Health Consequences of Caffeine Intake
In humans, caffeine is acentral nervous system stimulant, having the effect of temporarily warding offdrowsinessand restoring alertness. It also stimulates heart rate, and respiration, has psychotropic (mood altering) properties, and acts as a mild diuretic.
The methylxanthines have effects on the central nervous system, kidney, and cardiac and skeletal muscle as well as smooth muscle. Of these methylxanthines caffeine has the most marked central nervous system effects.
Effects on central nervous system
In low and moderate doses, the methylxanthines, especially caffeine mild cortical arousal with increased alertness deferral of fatigue. The caffeine contained in beverages (eg, 100 mg in a cup of coffee) is sufficient to cause nervousness and insomnia in unusually sensitive individuals and slight bronchodilation in patients with asthma. At very high doses, medullary stimulation and convulsions may occur and can lead to death. 
Effects on cardiovascular system
The methylxanthines have direct positive chronotropic and inotropic effects on the heart. At low concentrations, these effects appear to result from increased catecholamine release that is caused by inhibition of presynaptic adenosine receptors. At higher concentrations (>10 mol/L), calcium influx may be increased directly through the increase in cAMP that result from inhibition of phosphodiesterase. At very high concentrations (>100 mol/L), sequestration of calcium by the sarcoplasmic reticulum is impaired. In unusually sensitive individuals, consumption of a few cups of coffee may result in arrhythmias, but in most people even parenteral administration of higher doses of the methylxanthines produces only sinus tachycardia and increased cardiac output. In large doses, these chemical agenrs also relax vascular smooth muscle except in cerebral blood vessels, where they cause contraction. Ordinary consumption of coffee and other methylxanthine containing beverages, however, usually raises the peripheral vascular resistance and blood pressure slightly, probably through the release of catecholamines. Methylxanthines decrease blood viscosity and may improve blood flow under certain conditions. But the mechanism of this action is not well defined. 
Effects on gastrointestinal tract
The methylxanthines stimulate secretion of both gastric acid and digestive enzymes. However, because even decaffeinated coffee has an effect on stimulating the secretions, it is believed that caffeine does not have a direct prominent effect on this issue. 
Effects on kidney
The methylxanthines are weak diuretics. This effect may involve both increased glomerular filtration and reduced tubular sodium reabsorption. 
Effects on smooth muscle
Methylxanthines produce bronchodilation. In addition to this effect on the airway smooth muscle, these agents inhibit antigen-induced release of histamine from lung tissue. This bronchodilation action is a very important therapeutic action in asthma. 
Effects on skeletal muscle
Methylxanthines strengthen the contractions of isolated skeletal muscle in vitro and have potent effects in improving contractility and in reversing fatigue of the diaphragm in patients with chronic obstructive lung disease. 
01. Dr AD Smith et al, Oxford dictionary of biochemistry and molecular biology, revised edition. 2000. pp 86
02. Bertram G Katzung, Basic and clinical pharmacology, 9th edition, pp 325-327
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