Caffeine is the most commonly consumed stimulant in the world. It is metabolized in the liver and diffuses throughout the body including the brain within approximately 30 min. The stimulant effects of caffeine are mediated primarily by adenosinergic antagonism, modulating the activity of a variety of neurotransmitter systems (Kenemans and Lorist, 1995; Fredholm et al., 1999). Caffeine itself increases the basal metabolic rate and acts as a mild central nervous system stimulant, myocardial stimulant, and smooth-muscle relaxant (Christian & Brent, 2001; Zagnoni & Albano, 2002). Main effect of caffeine in coffee is to prevent sleepy and increase arousal.
The world’s primary source of caffeine is the coffee “bean”, from which coffee is brewed. In general, one serving of coffee ranges from 80 to 100 milligrams caffeine.
Caffeine is metabolized in the liver into three primary metabolites: paraxanthine (84%), theobromine (12%) and theophylline (4%).
Paraxanthine
After intaking, roughly 84% of caffeine is demethylated at the 3-position to yield paraxanthine, making it the chief metabolite of caffeine in the body.
Paraxanthine is a central nervous stimulant with similar activity, but lower toxicity and less anxiogenic effects than caffeine (Okuro et al., 2010).
Paraxanthine is a nonselective adenosine receptor antagonist, which raises plasma epinephrine and diastolic blood pressure and can help stimulate the central nervous system.
Theobromine
Theobromine, which is highly prevalent in cocoa and chocolate, also shows similar stimulating effects, although to a lesser degree than caffeine (Benowitz, Jacob, Mayan, & Denaro, 1995).
In the liver, theobromine is metabolized into xanthine and subsequently into methyluric acid. It can dilate blood vessels, increase urine volume and stimulate heart.
Theophylline
A small amount of theophylline is one of the products of caffeine metabolic processing in the liver. Theophylline is a widely used bronchodilating agent with a narrow serum therapeutic range (Shannon, 1999).
The main actions of theophylline involve increasing heart muscle efficiency, heart rate, blood pressure, renal blood flow, etc.
Reference
- Barry, R. J., Clarke, A. R., Johnstone, S. J., & Rushby, J. A. (2008). Timing of caffeine’s impact on autonomic and central nervous system measures: clarification of arousal effects. Biological psychology, 77(3), 304-316.
- Choi, E. J., Bae, S. H., Park, J. B., Kwon, M. J., Jang, S. M., Zheng, Y. F., … & Bae, S. K. (2013). Simultaneous quantification of caffeine and its three primary metabolites in rat plasma by liquid chromatography–tandem mass spectrometry. Food chemistry, 141(3), 2735-2742.
