General Principles and Phases of Satiety
The presence of food in the gastrointestinal tract is regarded in normal physiology as the second crucial useful adaptive result of the functional nutritional system. The processes belonging to the external self-regulation loop of this system ultimately determine the development of food satiety.
Physiologically, this complex process is typically divided into two successive stages:
- Sensory satiety (also known as primary or emotional satiety).
- Metabolic satiety (occurring later, as nutrients are digested and absorbed).
Sensory (Primary) Satiety
The sensory phase is an extremely rapid physiological process. In everyday life, this state is often described as "being full right at the dinner table," when a person or animal stops eating even before the food has time to be digested and absorbed into the bloodstream.
The biological significance of this phenomenon is critical for species survival in the wild. Rapid, anticipatory consumption of available food and the immediate relief of hunger allow the animal to quickly leave an open area and hide in a secure shelter before prolonged metabolic processes unfold. This is a reliable evolutionary mechanism to avoid predators during a moment of vulnerability.
Neurophysiological Mechanism
The cessation of feeding motivation is ensured by a strict sequence of reflex reactions:
- Activation of the receptor apparatus. Ingested food begins to exert mechanical and chemical effects on receptors located in the mucosa of the oral cavity and esophagus. However, the most significant signaling originates from gastric stretch receptors.
- Afferent pathway. The resulting nerve impulses travel via afferent pathways to the brain, reaching neurons of the ventromedial hypothalamus (traditionally referred to as the "satiety center").
- Reciprocal inhibition. Excited cells of the ventromedial hypothalamus exert a powerful inhibitory effect on neurons of the lateral hypothalamus (the so-called "hunger center," which was previously highly active due to nutritional need).
- Loss of motivation. As a result of the inhibition of the lateral hypothalamus, its ascending activating influences on other brain structures sharply decrease. Feeding motivation completely disappears, and the act of eating ceases.
Humoral Mechanisms and Satiety Factors
In addition to nerve impulses, humoral factors play a role in this process. Excitation of the ventromedial hypothalamus triggers a cascade of reactions (via hormonal and not yet fully understood pathways) that leads to the emergency mobilization of nutrient depots in the body.
Released nutrients rapidly enter the systemic circulation, depriving the blood of its previous "hungry" properties—which previously exerted an irritating effect on hypothalamic structures. Parallel to this, the blood is actively enriched with specific factors—oligopeptides—that reinforce the feeling of fullness.
The main satiety factors include:
- Cholecystokinin;
- Somatostatin;
- Bombesin;
- Glucagon;
- Leptin.
A special place in this series belongs to enterins. These are specific substances secreted by the duodenal mucosa that effectively reduce appetite.