Caffeine and Nutrition: an Overview - Caffeine: Chemistry, Analysis, Function and Effects - page 7

ase activity, the intracellular mobilization of calcium, and the binding to
benzodiazepine receptors (Chen et al 2010; Lee and Chung 2010).
When administered orally or intravenously in humans or rats, caffeine acts
with reinforcing properties in response to electrophysiological and behavioral
stimulation (Griffiths and Mumford 1995). However, the bitter taste of
caffeine can interfere with administration, especially in behavioral studies, as
experiments in rats have shown that the animals freely ingest only low doses of
caffeine (Heppner et al 1986). Thus, the immediate consequence of consuming
caffeine orally would be a trend to generate aversion (because of its bitter
taste). Other studies show that caffeine intake in the form of medication can
cause nausea and gastric irritation, especially in children (e.g., McKim, 1996).
Furthermore, caffeine can interfere with a subsequent reinforcing effect in
animal models of oral self-administration. Similar findings have been observed
in humans; the reinforcing effect of caffeine varies with dose, i.e., low and
medium doses maintain the behavior of oral self-administration and high doses
can even produce aversion (Griffiths and Mumford 1995). Other routes, while
less

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explored, may also be used for caffeine administration, such as

intramuscular injection and application in the form of suppositories.
Caffeine is absorbed primarily by the stomach and intestine (absorption
reaches 99% in the intestinal tract; see Fredholm et al., 1999, for details), with
higher affinity for lipids compared to water; the peak plasma of caffeine occurs
15-45 minutes after ingestion, and the half-life is approximately 5-6 hours
(Smith 2002). Caffeine is distributed through the blood, with 10 to 30 percent
being transported by proteins (McKim 1996). In addition to its relative facility in
passing the blood-brain barrier, caffeine has a lipophilic character and, for this
reason, is also found in all organs and can be present in breast milk (Arnaud
1993; Fredholm et al 1999; McKim 1996). The excretion of the products of
caffeine metabolism is performed by the liver (Fredholm et al 1999), and only 2%
is excreted by the kidneys without being metabolized (Arnaud 2011).
The selective antagonism of A 1 and A 2A receptors that is involved in the
mechanism of action of caffeine has been the focus of recent investigations
regarding behavioral outcomes (Randall et al 2011). Authors have shown that
increased locomotor activity and high frequency of lever pressing in rats result
from the caffeine stimulating effect mediated by blockade of A 2A receptors,
whereas the anxiogenic effect is probably related to action on the A 1 receptor
antagonist. However, additional investigation is required to better clarify the
specific action of adenosine receptors on behavioral alterations.

1.3 Caffeine-Nutrition Interaction: Effects on
Physiological Processes
In this part of the chapter, we address the possible interaction between caffeine
and changes in the nutritional status of the organism. This discussion is based
on data from our lab on an experimental model of attention designated as LI,