Caffeine is the bitter substance found in the beans of the coffee plant, leaves of the tea bush and the kola nut. It is the most widely consumed central nervous stimulant in the world. Sources of caffeine include coffee, tea, chocolate, energy drinks and some medications. Its stimulating effect impacts mood, energy levels, the cerebral vascular system, and gastric and colonic activity.
Increased Alertness
Caffeine stimulates the central nervous system by three main mechanisms. High concentrations of caffeine from dietary sources such as coffee mobilize the calcium in cells and inhibit specific enzymes. It increases energy metabolism throughout the brain while decreasing cerebral blood flow. Last, caffeine activates noradrenalin neurons and the release of dopamine. The stimulating effect of increased alertness may be related to its action on serotonin neurons. Caffeine is widely used as a mild stimulant to increase alertness and decrease fatigue. It can be habit-forming, resulting in dependency. In general, physical dependency develops with a minimum consumption of four cups of coffee each day.
Constricts Cerebral Blood Vessels
Constriction of the cerebral blood vessels is a stimulant effect of caffeine. It antagonizes the adenosine receptors in the brain. Adenosine controls the activity of neurotransmitters such as serotonin and dopamine. In the brain stem and spinal cord, adenosine acts as a pain reducer. In the head and neck, it dilates blood vessels. By blocking the action of adenosine, caffeine constricts cerebral blood vessels and increases the release of excitatory neurochemicals. Through this mechanism, caffeine relieves headaches.
Increases Gastric Acid Production
During the normal gastric phase of digestion, gastric secretions are stimulated as food, especially proteins, enter the stomach. Caffeine acts as a stimulant of highly acidic gastric secretions even in the absence of food. This abnormal and unnecessary production of gastric acid irritates the lining of the stomach and duodenum.
Increases Peristalsis
Peristalsis in the gastrointestinal system is the automatic series of organized muscle contractions that move food through the digestive system. In the colon, caffeine stimulates and increases peristalsis. This causes shorter transit times, which means a sooner than normal bowel movement after a meal, along with a decrease in the absorption of nutrients.
References
- NCBI: Caffeine and the Central Nervous System: Mechanisms of Action, Biochemical, Metabolic and Psychostimulant Effects
- UW: Effects of Caffeine on the Nervous System
- Caffeine and Migraines: Caffeine
- "Fundamentals of Physiology: A Human Perspective"; Lauralee Sherwood; 2006
- Pacific Northwest Foundation: Coffee: To Drink or Not To Drink
