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disrupt the behavior in the nonuser, they may produce desirable effects for the habitual user (62).
Numerous studies have attempted to link methylxanthine intake with behavioral and attention problems in children. One study compared the behavioral effects of caffeine in children who were either 'high' (500 mg/day or more) or 'low' caffeine consumers (64). Children received 5 mg/kg of caffeine or a placebo twice daily for 2 weeks. When not receiving caffeine, 'high' consumers scored higher on an anxiety questionnaire and showed lower autonomic arousal than 'low' consumers. Parents perceived the 'low' consumers as more emotional, inattentive and restless when consuming caffeine, while they rated 'high' consumers as not changed. It was suggested that these results indicate a possible physiological basis in children for dietary caffeine preference.
Results from a recent meta-analysis indicated that caffeine is not associated with any significant adverse effects on behavior or cognition (65). Additionally, the results support previous conclusions that caffeine may have a small, beneficial effect in some children, decreasing behavior that is characterized as aggressive.
Sudden withdrawals of caffeine among habitual consumers can result in headaches, fatigue, dysphoric mood changes and sometimes nausea and vomiting (61, 62, 6668). Among individuals whose typical daily caffeine intake averaged 235 mg per day, 52% reported moderate or severe headaches and 811% reported symptoms of anxiety and depression when caffeine intake was discontinued. Moderate to heavy caffeine consumers also reported irritability, inability to work effectively, nervousness, restlessness and lethargy. Medical experts agree that these 'withdrawal' symptoms can be avoided by progressively decreasing caffeine intake over a few days.
In that caffeine stimulates the CNS, alleviates fatigue and strengthens voluntary muscle contraction, it has been associated with enhanced mental and physical performance. Performance of rapid information processing tasks such as problem solving, logical reasoning and mental arithmetic tasks was improved after consumption of 150 mg of caffeine, but impaired after 600 mg (61, 69). Caffeine appears to negatively affect the performance of complex tasks. The literature indicates consistent benefits of moderate doses (32256 mg) of caffeine on auditory and visual vigilance, although high doses may initially impair vigilance (70, 71). Caffeine can counteract the effects of fatigue on reaction time. Subjects who consumed caffeine doses of 75300 mg demonstrated shortened auditory reaction times and reported feeling more alert and active. However, objective measures of alertness and psychomotor coordination were not improved (72, 73).
There are conflicting results and controversy as to caffeine's ergogenic benefits (74). No significant effects on short-term, high-intensity performance have been
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