Hunger - Physiological and Environmental Factors

Hunger Factors

People typically eat until fullness (satiation), after which they will not eat for a period of time (satiety). The next time of eating is determined both by physiological cues, as well as environmental cues. Some of these environmental cues are sensory stimulation (such as by a commercial), stress, peer pressure, and boredom.

Internal Hunger Factors (Hormones)

There are two types of hormones regulating food intake/hunger. There are long term regulators, such as insulin and leptin, which are released into the blood in proportion to the amount of bodyfat, and act to lower hunger while increasing energy expenditure. When the amount of bodyfat decreases, these hormones decrease, leading to an increase in appetite, and a decrease in metabolism.

The short term regulators provide satiety signals upon eating (such as cholecystokinin - CCK), and are released in the gastrointestinal tract. This causes a sense of fullness thus prompting one to stop eating. Another short term regulator (also from the gastrointestinal tract) is ghrelin. Ghrelin levels rise before meals (when the stomach is empty), and fall shortly after eating.

Satiety is found to be dependent on the weight or volume of a food, rather than on energy content. Thus, eating large amounts of low calorie foods, such as non-starch vegetables, is highly satiating, and helps to lower perceived hunger.

When the stomach is full, the gut hormones GLP-1 and CCK will act to cause delayed gastric emptying (delayed emptying of the stomach), and an increase in gastric distension (post meal bloating of the stomach).

The Effect of Glucose Ingestion

A decline in blood glucose levels was found to precede meal requests in a controlled study.

Response of blood glucose levels is dependent on the type of carbohydrate. The glycemic index (GI) of a carbohydrate tests the post-meal glucose response following the consumption of a standard amount of carbohydrates from a test food, in relation to a control food (typically glucose). Several studies have found an overall stronger suppression of hunger and energy intake after consumption of a low GI meal. High GI foods have a higher short term (less than 1 hour) appetite suppressing effect than the low GI foods.

Leptin

Leptin is made by the fat cells and signals to the brain the level of fat stores in the body. Thus, as you get leaner, less leptin is produced, and the appetite is increased. As well, an energy deficit decreases leptin concentrations, while an energy surplus increases leptin concentrations.

References

Keeping Hunger at Bay (PDF); Schwartz, Michael and Gregory Morton, Nature, Vol 418,8 August 2002, P. 595-597

Biomarkers of satiation and satiety; Cees de Graaf, Wendy AM Blom, Paul AM Smeets, Annette Stalfleu, and Henk FJ Hendriks; Am J Clin Nutr, 2004; 79:946-61