Cholecystokinin (CCK) is a group of similar proteins sharing rather specific hormonal activities in the digestive tract, and neurotransmitter functions in the central nervous system (CNS). The CCK hormones range in size from 4 to 55 amino acids and the two most studied forms are designated CCK4 and CCK8. CCK4 seems to be involved in CNS/cognitive processes like anxiety, memory, and learning. CCK8 appears to help modulate digestive function and is instrumental in causing the feeling of satiety. CCK8 is also linked to eating disorders like anorexia nervosa.
CCK and Digestion
After food is processed for a bit by the physical and chemical processes of the stomach it becomes a semi-liquid material called chyme. The contents of chyme include dietary fat, protein, and carbohydrate. Chyme is periodically squirted into the small intestine from the stomach in a process called gastric emptying. Epithelial cells in the upper third of the small intestine (duodenum) release CCK into the bloodstream when chyme high in fat and protein reaches them.
CCK in the bloodstream quickly travels to the CNS, stomach, liver, gallbladder, and pancreas. The hormone instructs the stomach to stop emptying by contracting the pyloric sphincter. It then acts as a neurotransmitter in the CNS, causing the sensation of satiety. CCK signals the liver to create more bile and for the pancreas to release digestive enzymes. It also signals the gallbladder to spasm and secrete stored bile into the small intestine. The combination of bile and pancreatic enzymes then breaks down the chyme and facilitates the absorption of nutrients.
Satiety: Feeling Full
The satiety effect of CCK seems to be a physiological mechanism to avoid overeating and obesity. In animal studies, males are affected more than females. Thin subjects also seem to be more sensitive than obese ones. When human subjects are administered forms of CCK they tend to decrease the size of their meals whether thin or obese. This is regardless of taste, smell, or other qualities. The effects of CCK seem to become stronger with age and may be the cause of wasting in the elderly. CCK has also been implicated in anorexia nervosa and is often found at much higher levels than expected in those with this condition.
Role in Weight Loss
Pharmaceutical strategies for weight loss often target the CCK component of the digestion pathway. For instance, a compound extracted from potatoes called protease inhibitor II (PI2) has been shown through various human studies to induce the release of CCK. This then leads to satiety and documented weight loss.
Supplementary Resources:
Principles and Practice of Endocrinology and Metabolism , 3rd ed., Becker, Lippincott Williams & Wilkins
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