate a fast glucose uptake and usage, that is high glycaemic index foods, are less satisfying. Low glycaemic index foods slow the rate of glucose uptake and usage resulting in greater satiety. However, after a mixed meal of several foods many factors affect the rate of absorption of carbohydrate.

Thermal stimuli

The central mechanism for the regulation of body temperature is also housed in the hypothalamus. When we are cold, we tend to feel hungry and eat to keep warm.

Adipose stimuli

The long-term regulation mechanism of appetite depends on the information and size of the fat stores, the nature of the signal being unknown. This is the most complicated of the regulatory mechanisms that determines and fixes the reserve of energy as body fat.

20/05/2009