Caffeine for the Sustainment of Mental Task Performance Formulations for Military Operations (2001) / Chapter Skim
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2 Pharmacology of Caffeine
2 Pharmacology of Caffeine
Pages 25-32
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From page 25... ...
However, caffeine's elimination half-life may range between 1.5 and 9.5 hours, while the total plasma clearance rate for caffeine is estimated to be 0.078 L/h/kg (Brachtel and Richter, 1992; Busto et al., 1989~. This wide range in the plasma mean half-life of caffeine is due to both innate individual variation, and a variety of physiological and environmental characteristics that influence caffeine metabolism (e.g., pregnancy, obesity, use of oral contraceptives, smoking, altitude)
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From page 26... ...
When it is consumed in beverages (most commonly coffee, tea, or soft drinks) caffeine is absorbed rapidly from the gastrointestinal tract and distributed throughout body water.
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From page 27... ...
Caffeine is cleared more quickly than paraxanthine, so 8 to 10 hours after caffeine ingestion, paraxanthine levels exceed caffeine levels in plasma (Arnaud, 1993~. The fact that the human body converts 70-80 percent of caffeine into paraxanthine with no apparent toxic effects following caffeine doses of 300-500 mg/day suggests that paraxanthine's toxicological potency is low.
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From page 28... ...
More recent studies, however, indicate no significant effects on caffeine pharmacokinetics across phases of the menstrual cycle in healthy, nonsmoking women who are not using oral contraceptives (Kamimori et al., 1999~. Decreased paraxanthine or caffeine metabolic rates in healthy postmenopausal women on estrogen replacement therapy suggest that exogenous estrogen in older women may inhibit caffeine metabolism through the P450 isozyme CYP1A2, an isozyme common to both estrogen and caffeine metabolism (Pollock et al., 1999~.
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From page 29... ...
However, caffeine is a fairly weak inhibitor of phosphodiesterase enzymes, and the in viva concentrations at which behavioral effects occur are probably too low to be associated with meaningful phosphodiesterase inhibition (Burg and Werner, 1975; Daly, 1993~. In contrast, phosphodiesterase inhibition may account for caffeine's (and theophylline's)
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From page 30... ...
General Effects of Caffeine on Physiological Functions The effects of caffeine on sodium-potassium adenosine triphosphate pump activity lead to a decrease in plasma potassium concentrations, and affect the depolarization-repolarization process during exercise with potential effects on fine motor coordination. The effects of caffeine on the heart are primarily stimulatory and are accompanied by increased coronary blood flow.
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From page 31... ...
With repeated caffeine dosing, paraxanthine may contribute to development of tolerance and withdrawal symptoms. Caffeine clearance rates are affected by both environmental and physiological factors, such as use of oral contraceptives, smoking, and pregnancy.
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From page 32... ...
COMPENSATION OF SLEEP DEPRIVATION IMPAIRMENTS Effects of Sleep Deprivation on Cognitive Behavior Military personnel face many situations in which extended wakefulness may be required, including sentry duty, deployment-related activities, air transportation during emergencies, submarine duty, and combat. As part of their duties in these situations, individuals may have to perform complex cognitive tasks.
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