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PHYSIOLOGICAL EFFECTS
Caffeine administration affects the functioning of the cardiovascular, respiratory, renal, and nervous systems. Proposed mechanisms of action differ for different physiological effects. Caffeine action is thought to be mediated via several mechanisms: the antagonism of adenosine receptors, the inhibition of phosphodiesterase, the release of calcium from intracellular stores, and antagonism of benzodiazepine receptors (Myers et al., 1999).
Caffeine and Adenosine Receptors
The ability of caffeine to inhibit adenosine receptors appears to be highly important in its effects on behavior and cognitive function. This ability results from the competitive binding of caffeine and paraxanthine to adenosine receptors and is of importance in contributing to CNS effects, especially those involving the neuromodulatory effects of adenosine. Due to the blocking of adenosine inhibitory effects through its receptors, caffeine indirectly affects the release of norepinephrine, dopamine, acetylcholine, serotonin, glutamate, gamma-aminobutyric acid (GABA), and perhaps neuropeptides (Daly et al., 1999).
There are two main classes of adenosine receptor: A1 and A2, caffeine and paraxanthine are nonselective antagonists at both, although they are not especially potent antagonists. The caffeine concentrations attained in vivo that cause mild CNS stimulation (5–10 µM) and that are associated with antiasthmatic effects (50 µM), are in the range associated with adenosine receptor blockade (as quantitated by in vitro receptor binding assays) (Daly, 1993).
Caffeine and Phosphodiesterase
In contrast, phosphodiesterase inhibition may account for caffeine's (and theophylline's) cardiostimulatory and antiasthmatic actions, since nonxanthine phosphodiesterases are cardiac stimulants (Schmitz et al., 1989) and are also effective as bronchiolar and tracheal relaxants. Indeed, in the latter case, the potency correlates with phosphodiesterase inhibition, not with affinity for adenosine receptors (Brackett et al., 1990, Persson et al., 1982, Polson et al., 1985).
Natural vs. Synthetic Caffeine: What Foods Naturally Contain Caffeine?
If you’re like most adults, you probably enjoy a cup of coffee or tea now and again. These drinks are natural sources of caffeine, the most commonly used stimulant in the world ( 1 ).
Although commonly added to many products, caffeine is found naturally in only a handful of foods and beverages.
Natural and synthetic types of caffeine tend to be very similar but can have slightly different effects on your body.
In this article, I’ll discuss the difference between natural and synthetic caffeine and review the foods and drinks most rich in natural caffeine.
Natural caffeine is the type of caffeine that is naturally found in foods like coffee, tea, and chocolate.
It works by blocking adenosine receptors in your brain. Adenosine is a neurotransmitter that relaxes your brain, making you feel tired ( 1 ).
By blocking adenosine receptors, caffeine prevents adenosine from binding to them. This keeps your brain from realizing that it’s tired, which is why caffeine is so effective at fighting off sleepiness and helping you remain more alert ( 1 ).
How is natural caffeine different from synthetic caffeine?
Unlike natural caffeine, which comes from foods found in nature, synthetic caffeine is produced in a lab. It’s commonly added to packaged foods and beverages such as soda, juice, chewing gum, candy, and energy drinks.
Synthetic caffeine is also often added to non-food products such as pain relievers, makeup, face creams, and other cosmetics ( 2 , 3 ).
The Food and Drug Administration requires that caffeine be listed on food labels when it has been added during production. This is not required of foods that naturally contain caffeine, such as coffee or tea ( 5 ).
So if you see caffeine on a food or beverage’s label, that product most likely contains synthetic caffeine.
The chemical structure of synthetic caffeine is almost indistinguishable from that of natural caffeine.
Despite this, many claim that synthetic caffeine is absorbed much more quickly than natural caffeine, causing a peak in energy followed by a steep crash. However, there is very little scientific backing to this claim.